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])(Ca1.64Na0.25Mn0.11)([VI])(Fe3.562+Fe1.463+Ti Mg0.25Nb0.038Ta0.007)([IV)](Si4.48Be0.91Al0.54 Fe0.07O18)O-2 (end-member Ca2Fe2+Fe42+TiS4BeAlO20) from electron-microprobe data, Mossbauer 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 hasting-site), 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 mu m 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(l) g/cm(3); calculated density 3.933 g/cm(3). It is optically biaxial, alpha = 1.835 +/- 0.014 and beta = 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 (1) over bar, Z = 2, Wr(F-2) (all) = 0.096 for 6497 reflections, a = 10.355(2), b = 10.751(3), and c = 8.873(2) angstrom, alpha = 105.707(8), beta = 96.227(6), gamma = 124.861(6)degrees, V = 735.7(3) angstrom(3). The eight strongest lines in the powder pattern [d-spacing (angstrom),(l),(hkl] are 7.997(57)(100), 4.779(29)(011), 3.120(32)(012), 2.924(69)(0 (1) over bar3), 2.676(77)((2) over bar 03), 2.530(100)((21) over bar3), 2.410(28)(2 (2) over bar3), 2.075(39)(4 (1) over bar1).Of the aenigmatite-sapphirine-surinamite group minerals, makarochkinite is compositionally closest to rhonite and hogtuvaite. It is distinguished from rhonite by Fe2+ > Mg and by the presence of 0.91-0.98 Be per 20 cations; Be occupies the two most polymerized T sites in roughly equal amounts. Distinction from hogtuvaite is based on occupancy of the M7 site, which is dominated by Fe3+ in hogtuvaite and by Ti in makarochkinite.