Lavinskyite-1M, K(LiCu)Cu6 (Si4 O11)2 (OH)4, the monoclinic MDO equivalent of lavinskyite-2O(formerly lavinskyite), from the Cerchiara manganese mine, Liguria, Italy

摘要

Lavinskyite-1 M, a monoclinic MDO (Maximum Degree of Order) polytype related to the orthorhombic MDO polytype lavinskyite-2 O (formerly lavinskyite, now redefined), was identified in samples from the Cerchiara manganese mine (Liguria, Italy). Both polytypes have the sameideal chemical formula, K(LiCu)Cu6 (Si4 O11)2 (OH)4. Lavinskyite-1M was originally approved as "liguriaite", but was subsequently redefined as lavinskyite-1 M (IMA proposal 16-E). Lavinskyite-1 M occurs as blue, micaceousaggregates embedded in calcite-filled microfractures and veinlets, where it is associated with calcite, quartz, norrishite and "schefferite" (a Mn-bearing variety of diopside). Lavinskyite-1 M is translucent to transparent, bluish to pale blue in colour with a very pale blue to whitishstreak and vitreous lustre; 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-1 M is brittle with perfect cleavage parallel to {100}, and uneven fracture. The estimated Mohshardness is ~5. The calculated density is 3.613 g/cm3 (for empirical formula). Optically, it is biaxial positive, with α = 1.674(2); β = 1.692(3) and γ = 1.730(3); 2 Vγ is very large, ~75°(est.), 2Vγ (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 elongationis 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 (Li0.89 Mg0.36Cu0.33 Na0.22 Mn2+0.04)Σ1.86 Cu6.00 Si8.08 O22 (OH)4 and (K1.08)Σ1.08 (Li0.89 Cu0.35 Mg0.28 Na0.22 Mn2+0.04)Σ1.78Cu6.00 Si8.12 O22 (OH)4. Strongest lines in the X-ray powder diffraction pattern are [d in ? (Icalc)hkl]): 10.216 (100) 100, 9.007 (20) 110, 4.934 (19) 210, 3.98