Lavinskyite, K(LiCu)Cu6(Si4O11)2(OH)4, isotypic with plancheite, a new mineral from the Wessels mine, Kalahari Manganese Fields, South Africa

A new mineral species, lavinskyite, ideally K(LiCu)Cu6(Si4O11)2(OH)4 (IMA 2012-028), has been found in the Wessels mine, Kalahari Manganese Fields, Northern Cape Province, South Africa. Associated minerals include wesselsite, pectolite, richterite, sugilite, and scottyite. Lavinskyite crystals are tabular [parallel to (010)]. The mineral is light blue, transparent with very pale blue streak and vitreous luster. It is brittle and has a Mohs hardness of ~5; cleavage is perfect on {010} and no parting was observed. The measured and calculated densities are 3.61(3) and 3.62 g/cm3, respectively. Optically, lavinskyite is biaxial (+), with a = 1.675(1), b = 1.686(1), g = 1.715(1), 2Vmeas = 64(2)°. An electron microprobe analysis produced an average composition (wt%) of SiO2 42.85(10), CuO 46.13(23), K2O 4.16(2), MgO 1.53(17), Na2O 0.27(4), BaO 0.18(6), and MnO 0.08(1), plus Li2O 1.38 from the LA-ICP-MS measurement and H2O 3.22 (added to bring the analytical total close to 100%), yielding a total of 99.79% and an empirical chemical formula (K0.99Ba0.01)S=1.00(Li1.04Cu0.93Na0.10)S=2.07 (Cu5.57Mg0.43Mn0.01)S=6.01(Si4.00O11)2(OH)4. Lavinskyite is isotypic with plancheite, Cu8(Si4O11)2(OH)4·H2O, an amphibole derivative. It is orthorhombic, with space group Pcnb and unit-cell parameters a = 19.046(2), b = 20.377(2), c = 5.2497(6) Å, and V = 2037.4(4) Å3. The key difference between lavinskyite and plancheite lies in the coupled substitution of K+ and Li+ in the former for H2O and Cu2+ in the latter, respectively. The structure of lavinskyite is characterized by the undulating, brucite-like layers consisting of three distinct octahedral sites occupied mainly by Cu. These layers are sandwiched by the amphibole-type double silicate chains extending along the c axis, forming a sheet structure of compact silicate-Cu-silicate triple layers. Adjacent sheets are linked together by K and M4 (= Cu + Li) cations, as well as hydrogen bonding. The M4 site is split, with Cu and Li occupying two different sites. Lavinskyite exhibits more amphibole-like structural features than plancheite, as a consequence of K in the large cavity between the two back-to-back double silicate chains.