Crystal chemistry and thermal behavior of B-, S- and Na-bearing spurrite

Spurrite from Negra Mine, Queretaro, Mexico is characterized by a complex chemical composition. Its empirical formula derived based on electron microprobe, wet chemical analyses and gas chromatography of annealing products is H_0.18Ca_5.01Na_0.05[(SiO₄)_1.91(SO₄)_0.08)][(CO₃)_0.71(BO₃)_0.28]O₁₁. The mineral was studied by single-crystal X-ray diffraction (SCXRD) as well as infrared (IR), Raman and nuclear magnetic resonance (NMR) spectroscopy. According to spectroscopic data, boron has three-fold coordination and sulfur occurs in the mineral in the sulfate form. A significant portion of carbonate groups is substituted by BO₃^3– anions. Charge compensation is achieved due to the substitution of a part of SiO₄^4– anions by SO₄^2– groups, as well as to the admixture of sodium. SCXRD shows that sodium occurs in its own site with a low occupancy. The studied sample is isotypic with the synthetic NaCa₅(SiO₄)₂(BO₃) compound. The IR spectrum shows possible partial protonation of the SiO₄ tetrahedra whereas bands of H₂O molecules and isolated OH^– anions are not observed. Thermal behavior of B,S,Na-bearing spurrite from Negra Mine has been studied using powder high-temperature X-ray diffraction (HTXRD) together with boron poor and S-free spurrite from Fuka Area (Japan). The studied samples are stable up to ~ 1200 °C and ~ 1100 °C, respectively, whereas synthetic B,S-free spurrite decomposes at about 900 °C. The thermal expansion is significantly anisotropic and is observed mainly in the direction perpendicular to the ac plane which is coplanar with the layers of calcium polyhedra and anionic pseudo-layers formed by (C,B)O₃ triangles and (Si,S)O₄ tetrahedra. Isomorphism and a similarity of the thermal, baric and compositional (C-B substitution) deformations of spurrite-like structures are discussed.