Pavlovskyite Ca₈(SiO₄)₂(Si₃O₁₀): A new mineral of altered silicate-carbonate xenoliths from the two Russian type localities, Birkhin massif, Baikal Lake area and Upper Chegem caldera, North Caucasus

Abstract

The new mineral pavlovskyite Ca₈(SiO₄)₂(Si₃O₁₀) forms rims together with dellaite Ca₆(Si₂O₇)(SiO₄)(OH)₂ around galuskinite Ca₇(SiO₄)₃CO₃ veins cutting calcio-olivine skarns in the Birkhin gabbro massif. In addition, skeletal pavlovskyite occurs in cuspidine zones of altered carbonate xenoliths in the ignimbrites of the Upper Chegem caldera (North Caucasus). The synthetic analog of pavlovskyite has been synthesized before and is known from cement-like materials. Isotypic to pavlovskyite is the synthetic germanate analog Ca₈(GeO₄)₂(Ge₃O₁₀). The crystal structure of pavlovskyite, space group Pbcn, a = 5.0851(1), b = 11.4165(3), c = 28.6408(8) Å, V = 1662.71(7) ų, Z = 4, has been refined from X-ray single-crystal data to R1 = 3.87%. The new colorless mineral has a Mohs hardness of 6-6.5, biaxial (-), α = 1.656(2), β = 1.658(2), γ = 1.660(2) (589 nm), 2V (meas) = 80(5)°, 2V (calc) = 89.9°, medium dispersion: r > v, optical orientation: X = b, Y = c, Z = a.

For comparison with pavlovskyite, the crystal structure of kilchoanite Ca₆(SiO₄)(Si₃O₁₀) from the Birkhin massif [space group I2cm, a = 11.4525(2), b = 5.0867(1), c = 21.996(3) Å, V = 1281.40(4) ų, Z = 4] has been refined from single-crystal X-ray data to R1 = 2.00%.

Pavlovskyite represents a 1:1 member of a polysomatic series with calcio-olivine γ-Ca₂SiO₄ and kilchoanite Ca₆(SiO₄)(Si₃O₁₀) as end-member modules. The structure is characterized by strongly folded trisilicate units (Si₃O₁₀) interwoven with a framework of CaO₆ and CaO₈ polyhedra. Olivine-like slices with orthosilicate groups are interstratified with the characteristic trisilicate module of Ca₄(Si₃O₁₀) composition. Although the optical properties of pavlovskyite and kilchoanite are similar, both minerals can be distinguished by chemical analyses (different Ca/Si ratio), X-ray diffraction, and Raman spectroscopy. The new mineral is named after V.E. Pavlovsky (1901-1982), an outstanding geologist in the area of Eastern Siberia, in particular of the Baikal region.