Qingheiite-(Fe+), Na2Fe2+MgAl(PO4)3, a new phosphate mineral from the Sebastiao Cristino pegmatite, Minas Gerais, Brazil

Abstract

Qingheiite-(Fe 2+ ), ideally Na 2 Fe 2+ MgAl(PO 4 ) 3 , is a new mineral species from the Sebastiao Cristino pegmatite, Minas Gerais, Brazil. It occurs as rims around frondelite grains, included in a matrix of quartz and albite. Frondelite is locally replaced by jahnsite, cyrilovite and Fe-Mn oxides. Qingheiite-(Fe 2+ ) is transparent and exhibits a dark green colour, with a resinous lustre and with a pale to bottle green streak. It is non-fluorescent, brittle, and shows a perfect {010} cleavage. The estimated Mohs hardness is 4. The measured density is 3.6(2) g/cm 3 ; the calculated density is 3.54 g/cm 3 . Qingheiite-(Fe 2+ ) is biaxial negative, with α = 1.692(5), β = 1.718(3), and γ = 1.720(5) (with λ = 590 nm). Pleochroism is from pale pinkish brown ( X ) to pale green ( Y ) and pale bluish grey ( Z ). The calculated 2 V angle is 31°, and a strong dispersion r > v has been observed. The β index is parallel to the b crystallographic axis; α and γ lie in the (010) plane. Electron microprobe analyses gave P 2 O 5 46.51, Al 2 O 3 6.94, Fe 2 O 3 10.58, FeO 11.46, MgO 6.32, MnO 11.23, CaO 0.24, Na 2 O 6.27, K 2 O 0.01, total 99.56 wt. %. The resulting empirical formula, calculated on the basis of 3 P, is (□ 0.65 Na 0.35 ) ∑1.00 (Na 0.58 Mn 2+ 0.40 Ca 0.02 ) ∑1.00 (Fe 2+ 0.68 Mn 2+ 0.32 ) ∑1.00 (Mg 0.72 Fe 3+ 0.23 Fe 2+ 0.05 ) ∑1.00 (Al 0.62 Fe 3+ 0.38 ) ∑1.00 [PO 4 ] 3 . The single-crystal unit-cell parameters are a = 11.910(2), b = 12.383(3), c = 6.372(1) A, β = 114.43(3)°, and V = 855.6(3) A 3 , space group P 2 1 / n . The eight strongest lines in the powder X-ray diffraction pattern [ d (in A)( I )( hkl )] are: 3.468(35)(310), 3.047(100)112, 2.849(80)312, 2.810(35)222, 2.711(40)(330), 2.688(90)(240), 2.500(40) 132; 112, 2.074(30) 313. Qingheiite-(Fe 2+ ) is the Fe 2+ analogue of qingheiite, and belongs to the wyllieite group of minerals. The crystal structure has been refined, based on single-crystal X-ray diffraction data, to R 1 = 2.91%. The mineral species and name were approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (CNMNC-IMA) under number 2009-076.