Makarochkinite, Ca2Fe42+Fe3+TiSi4BeAlO20, a new beryllosilicate member of the aenigm

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• journal_title：American Mineralogist
• Contributor：Edward S. Grew ; Jacques Barbier ; Jim Britten ; Martin G. Yates ; Vladislav O. Polyakov ; Elena P. Shcherbakova ; Ulf Hålenius ; Charles K. Shearer
• Publisher：Mineralogical Society of America
• Date：2005-
• Format：text/html
• Language：en
• Identifier：10.2138/am.2005.1759
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：90
• issue：8-9
• firstpage：1402
• section：Articles

摘要

<p id="p-1">Makarochkinite, p>[VII]p>(Ca_1.64Na_0.25Mn_0.11)p>[VI]p>(Fep>2+p>_3.56Fep>3+p>_1.46Ti_0.61Mg_0.25Mn_0.02Nb_0.038Ta_0.007)p>[IV]p>(Si_4.48Be_0.91Al_0.54 Fe_0.07O₁₈)O₂ (end-member Ca₂Fe₄p>2+p>Fep>3+p>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/cmp>3p>; calculated density 3.933 g/cmp>3p>. 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 P1̅, Z = 2, wR(Fp>2p>) (all) = 0.096 for 6497 reflections, a = 10.355(2), b = 10.751(3), and c = 8.873(2) Å, α = 105.707(8), β = 96.227(6), γ = 124.861(6)°, V = 735.7(3) Åp>3p>. 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)(01̅3), 2.676(77)( 2̅03), 2.530(100)( 2̅1̅3), 2.410(28)(22̅3), 2.075(39)(41̅1). p><p id="p-2">Of the aenigmatite-sapphirine-surinamite group minerals, makarochkinite is compositionally closest to rhönite and høgtuvaite. It is distinguished from rhönite by Fep>2+p> > 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 høgtuvaite is based on occupancy of the M7 site, which is dominated by Fep>3+p> in høgtuvaite and by Ti in makarochkinite. p>