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Mineralogical ClassificationCHISTYAKOVAITE (IMA 2005-003) Published
28th Oct 2006 13:37 UTCLuc Vandenberghe Expert
N.V. Chukanov, G.A. Sidorenko, I.S. Naumova, A.E. Zadov,
V.I. Kuz’min (2006) Chistyakovaite, a new mineral Al(UO2)2
(AsO4)2(F,OH)•6.5H2O. Dokl. Earth Sci., 407, 290–293 (in
English); Dokl. Akad. Nauk, 406, 816–819 (in Russian).
The new species chistyakovaite occurs in samples collected
in the 1950s from the supergene zone of the Bota-Burum deposit,
south of Alakol’ Lake, southwestern Balkhash area, Zhambyl
(Dzhambul) Region, Kazakhstan. This hydrothermal U deposit
consists of carbonatized felsite porphyries and tuffaceous breccias
of Devonian age that have undergone primary hydrothermal
alteration and subsequent supergene alteration. In particular, supergene
alteration of hydrothermal arsenopyrite and pitchblende
is responsible for liberating the U and As responsible for the
new species and many other uranyl-bearing arsenates found at
the deposit. The mineral is associated with arsenopyrite, pyrite,
galena, scorodite, arseniosiderite, mansÞeldite, metazeunerite,
trögerite, and sodium uranospinite. It forms imperfect, {100}
ßattened, yellow crystals to 2 mm across, which are sometimes
intergrown. Brittle; Mohs’hardness 2.5; Dmeas 3.62(2), Dcalc 3.585
g/cm3 ; perfect {100} and imperfect {001} cleavage; stepped
fracture. Bright green luminescence in UV light (365 nm). Optically biaxial (–), colorless, with α = 1.557(2), β = 1.580(1), γ =
1.580(1), 2V = –10(5)°, X = a, Z ≈ b, no dispersion. Dissolves
in HCl at room temperature with no liberated gas. The infrared
spectrum (KBr pellet, 400 to 4000 cm
–1
range) records vibration
and/or ßexing bands for H2O, OH, PO4, UO2, AsO4, and AlO6;
a large shoulder at 3130 cm
–1
implies the existence of some
strong hydrogen bonding. Heating in Ar to 850 °C gives a total
weight loss of 12.2%. This, combined with electron microprobe
analysis (20 kV, 10 nA, 50 μm, except for F at 15 kV, 70 nA,
50–60 μm beam) gave: Al2O3 4.99, UO3 58.34, As2O5 21.40, P2O5
1.23, F 1.1, H2O 12.2, O=F –0.46, total 98.80 wt%, yielding the
empirical formula (per 2 AsO4 + PO4), Al0.96(UO2)2.00<(AsO4)1.83
(PO4)0.17>•6.50H2O, or ideally Al(UO2)2(AsO4)2(F,
OH)•6.50H2O. Gladstone-Dale compatibility is good. Fracturing
and mosaic structure precluded single-crystal study, but powder
X-ray diffraction study gave space groups P2/m, P2 or Pm, a =
19.99(1), b = 9.79(1), c = 19.62(2) Å, β = 110.7(2) °, Z = 8. The
mineral, which would seem to be the arsenate-ßuorine analog
of threadgoldite, has also possibly been found at the Menzenschwand
deposit in the Schwarzwald, Germany. Chistyakovite is
named for N.I. Chistyakova, a senior assistant at the All-Russia
Research Institute of Mineral Resources (VIMS). Type samples
are in the E.V. Kopchenova collection of the Mineralogical Museum
at VIMS, (no. 350/59), and in the Fersman Mineralogical
Museum, Moscow (no. 3286/1).
Discussion. No mention is made of whether peak shape
corrections were applied to the FKα microprobe data. Errors
as large as 20% rel. can be incurred without such corrections
(Solberg, T.N., 1982, Microbeam Analysis, 17, 148–150), an
important concern when the (F,OH) site contains only 0.57 F
apfu.
29th Oct 2006 13:48 UTCPeter Haas
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