Synnøve Elvevold, Joachim Jacobs, Leif-Erik Rydland Pedersen, Øyvind Sunde, Ane K. Engvik, Per Inge Myhre
Abstract. Central Dronning Maud Land (cDML) is part of the late Mesoproterozoic Maud Belt in East Antarctica, which was metamorphosed and deformed during the Ediacaran–Cambrian Gondwana assembly. Here we study high-pressure (HP) mafic rocks in Gjelsvikfjella, cDML, which occur as lenses and pods transposed in highly strained, upper amphibolite-facies gneisses. We present a P–T–t history for the HP rocks based on mineral assemblages, reaction textures and new U–Pb zircon data. Relics that indicate an early HP granulite-facies stage have been identified in anhydrous garnet–clinopyroxene rocks. The peak-pressure assemblage was plagioclase-free and contained garnet, titanite, clinopyroxene and quartz. The HP assemblage has been extensively overprinted by lower-pressure phases and exhibits a variety of symplectite and corona textures that record the post-peak-pressure evolution of the rocks. Decompression and heating in the granulite-facies field resulted in the replacement of titanite by ilmenite–clinopyroxene symplectite, formation of clinopyroxene–plagioclase intergrowths and resorption of garnet by plagioclase–clinopyroxene kelyphite. Formation of late orthopyroxene in symplectites and kelyphites demonstrates that the P–T evolution entered the medium-pressure granulite-facies field. The peak metamorphic stage was followed by retrograde cooling into the amphibolite facies. In situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb dating of zircons indicate Mesoproterozoic protolith ages (1150–1000 Ma) and Ediacaran–Cambrian metamorphic reworking at ca. 568 and ca. 514 Ma. The HP granulites were formed and exhumed during a clockwise P–T evolution related to continental collision during Gondwana amalgamation, followed by post-collisional extension and orogenic collapse.
{"title":"Symplectite and kelyphite formation during decompression of mafic granulite from Gjelsvikfjella, central Dronning Maud Land, Antarctica","authors":"Synnøve Elvevold, Joachim Jacobs, Leif-Erik Rydland Pedersen, Øyvind Sunde, Ane K. Engvik, Per Inge Myhre","doi":"10.5194/ejm-35-969-2023","DOIUrl":"https://doi.org/10.5194/ejm-35-969-2023","url":null,"abstract":"Abstract. Central Dronning Maud Land (cDML) is part of the late Mesoproterozoic Maud Belt in East Antarctica, which was metamorphosed and deformed during the Ediacaran–Cambrian Gondwana assembly. Here we study high-pressure (HP) mafic rocks in Gjelsvikfjella, cDML, which occur as lenses and pods transposed in highly strained, upper amphibolite-facies gneisses. We present a P–T–t history for the HP rocks based on mineral assemblages, reaction textures and new U–Pb zircon data. Relics that indicate an early HP granulite-facies stage have been identified in anhydrous garnet–clinopyroxene rocks. The peak-pressure assemblage was plagioclase-free and contained garnet, titanite, clinopyroxene and quartz. The HP assemblage has been extensively overprinted by lower-pressure phases and exhibits a variety of symplectite and corona textures that record the post-peak-pressure evolution of the rocks. Decompression and heating in the granulite-facies field resulted in the replacement of titanite by ilmenite–clinopyroxene symplectite, formation of clinopyroxene–plagioclase intergrowths and resorption of garnet by plagioclase–clinopyroxene kelyphite. Formation of late orthopyroxene in symplectites and kelyphites demonstrates that the P–T evolution entered the medium-pressure granulite-facies field. The peak metamorphic stage was followed by retrograde cooling into the amphibolite facies. In situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb dating of zircons indicate Mesoproterozoic protolith ages (1150–1000 Ma) and Ediacaran–Cambrian metamorphic reworking at ca. 568 and ca. 514 Ma. The HP granulites were formed and exhumed during a clockwise P–T evolution related to continental collision during Gondwana amalgamation, followed by post-collisional extension and orogenic collapse.","PeriodicalId":11971,"journal":{"name":"European Journal of Mineralogy","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2023-11-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"134992995","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}
Abstract. Silica minerals constitute a main target to assess the origin of life or the possibility of its emergence. On Earth, ancient hydrothermal silica deposits have preserved the oldest forms of life. Beyond Earth, such silica-rich hydrothermal systems have been observed on Mars by orbital near-infrared (NIR) remote sensing and in situ rover exploration. This work investigates the variations of texture and NIR properties of opal with temperature, within a single geological context of hot springs. Silica sinters have been sampled in Icelandic hot-spring fields, in the Reykholt region, and at the Hveravellir site, with water temperature ranging from 14 to 101 ∘C. Variations in the NIR spectral features (concavity ratio criteria, CRC) vary with fluid temperature, lithofacies, and microtexture. Only high-temperature samples display high CRC values (CRC5200>0.85), but low CRC values (CRC5200 < 0.75) are measured for any temperature. Hence, temperature is not the only parameter controlling spectral properties of opal. Several other parameters such as the hydrodynamic context, the microbial activity, silica micro-textures, and porosity may also affect silica precipitation, the incorporation and speciation of water in it, and thus its NIR signature. The observations suggest a limitation in the use of NIR spectral features for the interpretation of the geological context of fossil opal on Earth or Mars: only opal with high CRC values can be inferred as being formed by hydrothermal activity. Low CRC values can be attributed to either low-temperature hydrothermal activity (< 50–60 ∘C) or to continental weathering.
{"title":"Near-infrared signature of hydrothermal opal: a case study of Icelandic silica sinters","authors":"Maxime Pineau, Boris Chauviré, Benjamin Rondeau","doi":"10.5194/ejm-35-949-2023","DOIUrl":"https://doi.org/10.5194/ejm-35-949-2023","url":null,"abstract":"Abstract. Silica minerals constitute a main target to assess the origin of life or the possibility of its emergence. On Earth, ancient hydrothermal silica deposits have preserved the oldest forms of life. Beyond Earth, such silica-rich hydrothermal systems have been observed on Mars by orbital near-infrared (NIR) remote sensing and in situ rover exploration. This work investigates the variations of texture and NIR properties of opal with temperature, within a single geological context of hot springs. Silica sinters have been sampled in Icelandic hot-spring fields, in the Reykholt region, and at the Hveravellir site, with water temperature ranging from 14 to 101 ∘C. Variations in the NIR spectral features (concavity ratio criteria, CRC) vary with fluid temperature, lithofacies, and microtexture. Only high-temperature samples display high CRC values (CRC5200>0.85), but low CRC values (CRC5200 < 0.75) are measured for any temperature. Hence, temperature is not the only parameter controlling spectral properties of opal. Several other parameters such as the hydrodynamic context, the microbial activity, silica micro-textures, and porosity may also affect silica precipitation, the incorporation and speciation of water in it, and thus its NIR signature. The observations suggest a limitation in the use of NIR spectral features for the interpretation of the geological context of fossil opal on Earth or Mars: only opal with high CRC values can be inferred as being formed by hydrothermal activity. Low CRC values can be attributed to either low-temperature hydrothermal activity (< 50–60 ∘C) or to continental weathering.","PeriodicalId":11971,"journal":{"name":"European Journal of Mineralogy","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2023-11-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"136346583","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}
Rosario Esposito, Daniele Redi, Leonid V. Danyushevsky, Andrey Gurenko, Benedetto De Vivo, Craig E. Manning, Robert J. Bodnar, Matthew Steele-MacInnis, Maria-Luce Frezzotti
Abstract. Mount Somma–Vesuvius is a stratovolcano that represents a geological hazard to the population of the city of Naples and surrounding towns in southern Italy. Historically, volcanic eruptions at Mt. Somma–Vesuvius (SV) include high-magnitude Plinian eruptions, such as the infamous 79 CE eruption that occurred after 295 years of quiescence and killed thousands of people in Pompeii and surrounding towns and villages. The last eruption at SV was in 1944 and showed a Volcanic Explosivity Index (VEI) of 3 (0.01 km3 of volcanic material erupted). Following the 1944 eruption, SV has been dormant for the past nearly 79 years, with only minor fumarolic and seismic activity. During its long history, centuries of dormancy at SV have ended with Plinian eruptions (VEI 6) that signal the beginning of a new cycle of eruptive activity. Thus, the current dormancy stage demands a need to better understand the mechanism involved in high-magnitude eruptions in order to better predict future eruption magnitude and style. Despite centuries of research on the SV volcanic system, many questions remain, including the evolution of magmatic volatiles from deep primitive magmas to shallower more evolved magmas. Developing a better understanding of the physical and chemical processes associated with volatile evolution at SV can provide insights into magma dynamics and the mechanisms that trigger highly explosive eruptions at SV. In this study, we present new data for the pre-eruptive volatile contents of magmas associated with four Plinian and two inter-Plinian eruptions at SV based on analyses of reheated melt inclusions (MIs) hosted in olivine. We correct the volatile contents of bubble-bearing MIs by taking into account the volatile contents of bubbles in the MIs. We recognize two groups of MIs: one group hosted in high-Fo olivine (Fo85–90) and relatively rich in volatiles and the other group hosted in low-Fo olivine (Fo70–69) and relatively depleted in volatiles. The correlation between volatile contents and compositions of host olivines suggests that magma fractionation took place under volatile-saturated conditions and that more differentiated magmas reside at shallower levels relative to less evolved/quasi-primitive magmas. Using the CO2 contents of corrected MIs hosted in Fo90 olivine from SV, we estimate that 347 to 686 t d−1 of magmatic CO2 exsolved from SV magmas during the last 3 centuries (38–75 Mt in total) of volcanic activity. Although this study is limited to only few SV magmas, we suggest that further study applying similar methods could shed light on the apparent lack of correlation between the volatile contents of MIs and the style and age of eruptions. Further, such studies could provide additional constraints on the origin of CO2 and the interaction between the carbonate platform and ascending magmas below SV.
摘要索玛-维苏威火山是一座层状火山,对意大利南部那不勒斯市和周边城镇的人口构成地质灾害。从历史上看,索玛-维苏威火山(SV)的火山爆发包括高震级的普林尼火山爆发,比如公元79年那次臭名昭著的火山爆发,那次火山爆发发生在295年的平静之后,杀死了庞贝和周围城镇和村庄的数千人。SV最后一次喷发是在1944年,火山爆发指数(VEI)为3(喷发了0.01 km3的火山物质)。在1944年喷发之后,SV火山在过去的79年里一直处于休眠状态,只有轻微的火山喷发和地震活动。在其漫长的历史中,SV几个世纪的休眠随着普林尼火山喷发(VEI 6)而结束,这标志着新一轮喷发活动的开始。因此,当前的休眠阶段需要更好地了解高震级喷发的机制,以便更好地预测未来的喷发规模和风格。尽管对SV火山系统进行了几个世纪的研究,但仍然存在许多问题,包括岩浆挥发物从深层原始岩浆到较浅更进化的岩浆的演变。更好地了解与SV挥发性演化相关的物理和化学过程,可以深入了解岩浆动力学和触发SV高爆炸性喷发的机制。在这项研究中,我们基于对橄榄石中再加热熔体包裹体(MIs)的分析,提供了与SV的4次普林尼期和2次普林尼期间喷发相关的岩浆喷发前挥发物含量的新数据。我们通过考虑MIs中气泡的挥发性含量来校正含气泡MIs的挥发性含量。我们发现了两组MIs:一组以高fo橄榄石(Fo85-90)为宿主,挥发物相对丰富;另一组以低fo橄榄石(Fo70-69)为宿主,挥发物相对较少。寄主橄榄石挥发物含量与组成的对比表明,岩浆分馏发生在挥发物饱和的条件下,相对于演化程度较低/准原始的岩浆,分馏程度较高的岩浆位于较浅的层位。利用SV的Fo90橄榄石中含有的校正后的MIs的CO2含量,我们估计在过去3个世纪(总共38-75 Mt)的火山活动中,SV岩浆溶解了347 ~ 686 t d−1的岩浆CO2。虽然这项研究仅限于少数SV岩浆,但我们建议采用类似的方法进行进一步的研究,可以阐明MIs挥发性含量与喷发风格和年龄之间明显缺乏相关性。此外,这些研究还可以对CO2的来源以及碳酸盐岩台地与SV下上升岩浆之间的相互作用提供额外的约束。
{"title":"Constraining the volatile evolution of mafic melts at Mt. Somma–Vesuvius, Italy, based on the composition of reheated melt inclusions and their olivine hosts","authors":"Rosario Esposito, Daniele Redi, Leonid V. Danyushevsky, Andrey Gurenko, Benedetto De Vivo, Craig E. Manning, Robert J. Bodnar, Matthew Steele-MacInnis, Maria-Luce Frezzotti","doi":"10.5194/ejm-35-921-2023","DOIUrl":"https://doi.org/10.5194/ejm-35-921-2023","url":null,"abstract":"Abstract. Mount Somma–Vesuvius is a stratovolcano that represents a geological hazard to the population of the city of Naples and surrounding towns in southern Italy. Historically, volcanic eruptions at Mt. Somma–Vesuvius (SV) include high-magnitude Plinian eruptions, such as the infamous 79 CE eruption that occurred after 295 years of quiescence and killed thousands of people in Pompeii and surrounding towns and villages. The last eruption at SV was in 1944 and showed a Volcanic Explosivity Index (VEI) of 3 (0.01 km3 of volcanic material erupted). Following the 1944 eruption, SV has been dormant for the past nearly 79 years, with only minor fumarolic and seismic activity. During its long history, centuries of dormancy at SV have ended with Plinian eruptions (VEI 6) that signal the beginning of a new cycle of eruptive activity. Thus, the current dormancy stage demands a need to better understand the mechanism involved in high-magnitude eruptions in order to better predict future eruption magnitude and style. Despite centuries of research on the SV volcanic system, many questions remain, including the evolution of magmatic volatiles from deep primitive magmas to shallower more evolved magmas. Developing a better understanding of the physical and chemical processes associated with volatile evolution at SV can provide insights into magma dynamics and the mechanisms that trigger highly explosive eruptions at SV. In this study, we present new data for the pre-eruptive volatile contents of magmas associated with four Plinian and two inter-Plinian eruptions at SV based on analyses of reheated melt inclusions (MIs) hosted in olivine. We correct the volatile contents of bubble-bearing MIs by taking into account the volatile contents of bubbles in the MIs. We recognize two groups of MIs: one group hosted in high-Fo olivine (Fo85–90) and relatively rich in volatiles and the other group hosted in low-Fo olivine (Fo70–69) and relatively depleted in volatiles. The correlation between volatile contents and compositions of host olivines suggests that magma fractionation took place under volatile-saturated conditions and that more differentiated magmas reside at shallower levels relative to less evolved/quasi-primitive magmas. Using the CO2 contents of corrected MIs hosted in Fo90 olivine from SV, we estimate that 347 to 686 t d−1 of magmatic CO2 exsolved from SV magmas during the last 3 centuries (38–75 Mt in total) of volcanic activity. Although this study is limited to only few SV magmas, we suggest that further study applying similar methods could shed light on the apparent lack of correlation between the volatile contents of MIs and the style and age of eruptions. Further, such studies could provide additional constraints on the origin of CO2 and the interaction between the carbonate platform and ascending magmas below SV.","PeriodicalId":11971,"journal":{"name":"European Journal of Mineralogy","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2023-11-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135242676","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}
Jiří Sejkora, Cristian Biagioni, Pavel Škácha, Silvia Musetti, Anatoly V. Kasatkin, Fabrizio Nestola
Abstract. Tetrahedrite-(Cd), Cu6(Cu4Cd2)Sb4S13, is a new IMA-approved mineral species from Radětice near the Příbram deposit, Czech Republic. It occurs as black metallic anhedral grains, up to 200 µm in size in quartz–calcite gangue, associated with galena, bournonite, sphalerite, pyrite, geocronite, silver, stromeyerite, tetrahedrite-(Zn), tetrahedrite-(Fe), tetrahedrite-(Cu), and an unknown Pb-rich tetrahedrite-like phase. In reflected light, tetrahedrite-(Cd) is isotropic and grey with greenish tints. Reflectance data for the four COM wavelengths in air are as follows (λ (nm) : R (%)): 470 : 30.8, 546 : 31.1, 589 : 31.1 and 650 : 28.8. Electron microprobe analysis gave (in wt % – average of 13 spot analyses) Cu 34.85, Ag 2.09, Fe 0.18, Zn 0.26, Cd 11.03, Hg 0.75, Pb 0.31, Sb 28.07, and S 23.38, with a total of 100.92. On the basis of ΣMe = 16 atoms per formula unit, the empirical formula of tetrahedrite-(Cd) is Cu9.65Ag0.34Cd1.73Zn0.07Hg0.07Fe0.06Pb0.03Sb4.06S12.84. Tennantite-(Cd) is cubic, I4‾3m, with unit-cell parameters a=10.504(3) Å, V=1158.9(9) Å3, and Z=2. Its crystal structure was refined by single-crystal X-ray diffraction data to a final R1=0.0252 on the basis of 257 unique reflections with Fo>4σ(Fo) and 23 refined parameters. Tetrahedrite-(Cd) is isotypic with other tetrahedrite-group minerals. Its crystal chemistry is discussed, and previous findings of Cd-rich tetrahedrite-group minerals are briefly reviewed.
{"title":"Tetrahedrite-(Cd), Cu<sub>6</sub>(Cu<sub>4</sub>Cd<sub>2</sub>)Sb<sub>4</sub>S<sub>13</sub>, from Radětice near Příbram, Czech Republic: the new Cd member of the tetrahedrite group","authors":"Jiří Sejkora, Cristian Biagioni, Pavel Škácha, Silvia Musetti, Anatoly V. Kasatkin, Fabrizio Nestola","doi":"10.5194/ejm-35-897-2023","DOIUrl":"https://doi.org/10.5194/ejm-35-897-2023","url":null,"abstract":"Abstract. Tetrahedrite-(Cd), Cu6(Cu4Cd2)Sb4S13, is a new IMA-approved mineral species from Radětice near the Příbram deposit, Czech Republic. It occurs as black metallic anhedral grains, up to 200 µm in size in quartz–calcite gangue, associated with galena, bournonite, sphalerite, pyrite, geocronite, silver, stromeyerite, tetrahedrite-(Zn), tetrahedrite-(Fe), tetrahedrite-(Cu), and an unknown Pb-rich tetrahedrite-like phase. In reflected light, tetrahedrite-(Cd) is isotropic and grey with greenish tints. Reflectance data for the four COM wavelengths in air are as follows (λ (nm) : R (%)): 470 : 30.8, 546 : 31.1, 589 : 31.1 and 650 : 28.8. Electron microprobe analysis gave (in wt % – average of 13 spot analyses) Cu 34.85, Ag 2.09, Fe 0.18, Zn 0.26, Cd 11.03, Hg 0.75, Pb 0.31, Sb 28.07, and S 23.38, with a total of 100.92. On the basis of ΣMe = 16 atoms per formula unit, the empirical formula of tetrahedrite-(Cd) is Cu9.65Ag0.34Cd1.73Zn0.07Hg0.07Fe0.06Pb0.03Sb4.06S12.84. Tennantite-(Cd) is cubic, I4‾3m, with unit-cell parameters a=10.504(3) Å, V=1158.9(9) Å3, and Z=2. Its crystal structure was refined by single-crystal X-ray diffraction data to a final R1=0.0252 on the basis of 257 unique reflections with Fo>4σ(Fo) and 23 refined parameters. Tetrahedrite-(Cd) is isotypic with other tetrahedrite-group minerals. Its crystal chemistry is discussed, and previous findings of Cd-rich tetrahedrite-group minerals are briefly reviewed.","PeriodicalId":11971,"journal":{"name":"European Journal of Mineralogy","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2023-11-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135272391","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}
Ian E. Grey, Stephanie Boer, Colin M. MacRae, Nicholas C. Wilson, William G. Mumme, Ferdinando Bosi
Abstract. A single-crystal structure determination and refinement has been conducted for the type specimen of paulkerrite. The structure analysis showed that the mineral has monoclinic symmetry, space group P21/c, not orthorhombic, Pbca, as originally reported. The unit-cell parameters are a=10.569(2), b=20.590(4), c=12.413(2) Å, and β=90.33(3)∘. The results from the structure refinement were combined with electron microprobe analyses to establish the empirical structural formula A1[(H2O)0.98K0.02]Σ1.00 A2K1.00 M1(Mg1.02Mn0.982+)Σ2.00 M2(Fe1.203+Ti0.544+Al0.24Mg0.02)Σ2.00 M3(Ti0.744+ Fe0.263+)Σ1.00 (PO4)4.02 X[O1.21F0.47(OH)0.32]Σ2.00(H2O)10 ⋅ 3.95H2O, which leads to the end-member formula (H2O)KMg2Fe2Ti(PO4)4(OF)(H2O)10 ⋅ 4H2O. A proposal for a paulkerrite group, comprising orthorhombic members benyacarite, mantiennéite, pleysteinite, and hochleitnerite and monoclinic members paulkerrite and rewitzerite, has been approved by the International Mineralogical Association's Commission for New Minerals, Nomenclature and Classification. The general formulae are A2M12M22M3(PO4)4X2(H2O)10 ⋅ 4H2O and A1A2M12M22M3(PO4)4X2(H2O)10 ⋅ 4H2O for orthorhombic and monoclinic species, respectively, where A= K, H2O, □ (= vacancy); M1 = Mn2+, Mg, Fe2+, Zn (rarely Fe3+); M2 and M3 = Fe3+, Al, Ti4+ (and very rarely Mg); X= O, OH, F. In monoclinic species, K and H2O show an ordering at the A1 and A2 sites, whereas O, (OH), and F show a disordering over the two non-equivalent X1 and X2 sites, which were hence merged as X2 in the general formula. In both monoclinic and orthorhombic species, a high degree of mixing of Fe3+, Al, and Ti occurs at the M2 and M3 sites of paulkerrite group members, making it difficult to get unambiguous end-member formulae from the structural determination of the constituents at individual sites. To deal with this problem an approach has been used that involves merging the compositions at the M2 and M3 sites and applying the site-total-charge method. The merged-site approach allows end-member formulae to be obtained directly from the chemical analysis without the need to conduct crystal-structure refinements to obtain the individual site species.
{"title":"Crystal chemistry of type paulkerrite and establishment of the paulkerrite group nomenclature","authors":"Ian E. Grey, Stephanie Boer, Colin M. MacRae, Nicholas C. Wilson, William G. Mumme, Ferdinando Bosi","doi":"10.5194/ejm-35-909-2023","DOIUrl":"https://doi.org/10.5194/ejm-35-909-2023","url":null,"abstract":"Abstract. A single-crystal structure determination and refinement has been conducted for the type specimen of paulkerrite. The structure analysis showed that the mineral has monoclinic symmetry, space group P21/c, not orthorhombic, Pbca, as originally reported. The unit-cell parameters are a=10.569(2), b=20.590(4), c=12.413(2) Å, and β=90.33(3)∘. The results from the structure refinement were combined with electron microprobe analyses to establish the empirical structural formula A1[(H2O)0.98K0.02]Σ1.00 A2K1.00 M1(Mg1.02Mn0.982+)Σ2.00 M2(Fe1.203+Ti0.544+Al0.24Mg0.02)Σ2.00 M3(Ti0.744+ Fe0.263+)Σ1.00 (PO4)4.02 X[O1.21F0.47(OH)0.32]Σ2.00(H2O)10 ⋅ 3.95H2O, which leads to the end-member formula (H2O)KMg2Fe2Ti(PO4)4(OF)(H2O)10 ⋅ 4H2O. A proposal for a paulkerrite group, comprising orthorhombic members benyacarite, mantiennéite, pleysteinite, and hochleitnerite and monoclinic members paulkerrite and rewitzerite, has been approved by the International Mineralogical Association's Commission for New Minerals, Nomenclature and Classification. The general formulae are A2M12M22M3(PO4)4X2(H2O)10 ⋅ 4H2O and A1A2M12M22M3(PO4)4X2(H2O)10 ⋅ 4H2O for orthorhombic and monoclinic species, respectively, where A= K, H2O, □ (= vacancy); M1 = Mn2+, Mg, Fe2+, Zn (rarely Fe3+); M2 and M3 = Fe3+, Al, Ti4+ (and very rarely Mg); X= O, OH, F. In monoclinic species, K and H2O show an ordering at the A1 and A2 sites, whereas O, (OH), and F show a disordering over the two non-equivalent X1 and X2 sites, which were hence merged as X2 in the general formula. In both monoclinic and orthorhombic species, a high degree of mixing of Fe3+, Al, and Ti occurs at the M2 and M3 sites of paulkerrite group members, making it difficult to get unambiguous end-member formulae from the structural determination of the constituents at individual sites. To deal with this problem an approach has been used that involves merging the compositions at the M2 and M3 sites and applying the site-total-charge method. The merged-site approach allows end-member formulae to be obtained directly from the chemical analysis without the need to conduct crystal-structure refinements to obtain the individual site species.","PeriodicalId":11971,"journal":{"name":"European Journal of Mineralogy","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2023-11-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135272230","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}