Crystallization induced by heterogeneous surfaces is an important process in geochemistry, biomineralization, and material synthesis, but the effects of heterogeneous surfaces on the transformation of metastable phases into new crystals remain poorly understood. In this work, we studied the transformation behaviors of ferrihydrite (Fhy) in the presence of hematite (Hem) nanoplates with specific exposed facets ({001} and {113}) at different pH (4, 7, and 12). Our results reveal that the Hem nanoplates can induce the transformation of Fhy to Hem/Gth (goethite) and accelerate the transformation rate. This effect is primarily achieved by modulating the dissolution-recrystallization process, i.e., accelerating the dissolution of Fhy and promoting the heterogeneous crystallization (to form new Hem/Gth) at the surface of added Hem nanoplates, and solution pH plays crucial roles in these processes. In specific, a relatively low supply of dissolved Fe3+ from Fhy at pH 4 favors island growth of new Hem at the {001} facets of Hem nanoplates and layer-by-layer growth at the {113} facets, which eventually results in the formation of thermodynamically stable pseudo-cubic morphology (exposing {012} facets). Because of the very low solubility of Fhy at pH 7, the induced transformation of Fhy by Hem nanoplates is rather weak. While at pH 12, a high supply of dissolved Fe3+ from Fhy benefits the layer-by-layer growth at {001} facets of Hem and the significant heteroepitaxial growth of Gth at the {113} facets. Besides the induced transformation, the direct solid-state transformation of Fhy into Hem and the homogeneous crystallization of dissolved Fe3+ also contribute to the transformation of Fhy. This study, for the first time, well reveals the mechanisms of induced transformation of Fhy in the presence of Hem nanoparticles, which will advance our understanding of the significant effects of heterogeneous surfaces in modulating the transformation of metastable phases, and supplement the transformation mechanisms of Fhy.
异质表面诱导的结晶是地球化学、生物矿化和材料合成中的一个重要过程，但异质表面对蜕变相转化为新晶体的影响仍然知之甚少。在这项工作中，我们研究了在不同 pH 值（4、7 和 12）下，具有特定暴露面（{001} 和 {113}）的赤铁矿（Hem）纳米板存在下的铁水物（Fhy）转化行为。我们的研究结果表明，Hem 纳米板可诱导 Fhy 向 Hem/Gth（鹅铁矿）转化，并加快转化速度。这种效果主要是通过调节溶解-再结晶过程实现的，即加速 Fhy 的溶解和促进添加 Hem 纳米板表面的异质结晶（形成新的 Hem/Gth），而溶液的 pH 值在这些过程中起着至关重要的作用。具体来说，在 pH 值为 4 时，Fhy 提供的溶解 Fe3+ 相对较少，这有利于新 Hem 在 Hem 纳米板的{001}面呈岛状生长，并在{113}面呈层状生长，最终形成热力学上稳定的伪立方体形态（露出{012}面）。由于 Fhy 在 pH 值为 7 时的溶解度非常低，因此 Hem 纳米板对 Fhy 的诱导转化相当微弱。而在 pH 值为 12 时，来自 Fhy 的大量溶解 Fe3+ 有利于 Hem 在{001}面上的逐层生长，以及 Gth 在{113}面上的显著异外延生长。除了诱导转化外，Fhy 向 Hem 的直接固态转化和溶解的 Fe3+ 的均匀结晶也有助于 Fhy 的转化。这项研究首次很好地揭示了在Hem纳米粒子存在下Fhy的诱导转化机制，这将推进我们对异质表面在调节蜕变相转化中的重要作用的理解，并补充了Fhy的转化机制。
{"title":"Coexisting hematite induces and accelerates the transformation of ferrihydrite: pathway and underlying mechanisms","authors":"Hongyan Wei, Jing Liu, Qingze Chen, Yixuan Yang, H. Xian, Xiaoliang Liang, Yiping Yang, Jianxi Zhu, Runliang Zhu","doi":"10.2138/am-2023-9281","DOIUrl":"https://doi.org/10.2138/am-2023-9281","url":null,"abstract":"\u0000 Crystallization induced by heterogeneous surfaces is an important process in geochemistry, biomineralization, and material synthesis, but the effects of heterogeneous surfaces on the transformation of metastable phases into new crystals remain poorly understood. In this work, we studied the transformation behaviors of ferrihydrite (Fhy) in the presence of hematite (Hem) nanoplates with specific exposed facets ({001} and {113}) at different pH (4, 7, and 12). Our results reveal that the Hem nanoplates can induce the transformation of Fhy to Hem/Gth (goethite) and accelerate the transformation rate. This effect is primarily achieved by modulating the dissolution-recrystallization process, i.e., accelerating the dissolution of Fhy and promoting the heterogeneous crystallization (to form new Hem/Gth) at the surface of added Hem nanoplates, and solution pH plays crucial roles in these processes. In specific, a relatively low supply of dissolved Fe3+ from Fhy at pH 4 favors island growth of new Hem at the {001} facets of Hem nanoplates and layer-by-layer growth at the {113} facets, which eventually results in the formation of thermodynamically stable pseudo-cubic morphology (exposing {012} facets). Because of the very low solubility of Fhy at pH 7, the induced transformation of Fhy by Hem nanoplates is rather weak. While at pH 12, a high supply of dissolved Fe3+ from Fhy benefits the layer-by-layer growth at {001} facets of Hem and the significant heteroepitaxial growth of Gth at the {113} facets. Besides the induced transformation, the direct solid-state transformation of Fhy into Hem and the homogeneous crystallization of dissolved Fe3+ also contribute to the transformation of Fhy. This study, for the first time, well reveals the mechanisms of induced transformation of Fhy in the presence of Hem nanoparticles, which will advance our understanding of the significant effects of heterogeneous surfaces in modulating the transformation of metastable phases, and supplement the transformation mechanisms of Fhy.","PeriodicalId":7768,"journal":{"name":"American Mineralogist","volume":null,"pages":null},"PeriodicalIF":2.7,"publicationDate":"2024-07-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141679617","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Chlorine-rich fluids play an important role in many geological processes, including the formation of economic deposits, crustal and mantle metasomatism, and high-grade metamorphism. Furthermore, the chlorine content of a fluid is often one of the main variables, dictating the fluid’s properties (i.e., the propensity for mass transport). Calcium amphiboles have the potential to be used to determine the chlorinity of paleo fluids, given sufficient knowledge of how Cl is partitioned between amphibole and fluid. Amphiboles with Fe# [= Fe2+/(Fe2+ + Mg)] = 1.0 were synthesized along the hastingsite–potassic-hastingsite join in the presence of variably concentrated FeCl2 brines, ranging from 1 molal to 100 molal. Syntheses were done at 700 °C and 3 kbar at fO2 values near Ni-NiO for durations of 96-132 hours. All amphiboles were characterized by powder X-ray diffraction and electron microprobe and several samples were analyzed by Mössbauer spectroscopy to determine ferric iron content. Results showed that amphibole Cl content increased linearly with the mole fraction of Cl in the brine and has no obvious relationship with the K# (= K/(K+Na)). Amphibole Cl contents varied from ~0.05 atoms per formula unit (apfu), synthesized in the most dilute brines, to ~1.05 apfu, synthesized in the most concentrated brines. Amphibole yield was related to the K#, with higher amphibole yields for the more K-rich bulk compositions. The amphibole ferric iron fraction was dependent on the brine FeCl2 concentration, increasing from 0.176 at 1 molal FeCl2, to 0.310 at 24 molal FeCl2. For brines more concentrated than 24 molal FeCl2, the ferric iron fraction significantly decreased to 0.116 at 50 molal FeCl2. The significant decrease in ferric iron fraction also coincided with a transition from magnetite to fayalite as a coexisting phase. The ferric iron fraction seemed to influence the total occupancy of the A site through the following reaction: A+ + Fe2+ ⇋ ☐ + Fe3+, where ☐ is a vacancy. Trends between Fe3+ and Cl display both positive and negative correlations, raising further questions on the role of Fe3+ on Cl incorporation. The findings of this study indicate that for Fe# = 1.0 amphiboles, the Cl concentration of the fluid plays the dominant, or perhaps only, role in amphibole Cl incorporation, with the occupant of the A site being inconsequential.
{"title":"The Effect of the ANa-AK ratio on Chlorine Incorporation into Hastingsitic Amphiboles","authors":"Jared P. Matteucci, David M. Jenkins, M. Dyar","doi":"10.2138/am-2023-9083","DOIUrl":"https://doi.org/10.2138/am-2023-9083","url":null,"abstract":"\u0000 Chlorine-rich fluids play an important role in many geological processes, including the formation of economic deposits, crustal and mantle metasomatism, and high-grade metamorphism. Furthermore, the chlorine content of a fluid is often one of the main variables, dictating the fluid’s properties (i.e., the propensity for mass transport). Calcium amphiboles have the potential to be used to determine the chlorinity of paleo fluids, given sufficient knowledge of how Cl is partitioned between amphibole and fluid. Amphiboles with Fe# [= Fe2+/(Fe2+ + Mg)] = 1.0 were synthesized along the hastingsite–potassic-hastingsite join in the presence of variably concentrated FeCl2 brines, ranging from 1 molal to 100 molal. Syntheses were done at 700 °C and 3 kbar at fO2 values near Ni-NiO for durations of 96-132 hours. All amphiboles were characterized by powder X-ray diffraction and electron microprobe and several samples were analyzed by Mössbauer spectroscopy to determine ferric iron content. Results showed that amphibole Cl content increased linearly with the mole fraction of Cl in the brine and has no obvious relationship with the K# (= K/(K+Na)). Amphibole Cl contents varied from ~0.05 atoms per formula unit (apfu), synthesized in the most dilute brines, to ~1.05 apfu, synthesized in the most concentrated brines. Amphibole yield was related to the K#, with higher amphibole yields for the more K-rich bulk compositions. The amphibole ferric iron fraction was dependent on the brine FeCl2 concentration, increasing from 0.176 at 1 molal FeCl2, to 0.310 at 24 molal FeCl2. For brines more concentrated than 24 molal FeCl2, the ferric iron fraction significantly decreased to 0.116 at 50 molal FeCl2. The significant decrease in ferric iron fraction also coincided with a transition from magnetite to fayalite as a coexisting phase. The ferric iron fraction seemed to influence the total occupancy of the A site through the following reaction: A+ + Fe2+ ⇋ ☐ + Fe3+, where ☐ is a vacancy. Trends between Fe3+ and Cl display both positive and negative correlations, raising further questions on the role of Fe3+ on Cl incorporation. The findings of this study indicate that for Fe# = 1.0 amphiboles, the Cl concentration of the fluid plays the dominant, or perhaps only, role in amphibole Cl incorporation, with the occupant of the A site being inconsequential.","PeriodicalId":7768,"journal":{"name":"American Mineralogist","volume":null,"pages":null},"PeriodicalIF":2.7,"publicationDate":"2024-07-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141677669","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
M. Lv, Shengcai Zhu, Jiachao Liu, Yi Hu, F. Zhu, X. Lai, Dongzhou Zhang, Bin Chen, Przemyslaw Dera, Jie Li, Susannah M Dorfman
Incorporation of ferric iron in mantle silicates stabilizes different crystal structures and changes phase transition conditions, thus impacting seismic wave speeds and discontinuities. In MgSiO3-Fe2O3 mixtures, recent experiments indicate the coexistence of fully oxidized iron-rich (Mg0.5Fe0.53+)(Fe0.53+Si0.5)O3 with Fe-poor silicate (wadsleyite or bridgmanite) and stishovite at 15 to 27 GPa and 1773 to 2000 K, conditions relevant to subducted lithosphere in the Earth’s transition zone and uppermost lower mantle. X-ray diffraction measurements show that (Mg0.5Fe0.53+)(Fe0.53+Si0.5)O3 recovered from these conditions adopts the R3c LiNbO3-type structure, which transforms to the bridgmanite structure again between 18.3 GPa and 24.7 GPa at 300 K. Diffraction observations are used to obtain the equation of state of the LiNbO3-type phase up to 18.3 GPa. These observations combined with multi-anvil experiments suggest that the stable phase of (Mg0.5Fe0.53+)(Fe0.53+Si0.5)O3 is bridgmanite at 15-27 GPa, which transforms on decompression to LiNbO3-type structure. Our calculation revealed that ordering of the ferric ion reduces the kinetic energy barrier of the transition between (Mg0.5Fe0.53+)(Fe0.53+Si0.5)O3 LiNbO3 structure and bridgmanite relative to the MgSiO3 akimotoite-bridgmanite system. Dense Fe3+-rich bridgmanite structure is thus stable at substantially shallower depths than MgSiO3 bridgmanite and would promote subduction.
{"title":"Phase transformation of ferric-iron-rich silicate in Earth’s mid-mantle","authors":"M. Lv, Shengcai Zhu, Jiachao Liu, Yi Hu, F. Zhu, X. Lai, Dongzhou Zhang, Bin Chen, Przemyslaw Dera, Jie Li, Susannah M Dorfman","doi":"10.2138/am-2024-9410","DOIUrl":"https://doi.org/10.2138/am-2024-9410","url":null,"abstract":"\u0000 Incorporation of ferric iron in mantle silicates stabilizes different crystal structures and changes phase transition conditions, thus impacting seismic wave speeds and discontinuities. In MgSiO3-Fe2O3 mixtures, recent experiments indicate the coexistence of fully oxidized iron-rich (Mg0.5Fe0.53+)(Fe0.53+Si0.5)O3 with Fe-poor silicate (wadsleyite or bridgmanite) and stishovite at 15 to 27 GPa and 1773 to 2000 K, conditions relevant to subducted lithosphere in the Earth’s transition zone and uppermost lower mantle. X-ray diffraction measurements show that (Mg0.5Fe0.53+)(Fe0.53+Si0.5)O3 recovered from these conditions adopts the R3c LiNbO3-type structure, which transforms to the bridgmanite structure again between 18.3 GPa and 24.7 GPa at 300 K. Diffraction observations are used to obtain the equation of state of the LiNbO3-type phase up to 18.3 GPa. These observations combined with multi-anvil experiments suggest that the stable phase of (Mg0.5Fe0.53+)(Fe0.53+Si0.5)O3 is bridgmanite at 15-27 GPa, which transforms on decompression to LiNbO3-type structure. Our calculation revealed that ordering of the ferric ion reduces the kinetic energy barrier of the transition between (Mg0.5Fe0.53+)(Fe0.53+Si0.5)O3 LiNbO3 structure and bridgmanite relative to the MgSiO3 akimotoite-bridgmanite system. Dense Fe3+-rich bridgmanite structure is thus stable at substantially shallower depths than MgSiO3 bridgmanite and would promote subduction.","PeriodicalId":7768,"journal":{"name":"American Mineralogist","volume":null,"pages":null},"PeriodicalIF":2.7,"publicationDate":"2024-07-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141678869","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Quartz is widely distributed in various magmatic-hydrothermal systems and shows variable textures and trace element contents in multiple generations, enabling quartz to serve as a robust tracer for monitoring hydrothermal fluid evolution. This study demonstrates that integrated high-resolution SEM-CL textures and trace element data of quartz can be used to constrain physicochemical fluid conditions and trace the genesis of quartz in porphyry ore-forming systems. The Bilihe deposit is a gold-only porphyry deposit located in the Central Asian orogenic belt, NE China. Four quartz generations were distinguished following a temporal sequence from early-stage dendritic quartz, unidirectional solidification textured quartz (UST quartz), gray banded vein quartz (BQ), to late-stage white calcite vein quartz (CQ), with the Au precipitation being mostly related to dendritic quartz, UST quartz, and BQ. The well-preserved dendritic quartz with sector-zoned CL intensities and euhedral oscillatory growth zones crystallized rapidly during the late magmatic stage. The relatively low Al contents of dendritic quartz were interpreted to be related to contemporaneous feldspar or mica crystallization, while the high-Ti contents indicate high-crystallization temperatures (~750 °C). The comb-layered UST quartz displays heterogeneous, patchy luminescence with weak zoning, hosts coeval melt and fluid inclusions, and retains the chemical characteristics of magmatic dendritic quartz. High-Ti and low-Al contents of UST quartz suggest a formation at relatively high temperatures (~700 °C) and high-pH conditions. Three sub-types can be defined for hydrothermal BQ (BQ1, BQ2, and BQ3) based on contrasting CL features and trace element contents. The Al contents increase from BQ1 to BQ2 followed by a drop in BQ3, corresponding to an initial decrease and subsequent increase in fluid acidity. Temperature estimates of BQ decrease from BQ1 (635 °C) to BQ3 (575 °C), which may, however, be disturbed by high growth rates and/or high-TiO2 activities. The CQ typically displays a CL-bright core and CL-dark rim with oscillating CL intensities and is characterized by the lowest Ti and highest Al, Li, and Sb contents compared to the other quartz types, which suggests a deposition from more acidic and lower temperature fluids (~250 °C). Trace element patterns indicate that a coupled Si4+ ↔ (Al3+) + (K+) element exchange vector is applicable to dendritic quartz, UST quartz, and BQ. By contrast, charge-compensated cation substitution of Si4+ ↔ (Al3+, Sb3+) + (Li+, Rb+) is favored for CQ. The comparison with compiled trace element data of quartz from other porphyry Au, Cu, and Mo deposits worldwide suggests that Ti, Al, Li, K, and Ge concentrations, as well as Al/Ti and Ge/Ti ratios, have the potential to discriminate the metal fertility of porphyry mineralization.
{"title":"Quartz texture and the chemical composition fingerprint of ore-forming fluid evolution at the Bilihe porphyry Au deposit, NE China","authors":"Jingxin Hong, Degao Zhai, Manuel Keith","doi":"10.2138/am-2022-8840","DOIUrl":"https://doi.org/10.2138/am-2022-8840","url":null,"abstract":"Quartz is widely distributed in various magmatic-hydrothermal systems and shows variable textures and trace element contents in multiple generations, enabling quartz to serve as a robust tracer for monitoring hydrothermal fluid evolution. This study demonstrates that integrated high-resolution SEM-CL textures and trace element data of quartz can be used to constrain physicochemical fluid conditions and trace the genesis of quartz in porphyry ore-forming systems. The Bilihe deposit is a gold-only porphyry deposit located in the Central Asian orogenic belt, NE China. Four quartz generations were distinguished following a temporal sequence from early-stage dendritic quartz, unidirectional solidification textured quartz (UST quartz), gray banded vein quartz (BQ), to late-stage white calcite vein quartz (CQ), with the Au precipitation being mostly related to dendritic quartz, UST quartz, and BQ. The well-preserved dendritic quartz with sector-zoned CL intensities and euhedral oscillatory growth zones crystallized rapidly during the late magmatic stage. The relatively low Al contents of dendritic quartz were interpreted to be related to contemporaneous feldspar or mica crystallization, while the high-Ti contents indicate high-crystallization temperatures (~750 °C). The comb-layered UST quartz displays heterogeneous, patchy luminescence with weak zoning, hosts coeval melt and fluid inclusions, and retains the chemical characteristics of magmatic dendritic quartz. High-Ti and low-Al contents of UST quartz suggest a formation at relatively high temperatures (~700 °C) and high-pH conditions. Three sub-types can be defined for hydrothermal BQ (BQ1, BQ2, and BQ3) based on contrasting CL features and trace element contents. The Al contents increase from BQ1 to BQ2 followed by a drop in BQ3, corresponding to an initial decrease and subsequent increase in fluid acidity. Temperature estimates of BQ decrease from BQ1 (635 °C) to BQ3 (575 °C), which may, however, be disturbed by high growth rates and/or high-TiO2 activities. The CQ typically displays a CL-bright core and CL-dark rim with oscillating CL intensities and is characterized by the lowest Ti and highest Al, Li, and Sb contents compared to the other quartz types, which suggests a deposition from more acidic and lower temperature fluids (~250 °C). Trace element patterns indicate that a coupled Si4+ ↔ (Al3+) + (K+) element exchange vector is applicable to dendritic quartz, UST quartz, and BQ. By contrast, charge-compensated cation substitution of Si4+ ↔ (Al3+, Sb3+) + (Li+, Rb+) is favored for CQ. The comparison with compiled trace element data of quartz from other porphyry Au, Cu, and Mo deposits worldwide suggests that Ti, Al, Li, K, and Ge concentrations, as well as Al/Ti and Ge/Ti ratios, have the potential to discriminate the metal fertility of porphyry mineralization.","PeriodicalId":7768,"journal":{"name":"American Mineralogist","volume":null,"pages":null},"PeriodicalIF":3.1,"publicationDate":"2024-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141520671","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
John M. Hughes, Daniel Harlov, John F. Rakovan, Jamshid Ahmadi, Melanie J. Sieber
Fluoride is one of the most consumed pharmaceuticals in the world, and its facility in preventing dental caries is recognized as one of the top 10 public health achievements of the 20th century. Although hydroxylapatite is often used as an analog of dental enamel, the details of the substitution of F for OH in the apatite anion column are not well known. Using new synthesis techniques, this study extends the structure work on P63/m apatites along the middle portion of the F-OH apatite join to compositions near the composition of fluoridated human teeth. The first F substituent in hydroxylapatite, near fluoridated dental enamel compositions, is dramatically underbonded by the surrounding Ca2 atoms (0.72 vu) in a hydroxylapatite matrix. However, the hydroxyl hydrogen is able to contribute 0.20 or 0.10 vu in hydrogen bonding, depending on whether the substitution creates a reversal site in the anion column; this hydrogen bonding alleviates the bonding requirements of the substituent F. As F concentrations increase along the join, the average hydroxyl contributes increasing amounts of hydrogen bonding to the F column anions; to mitigate the loss of its hydrogen bonding, the hydroxyl oxygen migrates toward the adjacent mirror plane that contains the bonded Ca2 atoms, and the triangle of bonded Ca2 ions concomitantly contracts. These two mechanisms increase bonding to the column hydroxyl oxygen from the adjoining Ca2 atoms to balance the loss of hydrogen bonding that stabilizes the substituent F column anion and the increasing concentration of underbonded F.
氟是世界上消耗量最大的药物之一，它在预防龋齿方面的作用被公认为是 20 世纪十大公共卫生成就之一。虽然羟基磷灰石经常被用作牙釉质的类似物，但人们对磷灰石阴离子柱中 F 取代 OH 的细节并不十分了解。本研究利用新的合成技术，沿着 F-OH 磷灰石连接的中间部分，将 P63/m 磷灰石的结构工作扩展到接近氟化人类牙齿成分的组成。羟基磷灰石中的第一个 F 取代基接近氟化牙釉质成分，在羟基磷灰石基质中与周围 Ca2 原子的结合力明显不足（0.72 vu）。然而，羟基氢能够提供 0.20 或 0.10 vu 的氢键，这取决于取代是否会在阴离子柱中产生一个反向位点；这种氢键减轻了取代基 F 的键合要求。随着连接处 F 浓度的增加，平均羟基对 F 柱阴离子的氢键作用越来越大；为了减轻氢键作用的损失，羟基氧向包含键合 Ca2 原子的相邻镜面迁移，键合 Ca2 离子的三角形同时收缩。这两种机制增加了邻近 Ca2 原子对柱羟基氧的键合，以平衡稳定取代基 F 柱阴离子的氢键损失和键合不足的 F 浓度的增加。
{"title":"REVISION 1: Structural variations along the apatite F-OH join: II. The role of hydrogen bonding in fluoridated teeth","authors":"John M. Hughes, Daniel Harlov, John F. Rakovan, Jamshid Ahmadi, Melanie J. Sieber","doi":"10.2138/am-2024-9393","DOIUrl":"https://doi.org/10.2138/am-2024-9393","url":null,"abstract":"\u0000 Fluoride is one of the most consumed pharmaceuticals in the world, and its facility in preventing dental caries is recognized as one of the top 10 public health achievements of the 20th century. Although hydroxylapatite is often used as an analog of dental enamel, the details of the substitution of F for OH in the apatite anion column are not well known. Using new synthesis techniques, this study extends the structure work on P63/m apatites along the middle portion of the F-OH apatite join to compositions near the composition of fluoridated human teeth. The first F substituent in hydroxylapatite, near fluoridated dental enamel compositions, is dramatically underbonded by the surrounding Ca2 atoms (0.72 vu) in a hydroxylapatite matrix. However, the hydroxyl hydrogen is able to contribute 0.20 or 0.10 vu in hydrogen bonding, depending on whether the substitution creates a reversal site in the anion column; this hydrogen bonding alleviates the bonding requirements of the substituent F. As F concentrations increase along the join, the average hydroxyl contributes increasing amounts of hydrogen bonding to the F column anions; to mitigate the loss of its hydrogen bonding, the hydroxyl oxygen migrates toward the adjacent mirror plane that contains the bonded Ca2 atoms, and the triangle of bonded Ca2 ions concomitantly contracts. These two mechanisms increase bonding to the column hydroxyl oxygen from the adjoining Ca2 atoms to balance the loss of hydrogen bonding that stabilizes the substituent F column anion and the increasing concentration of underbonded F.","PeriodicalId":7768,"journal":{"name":"American Mineralogist","volume":null,"pages":null},"PeriodicalIF":3.1,"publicationDate":"2024-06-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141347714","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Changhao Li, Ping Shen, R. Seltmann, Di Zhang, H. Pan, Eleonora Yusupovha Seitmuratova
The Paleozoic Aktogai Group in Kazakhstan ranks among the thirty largest porphyry Cu deposits globally. The Aktogai deposit is the largest one in the Aktogai Group and is characterized by intensive potassic alteration where the dominant orebody occurred. However, its mineralization processes remain unclear. Our investigation focused on the texture, trace elements, fluid inclusions, and in-situ oxygen isotopes of the quartz from the ore-related tonalite porphyry and associated potassic alteration at Aktogai to trace the deposit’s mineralization processes. Ti-in-quartz thermobarometry, fluid inclusion microthermometry, and geological characteristics indicate that the ore-related magma at Aktogai originated from a shallow magma chamber at ~1.9±0.5 kbar (~7.2±1.9 km) and intruded as the tonalite porphyry stock at ~1.7-2.4 km. The potassic alteration and associated Cu mineralization comprise five types of veins (A1, A2, B1, B2, and C) and two types of altered rocks (biotite and K-feldspar). Among them, nine types of hydrothermal quartz were identified from early to late: (1) VQA1 in A1 veins and RQbt in biotite altered rocks, (2) VQA2 in A2 veins and RQkfs in K-feldspar altered rocks, (3) VQB1 in B1 veins and VQB2E in B2 veins, and (4) quartz associated with Cu-Fe sulfides (VQB2L, VQBC, and VQC) in B and C veins. Titanium contents of the quartz decreased, while Al/Ti ratios increased from early to late. Fluid inclusion microthermometry and mineral thermometers reveal that VQA1, RQbt, and hydrothermal biotite formed under high-temperature (~470-560 °C) and ductile conditions. VQA2, RQkfs, VQB1, and hydrothermal K-feldspar formed during the transition stage from ductile to brittle, with temperatures of ~350-540 °C. The rapid decrease in pressure from lithostatic to hydrostatic pressure led to fluid boiling and minor involvement of meteoric water (~11-14 %) in the mineralizing fluid. Extensive recrystallization in VQA1 to VQB1 was associated with repeated cleavage and healing of the intrusion. With cooling, K-feldspar decomposition and hydrolysis increased. Fluid cooling and water-rock reactions resulted in the co-precipitation of Cu-Fe sulfides, white mica, chlorite, VQBC, and VQC, with temperatures of ~275-370 °C and brittle conditions. The Paleozoic Aktogai deposit exhibits formation depths and fluid evolution processes similar to Mesozoic and Cenozoic PCDs worldwide. The close association between Cu-Fe sulfides and later quartz formed under intermediate-temperature conditions at Aktogai implies that Cu-Fe sulfides are not precipitated during early high-temperature conditions in porphyry Cu deposits.
{"title":"Quartz textures, trace elements, fluid inclusions, and in-situ oxygen isotopes from Aktogai porphyry Cu deposit, Kazakhstan","authors":"Changhao Li, Ping Shen, R. Seltmann, Di Zhang, H. Pan, Eleonora Yusupovha Seitmuratova","doi":"10.2138/am-2023-8931","DOIUrl":"https://doi.org/10.2138/am-2023-8931","url":null,"abstract":"\u0000 The Paleozoic Aktogai Group in Kazakhstan ranks among the thirty largest porphyry Cu deposits globally. The Aktogai deposit is the largest one in the Aktogai Group and is characterized by intensive potassic alteration where the dominant orebody occurred. However, its mineralization processes remain unclear. Our investigation focused on the texture, trace elements, fluid inclusions, and in-situ oxygen isotopes of the quartz from the ore-related tonalite porphyry and associated potassic alteration at Aktogai to trace the deposit’s mineralization processes. Ti-in-quartz thermobarometry, fluid inclusion microthermometry, and geological characteristics indicate that the ore-related magma at Aktogai originated from a shallow magma chamber at ~1.9±0.5 kbar (~7.2±1.9 km) and intruded as the tonalite porphyry stock at ~1.7-2.4 km. The potassic alteration and associated Cu mineralization comprise five types of veins (A1, A2, B1, B2, and C) and two types of altered rocks (biotite and K-feldspar). Among them, nine types of hydrothermal quartz were identified from early to late: (1) VQA1 in A1 veins and RQbt in biotite altered rocks, (2) VQA2 in A2 veins and RQkfs in K-feldspar altered rocks, (3) VQB1 in B1 veins and VQB2E in B2 veins, and (4) quartz associated with Cu-Fe sulfides (VQB2L, VQBC, and VQC) in B and C veins. Titanium contents of the quartz decreased, while Al/Ti ratios increased from early to late. Fluid inclusion microthermometry and mineral thermometers reveal that VQA1, RQbt, and hydrothermal biotite formed under high-temperature (~470-560 °C) and ductile conditions. VQA2, RQkfs, VQB1, and hydrothermal K-feldspar formed during the transition stage from ductile to brittle, with temperatures of ~350-540 °C. The rapid decrease in pressure from lithostatic to hydrostatic pressure led to fluid boiling and minor involvement of meteoric water (~11-14 %) in the mineralizing fluid. Extensive recrystallization in VQA1 to VQB1 was associated with repeated cleavage and healing of the intrusion. With cooling, K-feldspar decomposition and hydrolysis increased. Fluid cooling and water-rock reactions resulted in the co-precipitation of Cu-Fe sulfides, white mica, chlorite, VQBC, and VQC, with temperatures of ~275-370 °C and brittle conditions. The Paleozoic Aktogai deposit exhibits formation depths and fluid evolution processes similar to Mesozoic and Cenozoic PCDs worldwide. The close association between Cu-Fe sulfides and later quartz formed under intermediate-temperature conditions at Aktogai implies that Cu-Fe sulfides are not precipitated during early high-temperature conditions in porphyry Cu deposits.","PeriodicalId":7768,"journal":{"name":"American Mineralogist","volume":null,"pages":null},"PeriodicalIF":3.1,"publicationDate":"2024-06-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141346277","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
V. V. Gurzhiy, A. Kasatkin, N. Chukanov, J. Plášil
Uramphite, (NH4)(UO2)(PO4) · 3H2O, was found at Beshtau uranium deposit, Northern Caucasus, Russia, as the second world occurrence besides its type locality, Tura-Kavak uranium-coal deposit in Kyrgyzstan. In Beshtau, it occurs as yellow tabular crystals up to 0.3 mm grouped in crusts on a matrix composed of albite, microcline, quartz and chamosite in association with liebigite, meta-autunite and plumbogummite. The empirical formula calculated on the basis of 6 O apfu and 3 H2O is [(NH4)0.91K0.08]Σ0.99P0.99U6+1.01O6 · 3H2O. According to single-crystal X-ray diffraction, uramphite is tetragonal, P4/nmm, with a = 6.9971(3), c = 8.9787(9) Å, V = 439.59(6) Å3 and Z = 2. The crystal structure was refined to R1 = 3.28% for 255 unique observed reflections with |Fo| ≥ 4σF. A model for the distribution of H2O and NH4+ molecules in the interlayer space based on the electron density distribution data is proposed. The mineral belongs to the meta-autunite group. The IR spectrum shows the splitting of the band of H–N–H bending vibrations into four components, which is explained here by the resonance splitting of a group of NH4+ cations occurring around the four-fold axis at close distances from each other. Uramphite is related to uramarsite, (NH4)(UO2)(AsO4)·3H2O. Both minerals are very similar, which is apparent from crystal structures and IR spectra. However, they are not isostructural. Uramarsite is triclinic, contains a significant amount of P in arsenate sites, and significantly differs from uramphite by the arrangement of H2O and NH4+ molecules in the interlayer space (planar and well organized in uramphite vs. chaotic manner in uramarsite).
{"title":"Uramphite, (NH4)(UO2)(PO4) · 3H2O, from the second world occurrence, Beshtau uranium deposit, Northern Caucasus, Russia: crystal-structure refinement, infrared spectroscopy, and relation to uramarsite","authors":"V. V. Gurzhiy, A. Kasatkin, N. Chukanov, J. Plášil","doi":"10.2138/am-2024-9313","DOIUrl":"https://doi.org/10.2138/am-2024-9313","url":null,"abstract":"\u0000 Uramphite, (NH4)(UO2)(PO4) · 3H2O, was found at Beshtau uranium deposit, Northern Caucasus, Russia, as the second world occurrence besides its type locality, Tura-Kavak uranium-coal deposit in Kyrgyzstan. In Beshtau, it occurs as yellow tabular crystals up to 0.3 mm grouped in crusts on a matrix composed of albite, microcline, quartz and chamosite in association with liebigite, meta-autunite and plumbogummite. The empirical formula calculated on the basis of 6 O apfu and 3 H2O is [(NH4)0.91K0.08]Σ0.99P0.99U6+1.01O6 · 3H2O. According to single-crystal X-ray diffraction, uramphite is tetragonal, P4/nmm, with a = 6.9971(3), c = 8.9787(9) Å, V = 439.59(6) Å3 and Z = 2. The crystal structure was refined to R1 = 3.28% for 255 unique observed reflections with |Fo| ≥ 4σF. A model for the distribution of H2O and NH4+ molecules in the interlayer space based on the electron density distribution data is proposed. The mineral belongs to the meta-autunite group. The IR spectrum shows the splitting of the band of H–N–H bending vibrations into four components, which is explained here by the resonance splitting of a group of NH4+ cations occurring around the four-fold axis at close distances from each other. Uramphite is related to uramarsite, (NH4)(UO2)(AsO4)·3H2O. Both minerals are very similar, which is apparent from crystal structures and IR spectra. However, they are not isostructural. Uramarsite is triclinic, contains a significant amount of P in arsenate sites, and significantly differs from uramphite by the arrangement of H2O and NH4+ molecules in the interlayer space (planar and well organized in uramphite vs. chaotic manner in uramarsite).","PeriodicalId":7768,"journal":{"name":"American Mineralogist","volume":null,"pages":null},"PeriodicalIF":3.1,"publicationDate":"2024-06-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141374142","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Sha Wang, Qingbo Wang, Yu Ye, Dan Liu, Xi Zhu, Yancheng Hu, Yunfan Miao, Zhen Wu, Yanming Pan
Rutile is the most common TiO2 mineral on Earth’s surface and transforms to CaCl2- and α-PbO2-type structures at elevated pressures in subducted basaltic crusts. In this study, we synthesized hydrous CaCl2- and α-PbO2-TiO2 crystals with various Al3+ concentrations using a multi-anvil press. Al3+ is incorporated into the CaCl2- and rutile-type phases mainly in the form of 3Ti4+ = 4Al3+, while the coupled substitution of Ti4+ = Al3+ + H+ is dominant in the α-PbO2-type structure, forming Ti1-x(AlH)xO2 solid solutions. Consequently, the water solubility in Al-bearing α-PbO2-TiO2 is at least one order of magnitude greater than those in rutile- and CaCl2-phases, making TiO2 a significant water carrier at the pressure-temperature (P–T) conditions in the mantle transition zone (410 to 660 km depth in deep Earth’s interior), when coexisting with Al3+ and Fe3+. High-P and high-T Raman spectra were collected for these synthetic samples. The CaCl2- and α-PbO2-type phases irreversibly transform to a rutile-type structure at 950 K and ambient pressure. A reversible α-PbO2 → baddeleyite phase transition in TiO2 is detected at approximately P = 10 GPa and T = 300 K, and the incorporation of smaller amounts of Al3+ cations postpones the phase transition pressure. The lattice vibrational modes typically shift to lower frequencies at elevated temperature and to higher frequencies with increasing pressure due to variations in Ti(Al)-O bond length with temperature or pressure. Fourier transform infrared (FTIR) spectroscopic measurements were conducted on the samples under high-T or high-P conditions. Both T- and P-dependences are negative for the OH stretching vibrations in these TiO2 polymorphs, except that the OH bands in the α-PbO2-type samples exhibit a blueshift at elevated temperature. A negative linear correlation can be drawn between the measured OH stretching frequencies and the incorporated M3+O6 quadratic elongation, which were computed based on first-principles calculations. The local octahedral distortion can provide useful insights for understanding the M3+ and H+ incorporation mechanism in TiO2 and SiO2 structures.
{"title":"Al3+ and H+ substitutions in TiO2 polymorphs: structural and vibrational investigations","authors":"Sha Wang, Qingbo Wang, Yu Ye, Dan Liu, Xi Zhu, Yancheng Hu, Yunfan Miao, Zhen Wu, Yanming Pan","doi":"10.2138/am-2024-9316","DOIUrl":"https://doi.org/10.2138/am-2024-9316","url":null,"abstract":"\u0000 Rutile is the most common TiO2 mineral on Earth’s surface and transforms to CaCl2- and α-PbO2-type structures at elevated pressures in subducted basaltic crusts. In this study, we synthesized hydrous CaCl2- and α-PbO2-TiO2 crystals with various Al3+ concentrations using a multi-anvil press. Al3+ is incorporated into the CaCl2- and rutile-type phases mainly in the form of 3Ti4+ = 4Al3+, while the coupled substitution of Ti4+ = Al3+ + H+ is dominant in the α-PbO2-type structure, forming Ti1-x(AlH)xO2 solid solutions. Consequently, the water solubility in Al-bearing α-PbO2-TiO2 is at least one order of magnitude greater than those in rutile- and CaCl2-phases, making TiO2 a significant water carrier at the pressure-temperature (P–T) conditions in the mantle transition zone (410 to 660 km depth in deep Earth’s interior), when coexisting with Al3+ and Fe3+. High-P and high-T Raman spectra were collected for these synthetic samples. The CaCl2- and α-PbO2-type phases irreversibly transform to a rutile-type structure at 950 K and ambient pressure. A reversible α-PbO2 → baddeleyite phase transition in TiO2 is detected at approximately P = 10 GPa and T = 300 K, and the incorporation of smaller amounts of Al3+ cations postpones the phase transition pressure. The lattice vibrational modes typically shift to lower frequencies at elevated temperature and to higher frequencies with increasing pressure due to variations in Ti(Al)-O bond length with temperature or pressure. Fourier transform infrared (FTIR) spectroscopic measurements were conducted on the samples under high-T or high-P conditions. Both T- and P-dependences are negative for the OH stretching vibrations in these TiO2 polymorphs, except that the OH bands in the α-PbO2-type samples exhibit a blueshift at elevated temperature. A negative linear correlation can be drawn between the measured OH stretching frequencies and the incorporated M3+O6 quadratic elongation, which were computed based on first-principles calculations. The local octahedral distortion can provide useful insights for understanding the M3+ and H+ incorporation mechanism in TiO2 and SiO2 structures.","PeriodicalId":7768,"journal":{"name":"American Mineralogist","volume":null,"pages":null},"PeriodicalIF":3.1,"publicationDate":"2024-06-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141375066","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Benedetta Chrappan Soldavini, D. Comboni, M. Hanfland, Marco Merlini
Natural clinochlore structural variations with pressure have been studied by in-situ single crystal X-ray diffraction in diamond-anvil cell in the pressure interval 0-20 GPa at room temperature. High resolution data allowed for the identification of a polytypic phase transition at about 9 GPa. Around 4.32(5) GPa, the sample showed a significant deviation from linear behavior of the unit cell parameters, particularly in the c and β values, abruptly interrupted when the phase transition occurs. The X-ray diffraction patterns showed a drastic reduction of diffuse scattering due to the stabilization of the high-pressure structure, suggesting that the atomic reorganization of the layers led to a disorder reduction. The phase transition showed complete reversibility during the experiment. Ab-initio structural refinements identified the transition as polytypic, from the initial IIb-4 triclinic polytype (space group C1) to the IIa-1 monoclinic structure (space group C2/m), with unit cell parameters a=5.2058(6) Å, b=9.0208(4) Å, c=13.560(7) Å, β=97.34(3)°. The latter was theoretically derived back in the ’60s as the least stable chlorite polytype and has never been observed before in natural chlorites. The phase transition also has a significative effect on the bulk modulus, with a reduction from K0=81.2(13) GPa to K0=56.0(6), for the high-pressure structure. An isothermal run at 600 K from ambient pressure to 14 GPa showed the same phase transition at 7.8(5) GPa. Its occurrence at lower pressures suggests a negative P/T slope for this transition. Therefore, at high-temperature and high-pressure conditions compatible with impact phenomena, the polytypic phase transition could prevent chlorite from early destabilization and dehydration.
{"title":"High-pressure phase transition in clinochlore: IIa polytype stabilization","authors":"Benedetta Chrappan Soldavini, D. Comboni, M. Hanfland, Marco Merlini","doi":"10.2138/am-2023-9277","DOIUrl":"https://doi.org/10.2138/am-2023-9277","url":null,"abstract":"\u0000 Natural clinochlore structural variations with pressure have been studied by in-situ single crystal X-ray diffraction in diamond-anvil cell in the pressure interval 0-20 GPa at room temperature. High resolution data allowed for the identification of a polytypic phase transition at about 9 GPa. Around 4.32(5) GPa, the sample showed a significant deviation from linear behavior of the unit cell parameters, particularly in the c and β values, abruptly interrupted when the phase transition occurs. The X-ray diffraction patterns showed a drastic reduction of diffuse scattering due to the stabilization of the high-pressure structure, suggesting that the atomic reorganization of the layers led to a disorder reduction. The phase transition showed complete reversibility during the experiment. Ab-initio structural refinements identified the transition as polytypic, from the initial IIb-4 triclinic polytype (space group C1) to the IIa-1 monoclinic structure (space group C2/m), with unit cell parameters a=5.2058(6) Å, b=9.0208(4) Å, c=13.560(7) Å, β=97.34(3)°. The latter was theoretically derived back in the ’60s as the least stable chlorite polytype and has never been observed before in natural chlorites. The phase transition also has a significative effect on the bulk modulus, with a reduction from K0=81.2(13) GPa to K0=56.0(6), for the high-pressure structure. An isothermal run at 600 K from ambient pressure to 14 GPa showed the same phase transition at 7.8(5) GPa. Its occurrence at lower pressures suggests a negative P/T slope for this transition. Therefore, at high-temperature and high-pressure conditions compatible with impact phenomena, the polytypic phase transition could prevent chlorite from early destabilization and dehydration.","PeriodicalId":7768,"journal":{"name":"American Mineralogist","volume":null,"pages":null},"PeriodicalIF":3.1,"publicationDate":"2024-06-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141375515","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Shangqin Hao, Dapeng Yang, Wenzhong Wang, F. Zou, Zhongqing Wu
The distribution of water reservoirs in the deep Earth is critical to understanding geochemical evolution and mantle dynamics. Phase D is a potential water carrier in the slab subducted to the uppermost lower mantle (ULM) and its seismic velocity and density characteristics are important for seismological detection on water reservoirs, but these properties remain poorly constrained. Here we calculate the seismic velocities and density of Mg-endmember phase D (MgSi2H2O6) under the ULM conditions using first-principles calculations based on the density functional theory. The velocities of phase D are higher than those of periclase and slightly lower than those of bridgmanite by 0.5–3.4% for VP and by 0–1.9% for VS between 660- and 1000-km depths. Considering its relatively low content, phase D can hardly produce a low-velocity anomaly in the ULM observed by seismological studies. However, due to its strong elastic anisotropy, it may contribute significantly to the observed seismic anisotropy at a similar depth. Additionally, phase D dehydrates into bridgmanite and stishovite at the ULM, producing insignificant velocity changes but a substantial density increase of ~14%. Therefore, the dehydration is probably too weak to generate discontinuities associated with velocity jumps, whereas it may account for seismic discontinuities that are sensitive to impedance changes, and particularly density jumps, near the dehydration depth observed in some subduction zones.
{"title":"Thermoelasticity of phase D and implications for low-velocity anomalies and local discontinuities at the uppermost lower mantle","authors":"Shangqin Hao, Dapeng Yang, Wenzhong Wang, F. Zou, Zhongqing Wu","doi":"10.2138/am-2024-9305","DOIUrl":"https://doi.org/10.2138/am-2024-9305","url":null,"abstract":"\u0000 The distribution of water reservoirs in the deep Earth is critical to understanding geochemical evolution and mantle dynamics. Phase D is a potential water carrier in the slab subducted to the uppermost lower mantle (ULM) and its seismic velocity and density characteristics are important for seismological detection on water reservoirs, but these properties remain poorly constrained. Here we calculate the seismic velocities and density of Mg-endmember phase D (MgSi2H2O6) under the ULM conditions using first-principles calculations based on the density functional theory. The velocities of phase D are higher than those of periclase and slightly lower than those of bridgmanite by 0.5–3.4% for VP and by 0–1.9% for VS between 660- and 1000-km depths. Considering its relatively low content, phase D can hardly produce a low-velocity anomaly in the ULM observed by seismological studies. However, due to its strong elastic anisotropy, it may contribute significantly to the observed seismic anisotropy at a similar depth. Additionally, phase D dehydrates into bridgmanite and stishovite at the ULM, producing insignificant velocity changes but a substantial density increase of ~14%. Therefore, the dehydration is probably too weak to generate discontinuities associated with velocity jumps, whereas it may account for seismic discontinuities that are sensitive to impedance changes, and particularly density jumps, near the dehydration depth observed in some subduction zones.","PeriodicalId":7768,"journal":{"name":"American Mineralogist","volume":null,"pages":null},"PeriodicalIF":3.1,"publicationDate":"2024-06-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141374121","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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