Discovery of Low Aluminium Nevadaite from the Kara-Chagyr Area, Kyrgyzstan

摘要

The rare phosphate-nevadaite has been found at Kara-Chagyr (Batken region, Kyrgyzstan) in a zone of alteration of vanadium bearing "black shales". It occurs as blue crusts of spherulitic aggregates of tiny tabular crystals (0.1-10 μm). It is associated with metahewettite, hummerite, carnotite, minyulite, fluellite, cran-dallite, variscite, and woodhouseite. Optical properties: n = 1.542-1.555, D_(meas) (for aggregates) = 2.58(1) g/cm~3, D_(calc) = 2.582 g/cm~3. The most intense X-ray powder reflections are as follows: [d, A, (I_(meas)), (hkl)]: 9.54 (80) (020), 6.03 (100) (200), 5.61 (100) (130), 3.91 (60) (310), 3.41 (80) (041), 2.982 (100) (241), 2.804 (60) (331), 2.672 (70) (061), 1.845 (60) (352) 1.507 (70) (243). Calculated cell dimensions are: a = 12.072(10) A, b = 18.958(15) A, c = 4.969(5) A, α= β =γ= 90°, V = 1137.2 A~3. Electron microprobe analyses gives (wt %): (observed (average of 8 analyses); (calculated for 22H2O)): P2O5 34.69 (31.85), SiO2 0.25 (0.24), Al2O3 25.61 (23.50), V2O 5.58 (5.13), Fe2O3 0.48 (0.46), MnO 0.03 (0.03), CuO 10.79 (9.90), ZnO 0.69 (0.65), CaO 0.18 (0.15), MgO 0.17 (0.17), K2O 0.08 (0.08), F 7.40 (6.79), H2O 17.16 (by diff.) (23.90), -F2=O 3.11 (2.86), total 100.00 (100.00).The crystal-chemical formula of the mineral is (CU_(2.2)~(+2)□_(2.03) V_(1.21)~(+3)Al_(0.15)Zn_(0.14)Fe_(0.40)Mg_(0.07)Ca_(0.05)K_(0.03)Mn_(0.01))_(6.00)(Al_(8.00)(P_(7.93)Si_(0.07)O_(32))F_(6.32)(OH)_(2.98)·22(H2O) for the ideal number of water molecules. Nevadaite from Kara-Chagyr differs from that from the type locality, Gold Quarry (Nev., USA), by its lower Al content. The IR-spectrum, and microphotographs of nevadaite and associated minerals are given.