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Zimbabweite occurs as honey yellow-brown crystals (= or <1 X 1 X 2 cm) in kaolinized granite pegmatite and dump material at St Anns mine, Karoi (formerly Urungwe) district. It is clear pale yellow in thin fragments and has one excellent cleavage (010), H. 5-51/2, adamantine lustre, non-magnetic, D 6.20 g/cm3; optically biaxial, all refr. ind. > 2.10, 2Vgamma approx 80o, dispersion very strong v > r; alpha (pale yellow brown) = c, beta (light reddish brown = b, gamma (reddish brown) = a. Mean reflectance in air for an (010) = alpha -gamma cleavage plate is 16.6% at 589 nm. Chemical analysis gave Ta2O5 46.5, As2O3 26.5, PbO 15.0, Nb2O5 4.8, Na2O 3.1 K2O 1.5, TiO2 1.4, BaO 0.4, UO2 0.3, Bi2O3 0.2, H2O 0.19, SnO2 0.1, F 0.04, SrO 0.02, less O = F 0.02, = 100.05, leading to the idealized formula (Na,K)2PbAs4(Ta,Nb,Ti)4O18. Indexed XRD powder data are tabulated; strongest lines 3.195(100), 2.990(70), 2.882(70), 3.033(60), 3.823(55), 2.548(50), 1.913(50) A; a 12.233, b 15.292, c 8.665; A; space group Ccma or Cc2a, Z = 4. The TG curve shows a rapid weight loss of approx 22.5% at 840-900oC, due to the loss of As2O5.-R.A.H.
Additional Publication Details
Zimbabweite, a new alkali-lead arsenic tantalate from St Anns mine, Karoi district, Zimbabwe.