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

Abstract

Makarochkinite, [VII](Ca1.64Na0.25Mn0.11)[VI](Fe2+3.56Fe3+1.46Ti0.61Mg0.25Mn0.02Nb0.038Ta0.007)[IV](Si4.48Be0.91Al0.54 Fe0.07O18)O2 (end-member Ca2Fe42+Fe3+TiSi4BeAlO20) 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 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, α = 1.835 ± 0.014 and γ = 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, Z = 2, wR ( F 2) (all) = 0.096 for 6497 reflections, a = 10.355(2), b = 10.751(3), and c = 8.873(2) A, α = 105.707(8), β = 96.227(6), γ = 124.861(6)°, V = 735.7(3) A3. The eight strongest lines in the powder pattern [ d -spacing (A),( I ),( hkl )] are 7.997(57)(100), 4.779(29)(011), 3.120(32)(012), 2.924(69)(013), 2.676(77)( 203), 2.530(100)( 213), 2.410(28)(223), 2.075(39)(411). 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.