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

Autor(en)

Uwe Kolitsch, Stefano Merlino, Donato Belmonte, Cristina Carbone, Roberto Cabella, Gabriella Lucchetti, Marco E. Ciriotti

Abstrakt

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(Si

₄O

₁₁)

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 ∼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/cm

³ (for empirical formula). Optically, it is biaxial positive, with a = 1.674(2); b = 1.692(3) and g = 1.730(3); 2V

_g is very large, ∼75° (est.), 2V

_g (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) (K

_1.08)S

_1.08(Li

_0.89Mg

_0.36Cu

_0.33Na

_0.22Mn

²⁺

_0.04)S

_1.86Cu

_6.00Si

_8.08O

₂₂(OH)

₄ and (K

_1.08)S

_1.08(Li

_0.89Cu

_0.35Mg

_0.28Na

_0.22Mn

²⁺

_0.04)S

_1.78 Cu

_6.00Si

_8.12O22(OH)

₄. Strongest lines in the X-ray powder diffraction pattern are [d in A _ (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 (R

_int = 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>4σ(Fo), 199 parameters]. Refined unit-cell parameters are: a = 10.224(2), b = 19.085(4), c = 5.252(1) Ǻ, β = 92.23(3)°, V = 1024.0(4)Ǻ

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 (CuO

₂)

_n layers with amphibole-type (SiO

₃)

_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.

Organisation(en)

Institut für Mineralogie und Kristallographie

Externe Organisation(en)

Università degli Studi di Genova, Naturhistorisches Museum Wien (NHM), Università degli Studi di Pisa

Journal

European Journal of Mineralogy: an international journal of mineralogy, geochemistry and related sciences

Band

30

Seiten

811-820

Anzahl der Seiten

10

ISSN

0935-1221

DOI

https://doi.org/10.1127/ejm/2018/0030-2731

Publikationsdatum

10-2018

Peer-reviewed

Ja

ÖFOS 2012

105116 Mineralogie, 105113 Kristallographie

Schlagwörter

ASJC Scopus Sachgebiete

Geochemistry and Petrology

Link zum Portal

https://ucrisportal.univie.ac.at/de/publications/382bd958-b1e4-4120-a0e3-4023601324ca