Stracherite, BaCa6(SiO4)2[(PO4)(CO3)]F, the first CO3-bearing intercalated hexagonal antiperovskite from Negev Desert, Israel

Abstract

The new mineral stracherite, BaCa₆(SiO₄)₂[(PO₄)(CO₃)]F [<em>R</em>3<em>¯m</em>, <em>a</em> = 7.0877(5) Å, <em>c</em> = 25.201(2) Å, <em>V</em> = 1096.4(1) ų, <em>Z</em> = 3], belongs to the zadovite group, which also includes zadovite, BaCa₆[(SiO₄) (PO₄)](PO₄)₂F; aradite, BaCa₆[(SiO₄)(VO₄)](VO₄)₂F; and gazeevite, BaCa₆(SiO₄)₂(SO₄)₂O. All minerals of this group exhibit single-layer antiperovskite modules, which are intercalated with tetrahedral layers. In stracherite, the first CO₃-bearing intercalated hexagonal antiperovskite, about 38% of the (PO₄)^3– tetrahedra are randomly substituted by planar (CO₃)^2– groups. The mineral was discovered in spurrite rocks of the Hatrurim Complex in the Negev Desert near Arad, Israel. Associated minerals are spurrite, calcite, brownmillerite, shulamitite, CO₃-bearing fluorapatite, fluormayenite-fluorkyuygenite, and ariegilatite. The empirical formula of stracherite is: (Ba_0.96K_0.02Na_0.01)_∑0.99Ca_6.01[(SiO₄)_1.86 (PO₄)_0.12(AlO₄)_0.01(TiO₄)_0.01]_∑2[(PO₄)_1.05(CO₃)_0.75(SO₄)_0.18(VO₄)_0.02]_∑2(F_0.95O_0.03)_∑0.98. Poikilitic crystals of stracherite are up to 0.5 mm in size and are confined to re-crystallization zones of spurrite marbles under the influence of by-products (gases, fluids) of combustion metamorphism.