Makarochkinite, Ca2Fe42+Fe3+TiSi4BeAlO20, a new beryllosilicate member of the aenigmatite-sapphirine-surinamite group from the Il'men Mountains (southern Urals), Russia

摘要

Makarochkinite, [VII](Ca_1.64Na_0.25Mn_0.11)[VI](Fe2+_3.56Fe3+_1.46Ti_0.61Mg_0.25Mn_0.02Nb_0.038Ta_0.007)[IV](Si_4.48Be_0.91Al_0.54 Fe_0.07O₁₈)O₂ (end-member Ca₂Fe₄2+Fe3+TiSi₄BeAlO₂₀) from electron-microprobe data, Mössbauer spectroscopy, and single-crystal structure refinement, occurs in a granitic pegmatite near Lake Ishkul., Il’men Mountains (Southern Urals), Il’men State Reserve, Chelyabinsk Oblast., Russia. Associated minerals include danalite, phenakite, titanite, potassian calcic amphibole (ferro-edenite and hastingsite), biotite, ilmenite, magnetite, ferrocolumbite, fergusonite-(Y), and samarskite-(Y). Makarochkinite forms equant masses 5–50 mm across, black in hand specimen; luster vitreous; it is opaque except in slivers <1 µm thick. It is brittle; Moh’s hardness 5.5–6, and has no discernable cleavage; fracture is uneven. Twinning is absent. The measured density is 3.93(1) g/cm3; calculated density 3.933 g/cm3. It is optically biaxial, = 1.835 ± 0.014 and = 1.865 ± 0.015 from reflectance data, pleochroism marked: X = greenish brown, Y = yellowish brown, Z = reddish brown; Y X < Z. Single-crystal X-ray diffraction gives triclinic symmetry, space group P, Z = 2, wR(F2) (all) = 0.096 for 6497 reflections, a = 10.355(2), b = 10.751(3), and c = 8.873(2) Å, = 105.707(8), ß = 96.227(6), = 124.861(6)°, V = 735.7(3) Å3. The eight strongest lines in the powder pattern [d-spacing (Å),(I),(hkl)] are 7.997(57)(100), 4.779(29)(011), 3.120(32)(012), 2.924(69)(03), 2.676(77)( 03), 2.530(100)( 3), 2.410(28)(23), 2.075(39)(41).