Lavinskyite-1M, K(LiCu)Cu6(Si4O11)2(OH)4, the monoclinic MDO equivalent of lavinskyite-2O (formerly lavinskyite), from the Cerchiara manganese mine, Liguria, Italy

Lavinskyite-1M, a monoclinic MDO (Maximum Degree of Order) polytype related to the orthorhombic MDO polytype lavinskyite-2O (formerly lavinskyite, now redefined), was identified in samples from the Cerchiara manganese mine (Liguria, Italy). Both polytypes have the same ideal chemical formula, K(LiCu)Cu6(Si4O11)2(OH)4. Lavinskyite-1M was originally approved as “liguriaite”, but was subsequently redefined as lavinskyite-1M (IMA proposal 16-E). Lavinskyite-1M occurs as blue, micaceous aggregates embedded in calcite-filled microfractures and veinlets, where it is associated with calcite, quartz, norrishite and “schefferite” (a Mn-bearing variety of diopside). Lavinskyite-1M is translucent to transparent, bluish to pale blue in colour with a very pale blue to whitish streak and vitreous luster; it is non-fluorescent. Individual, always indistinct platelets are up to ~0.15 mm in length. The crystals are tabular (100) and elongate along [001]. Lavinskyite-1M is brittle with perfect cleavage parallel to {100}, and uneven fracture. The estimated Mohs hardness is ~5. The calculated density is 3.613 g/cm3 (for empirical formula). Optically, it is biaxial positive, with *= 1.674(2); * = 1.692(3) and * = 1.730(3); 2V* is very large, ~75° (est.)°, 2V* (calc.) = 70°. Pleochroism is moderate: X (pale) blue, Y pale blue and Z pale blue with faint greenish tint; absorption X ≥ Z ≥ Y. Orientation: X ^ a ~20° (probably in obtuse beta), Y = b, Z ~ c; optical elongation is positive and the optical axis plane is parallel to (010). No dispersion was observed. Chemical analysis (quantitative SEM-EDS and LA-ICP-MS) of two samples yielded the empirical formulae (based on 26 O atoms) (K1.08)Σ1.08(Li0.89Mg0.36Cu0.33Na0.22Mn2+0.04)Σ1.86Cu6.00Si8.08O22(OH)4 and (K1.08)Σ1.08(Li0.89Cu0.35Mg0.28Na0.22Mn2+0.04) Σ1.78Cu6.00Si8.12O22(OH)4. Strongest lines in the X-ray powder diffraction pattern are [d in Å (Icalc) hkl]): 10.216 (100) 100, 9.007 (20) 110, 4.934 (19) 210, 3.983 (19) 230, 3.353 (33) 310, 2.8693 (22) 241, 2.6155 (35) 161, 2.3719 (23) 20-2. The crystal structure has been solved, using single-crystal X-ray diffractometer data (Rint = 4.60 %), by direct methods and refined in space group P21/c (no. 14) to R1 = 5.10 % and wR2all = 13.92 % [1786 'observed' reflections with Fo > 4*(Fo), 199 parameters]. Refined unit-cell parameters are: a = 10.224(2), b = 19.085(4), c = 5.252(1) Å, β = 92.23(3)o, V = 1024.0(4) Å3 (Z = 2). The chemical composition and crystal structure are supported by micro-Raman spectra. Lavinskyite-1M has a sheet structure consisting of corrugated brucite-like (CuO2)n layers with amphibole-type (SiO3)n chains joined to both their upper and lower surfaces. Adjacent complex sheets are linked by [5]-coordinated Li atoms and Cu atoms in square coordination (nearly planar) and interlayer K atoms. Lavinskyite-1M is isostructural with a hypothetical monoclinic MDO polytype of plancheite, not yet found in nature, while lavinskyite-2O is isostructural with plancheite. It appears that a complex and delicate interplay between the Li:Cu and Cu:Mg ratios (lower in lavinskyite-1M), along with an additional influence of impurity cations such as Na and different conditions of formation, results in a stabilisation of the 1M polytype. The origin of lavinskyite-1M can be related to a complex, multi-stage hydrothermal evolution of the primary Fe-Mn ore at Cerchiara, which experienced a diffuse alkali metasomatism under strongly oxidising conditions and produced mineral assemblages enriched in Na, K and Li, while providing also appreciable amounts of Ba, Sr, Ca and Cu.