Potassic-hastingsite, KCa2(Fe2+4Fe3+)(Si6Al2)O22(OH)2, from the Keshiketeng Banner, Inner Mongolia, China: description of the neotype and its implication.

Potassic-hastingsite (IMA2018–160), ideally KCa2(Fe2+4Fe3+)(Si6Al2)O22(OH)2,was discovered in Danailingou, Inner Mongolia, China. Potassic-hastingsite is black to dark green with a vitreous luster, brittle with conchoidal fracture. The {110} cleavage is perfect and the calculated density is 3.541 g·cm−3. It is biaxial negative with α = 1.708 (2), β = 1.716 (3) and γ = 1.718 (3). Combined electron probe,icro-analyser (EPMA) and Mössbauer spectroscopy data yielded 35.02 wt% SiO2, 0.35 wt% TiO2, 10.69 wt% Al2O3, 0.03 wt% Cr2O3, 7.33 wt% Fe2O3, 26.08 wt% FeO, 11.35 wt% CaO, 0.43 wt% MnO, 0.23 wt% MgO, 0.061 wt% Na2O, 2.79 wt% K2O, F 0.00 wt%, Cl 2.79 wt%, Cl ≡ O − 0.67 wt%, and H2O+ 1.43 wt%, resulting in an analytical total of 98.66 wt%. The empirical structural formula, calculated based of the assumption of a total of 24 anions per formula unit and OH + F + Cl = 2 atoms per formula unit (apfu) at the W site is A(K0.61Na0.20Ca0.07)0.88BCa2.00C(Fe2+3.72Fe3+0.94Al0.12Mn0.06Mg0.06Ti0.04Ni0.01)4.96T(Si5.97Al2.03)O22(OH1.14Cl0.86). The derived simplified formula is (K,Na,□)Ca2(Fe2+,Fe3+)5[(Si,Al)6Al2]O22(OH,Cl)2. Potassic-hastingsite is monoclinic, space group C2/m, with the unit-cell parameters a = 9.9405(7) Å; b = 18.2561(19) Å; c = 5.3501(3) Å; β = 105.117(5)°; V = 937.304(11) Å3; Z = 2. The structure was solved (R1 0.044) using all measured independent data and the reflections with I>2σ (I). The strongest six X-ray powder-diffraction lines [d in Å; (I/I0); (hkl)] are 8.504 (100) (110), 3.417 (35) (131), 3.155 (60) (310), 2.735 (70) (151), 2.623 (35) (061) and 2.570 (35) ( 2 ¯ 02). [ABSTRACT FROM AUTHOR]