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Abstract:

The rare minerals, inesite, Ca₂Mn²⁺₇Si₁₀O₂₈(OH)₂·5H₂O, hubeite, Ca₂Mn²⁺Fe³⁺Si₄O₁₂(OH)·2H₂O, and ruizite, Ca₂Mn³⁺Si₄O₁₁(OH)₄·2H₂O, were determined, for the first time, by optical and XRD analyses, at Cavnic (Baia Mare Neogene metallogenetic district, Eastern Carpathians, Romania) in a gold-bearing, base metal deposit. Inesite was refined as triclinic, in space group P1 (2), and has the following unit-cell parameters (refined from X-ray powder diffraction data): a = 8.927(7) Å, b = 9.245(8) Å, c = 11.954(9) Å; α = 91.80(2)°, β = 132.60(1)°, γ = 94.37(3)°, V = 719.45(12)ų. Hubeite was also refined as triclinic, in space group P1, and has as unit-cell parameters: a = 9.960(7) Å, b = 13.870(9) Å, c = 6.562(4) Å, V = 601.19(17) ų; α = 133.19(5)°, β =101.50(3)°, γ = 66.27(4)°. Ruizite was refined on a monoclinic cell, space group A2 (5) and has the following unit-cell parameters: a = 11.974(9)Å, b = 6.175(5)Å, c = 9.052(7)Å, V = 669.12 (9)ų; β = 91.34(3)°. Apophyllite, KCa₄Si₈O₂0(F,OH)·8H₂O, is tetragonal, space group P4/mnc (128), and has a = 8.960(3) Å, c = 15.767(7) Å and V = 1265.80(11) ų. The terms identified represent a solid solutions series between the two end-members, fluorapophyllite and hydroxyapophyllite. Manganoan ilvaite, CaFe²⁺Fe³⁺(Fe²⁺, Mn)(Si₂O₇)(O)(OH), crystallizes in the space group Pnam (62), and has as unit-cell parameters: a = 13.008(7) Å; b = 8.865(5) Å; c = 5.845(3) Å, V = 674.02(9) ų. The textural relations of inesite /hubeite/ruizite /apophyllite/manganoan ilvaite assemblage show that the oldest mineral is manganoan ilvaite, as inesite and hubeite overgrow and engulf this mineral. Hubeite is indicative for mildly oxidizing crystallization, conditions. Both hubeite and ruizite are Ca-Mn hydrated silicates. However, all Mn is divalent in hubeite and trivalent in ruizite. In addition, ruizite is Fe-free, whereas hubeite contains significant Fe³⁺. Ruizite Ca₂Mn³⁺Si₄O₁₁(OH)₄.2H₂O was formed under higher oxidizing conditions as compared with hubeite. Sudoite and specularite also occurs in this oxidized assemblage. Sudoite, Mg₂(Al.Fe³⁺)3Si₃O10(OH)₈, is monoclinic and has as space group C2m (12) and as unit-cell parameters a = 5.237(2) Å, b = 9.070(5) Å, c = 14.285(8), β = 97.00, V = 673.47(14) ų. The carbonate closely associated to this assemblage is a Mn- and Ca-bearing siderite, which crystallizes in space group R-3c (167), having as unit-cell parameters: a = 4.694(1) Å; c = 15.386(5) Å, and V = 293.53(13) ų. The secondary minerals occurring in this assemblage as determined by X-ray powder diffraction are: gold, sphalerite, pyrite, and galena. The origin of vein assemblage inesite /hubeite /ruizite /apophyllite is hydrothermal (epithermal); there exist indications of a genesis at low temperature, around 200°C. The assemblage indicated before is grown on manganoan ilvaite, which is the oldest mineral in this association. There are no signs of the genesis of manganoan ilvaite /inesite /hubeite /ruizite assemblage at the expense of higher T anhydrous manganese minerals such as rhodonite or johannsenite.