AbstractTangdanite, ideally
Ca2Cu9(AsO4)4(SO4)0.5(OH)9·9H2O
and monoclinic, is a new mineral species (IMA No. 2011-096) occurring in the
Tangdan and Nanniping mines, southeast Dongchuan copper mining district,
Dongchuan County, Kunming City Prefecture, Yunnan Province, P. R. China
(26°11’N 103°51’E). The mineral is found in the oxidized zone (gossan) of an
As-bearing Cu sulfide deposit and is clearly of supergene origin. Associated
minerals are chalcopyrite, bornite, chalcocite, covellite, tennantite,
enargite, cuprite, malachite, azurite, copper and brochantite. Crystals form
radiating or foliated aggregates of flaky crystals up to 3 mm, flattened
parallel to (100) and elongated along [001]. It is emerald green with a
light green streak, translucent and has a pearly to silky lustre. It is
sectile having perfect cleavage on {100} although neither parting nor
fracture was observed. No fluorescence in long- or short-wave ultraviolet
radiation was observed. The hardness is VHN50 42.0−43.6, mean
42.8 kg mm−2 (2−2½ on the Mohs scale). The density measured by
pycnometry is 3.22 g cm−3 (Ma et al., 1980). The
calculated density from the empirical chemical formula is 3.32 g
cm−3. The compatability index gives 1 −
(Kp/Kc) = −0.041 (good). The empirical formula
(based on 36 O a.p.f.u) of tangdanite is
Ca2.05Cu9.08(As1.03O4)4(S0.63O4)0.5(OH)9·9H2.04O.
The simplified formula is
Ca2Cu9(AsO4)4(SO4)0.5(OH)9·9H2O.
The strongest five reflections in the X-ray powder-diffraction pattern
[d in Å(I) (hkl)] are:
4.782(100) ( 1 1),
4.333(71) (6 0 2), 5.263(54) ( 0 2),
3.949(47) (8 0 2) and 2.976(46) ( 1 1). The
unit-cell parameters are a = 54.490(9), b
= 5.5685(9), c = 10.4690(17) Å, β = 96.294(3)o,
V = 3157.4(9) Å3, Z = 4. Its
structure was solved and refined in space group C2/c, with
R = 0.110.