Chlorellestadite, Ca5(SiO4)1.5(SO4)1.5Cl, a new ellestadite- group mineral from the Shadil-Khokh volcano, South Ossetia

Abstract

Chlorellestadite (IMA2017–013), ideally Ca5(SiO4)1.5(SO4)1.5Cl, the Cl-end member of the ellestadite group was discovered in a calcium-silicate xenolith in rhyodacite lava from the Shadil Khokh volcano, Greater Caucasus, South Ossetia. Chlorellestadite forms white, tinged with blue or green, elongate crystals up to 0.2–0.3 mm in length. Associated minerals include spurrite, larnite, chlormayenite, rondorfite, srebrodolskite, jasmundite and oldhamite. The empirical crystal chemical formula of the holotype specimen is Ca4.99Na0.01(SiO4)1.51(SO4)1.46(PO4)0.03(Cl0.61OH0.21F0.11)Σ0.93. Unit-cell parameters of chlorellestadite are: P63/m, a = 9.6002(2), c = 6.8692(2) Å, V = 548.27(3)Å3, Z = 2. Chlorellestadite has a Mohs hardness of 4–4.5 and a calculated density of 3.091 g/cm3. The cleavage is indistinct, and the mineral shows irregular fracture. The Raman spectrum of chlorellestadite is similar to the spectra of other ellestadite group minerals, with main bands located at 267 cm−1 (Ca–O vibrations), and between 471 and 630 cm−1 (SiO4 4− and SO4 2− bending vibrations) and 850–1150 cm−1 (SiO4 4− and SO4 2− stretching modes). Chlorellestadite forms in xenoliths of calcium-silicate composition when they are exposed to Cl-bearing volcanic exhalations at about 1000 °C under low pressure conditions.