Yangzhumingite, KMg2.5Si4O10F2, a new mineral in the mica group from Bayan Obo, Inner Mongolia, China

Abstract

Yangzhumingite, ideally KMg2.5Si4O10F2, occurs as subhedral to euhedral platy crystals up to ca . 0.5 mm across in metamorphosed carbonate rock from Bayan Obo, Inner Mongolia, China. It is associated with dolomite, calcite, tremolite, norbergite and huanghoite-(Ce) with minor phlogopite, barite, bastnasite-(Ce), parisite-(Ce), and fluorite. Yangzhumingite is transparent and colorless with white streak and pearly luster. It is optically biaxial (−), n = 1.532–1.537 (589 nm), and 2 V = 5–10°. The hardness is 3 on Mohs’ scale (measured on a synthetic equivalent). It has a monoclinic unit cell with a = 5.249(4), b = 9.095(5), c = 10.142(5) A, β = 99.96(6)°, V = 476.9(5) A3, Z = 2, space group C 2/ m , and D calc. = 2.807 g/cm3. The five strongest lines in the powder XRD pattern are [ d (A), I / I , hkl ]: (10.03, 95, 001); (3.37, 48, 022), (2.90, 49, 113), (2.59, 67, 131200) and (2.41, 100, 132201). The mean of 15 electron microprobe analyses and SIMS analysis for Li lead to the empirical formula (K0.70Li0.30Na0.01)(Mg2.48Fe0.06) (Si3.96Al0.03)O10[F1.92(OH)0.08] on the basis of 12 anions with (F + OH) = 2. Yangzhumingite is a member of the mica group (Nickel-Strunz grouping 9.EC.10), and it represents the Mg-dominant analogue of montdorite.