• journal_title：American Mineralogist
• Contributor：Frank Gfeller ; Thomas Armbruster ; Evgeny V. Galuskin ; Irina O. Galuskina ; Biljana Lazic ; Valentina B. Savelyeva ; Aleksandr E. Zadov ; Piotr Dzierżanowski ; Viktor M. Gazeev
• Publisher：Mineralogical Society of America
• Date：2013-02-01
• Format：text/html
• Language：en
• Identifier：10.2138/am.2013.4257
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：98
• issue：2-3
• firstpage：493
• section：Article

id="p-1">Three samples of the skarn mineral rustumite Ca₁₀(Si₂O7)2(SiO4)(OH)2Cl2, space group C2/c, a ≈7.6, b ≈ 18.5, c ≈ 15.5 Å, β ≈ 104°, with variable OH, Cl, F content were investigated by electron microprobe, single-crystal X-ray structure refinements, and Raman spectroscopy. “Rust1_LCl” is a low chlorine rustumite Ca₁₀(Si₂O7)2(SiO₄)(OH_1.88F_0.12)(Cl_1.28,OH_0.72) from skarns associated with the Rize batholith near Ikizedere, Turkey. “Rust2_F” is a F-bearing rustumite Ca₁₀(Si₂O₇)₂(SiO₄)(OH_1.13F_0.87) (Cl_{1 96}OH_0.04) from xenoliths in ignimbrites of the Upper Chegem Caldera, Northern Caucasus, Russia. “Rust3_LCl_F” represents a low-Cl, F-bearing rustumite Ca10(Si₂O7)₂[(SiO4)0.₈7(H4O₄)0.13](OH1.01F0.99) (Cl_1.00 OH_1.00) from altered merwinite skarns of the Birkhin massif, Baikal Lake area, Eastern Siberia, Russia. Rustumite from Birkhin massif is characterized by a significant hydrogarnet-like or fluorine substitution at the apices of the orthosilicate group, leading to specific atomic displacements. The crystal structures including hydrogen positions have been refined from single-crystal X-ray data to R1 = 0.0205 (Rust1_LCl), R1 = 0.0295 (Rust2_F), and R1 = 0.0243 (Rust3_LCl_F), respectively. Depletion in Cl and replacement by OH is associated with smaller unit-cell dimensions. The substitution of OH by F leads to shorter hydrogen bonds O-H⋯F instead of O-H⋯OH. Raman spectra for all samples have been measured and confirm slight strengthening of the hydrogen bonds with uptake of F.

id="p-2">This study discusses the complex crystal chemistry of the skarn mineral rustumite and may provide a wider understanding of the chemical reactions related to contact metamorphism of limestones.