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)Cu-6(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 lustre; it is non-fluorescent. Individual, always indistinct platelets are up to similar 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 similar to 5. The calculated density is 3.613 g/cm(3) (for empirical formula). Optically, it is biaxial positive, with alpha = 1.674(2); beta = 1.692(3) and gamma = 1.730(3); 2 V-gamma is very large, similar to 75 degrees (est.), 2 V gamma (calc.) = 70 degrees. Pleochroism is moderate: X (pale) blue, Y pale blue and Z pale blue with faint greenish tint; absorption X >= Z >= Y Orientation: X<^>a similar to 20 degrees (probably in obtuse beta), Y= b, Z similar to 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) (K-1.08)(Sigma 1.08)(Li0.89Mg0.36CU0.33Na0.22Mn0.042+)(Sigma 1.86)CU6.00Si8.08O22(OH)(4) and (K-1.08)(Sigma 1.08)(Li0.89CU0.35Mg0.28Na0.22Mn0.042+)(Sigma 1.78)CU6.00Si8.12O22 (OH)(4). Strongest lines in the X-ray powder diffraction pattern are [d in A (I-calc) 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 (R-int = 4.60%), by direct methods and refined in space group P2(1)/c (no. 14) to R1= 5.10% and wR2(all) = 13.92% [1786 'observed' reflections with F-o > 4 sigma(F-o), 199 parameters]. Refined unit-cell parameters are: a= 10.224(2), b= 19.085(4), c = 5.252(1) angstrom, = 92.23 (3)degrees, V= 1024.0(4)angstrom(3) (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.
