Fluorluanshiweiite, KLiAl1.5 square(0.5)(Si3.5Al0.5)O10F2, a New Mineral of the Mica Group from the Nanyangshan LCT Pegmatite Deposit, North Qinling Orogen, China

A new mineral species of the mica group, fluorluanshiweiite, ideally KLiAl1.5 square(0.5)(Si3.5Al0.5)O10F2, has been found in the Nanyangshan LCT (Li, Cs, Ta) pegmatite deposit in North Qinling Orogen (NQO), central China. Fluorluanshiweiite can be regarded as the F-dominant analogue at theAsite of luanshiweiite or the K-dominant analogue at the I site of voloshinite. It appears mostly in cookeite as a flaky residue, replaced by Cs-rich mica, or in the form of scale aggregates. Most individual grains are <1 mm in size, with the largest being ca. 1 cm, and the periphery is replaced by cookeite. No twinning is observed. The mineral is silvery white as a hand specimen, and in a thin section, it appears grayish-white to colorless, transparent with white streaks, with vitreous luster and pearliness on cleavage faces. It is flexible with micaceous fracture; the Mohs hardness is approximately 3; the cleavage is perfect on {001}; and no parting is observed. The measured and calculated densities are 2.94(3) and 2.898 g/cm(3), respectively. Optically, fluorluanshiweiite is biaxial (-), with alpha = 1.554(1), beta = 1.581(1), gamma = 1.583(1) (white light), 2V(meas.) = 25 degrees to 35 degrees, 2V(calc.) = 30.05 degrees. The calculated compatibility index based on the empirical formula is -0.014 (superior). An electron microprobe analysis yields the empirical formula calculated based on 10 O atoms and 2 additional anions of (K0.85Rb0.12Cs0.02Na0.03)(Sigma 1.02)[Li1.05Al1.44(square 0.47Fe0.01Mn0.02)(Sigma 0.5)] (Sigma 2.99)(Si3.55Al0.45) Sigma 4O10F2, which can be simplified to KLiAl1.5 square(0.5)(Si3.5Al0.5)O10F2. Fluorluanshiweiite is monoclinic with the space group C2/m and unit cell parameters a = 5.2030(5), b = 8.9894(6), c = 10.1253(9) angstrom, beta = 100.68(1)degrees, and V = 465.37(7) angstrom(3). The strongest eight lines in the X-ray diffraction data are [d in angstrom(I)(hkl)]: 8.427(25) (001), 4.519(57) (020), 4.121(25) (021), 3.628(61) (<(1)over bar>12), 3.350(60) (022), 3.091(46) (112), 2.586(100) ((1) over bar 30), and 1.506(45) (312).