Karwowskiite, Ca₉(Fe²⁺_0.5□_0.5)Mg(PO₄)₇—A New Merrillite Group Mineral from Paralava of the Hatrurim Complex, Daba-Siwaqa, Jordan

Crystals of karwowskiite, Ca₉Mg(Fe²⁺_0.5□_0.5)(PO₄)₇, a new mineral of the merrillite group, were found on an amygdule wall in the central part of an anorthite–tridymite–diopside paralava of the Hatrurim Complex, Daba-Siwaqa, Jordan. The amygdule was filled with a sulfide melt, which after crystallization gave a differentiated nodule, consisting of troilite and pentlandite parts and containing tetrataenite and nickelphosphide inclusions. Karwowskiite crystals are colorless, although sometimes a greenish tint is observed. The mineral has a vitreous luster. The microhardness VHN₂₅ is 365 (12), corresponding to 4 on the Mohs hardness scale. Cleavage is not observed, and fracture is conchoidal. The calculated density is 3.085 g/cm³. Karwowskiite is uniaxial (−): ω = 1.638 (3), ε = 1.622 (3) (λ = 589 nm), and pleochroism is not observed. The composition of karwowskiite is described by the empirical formula: Ca_9.00(□_0.54Fe²⁺_0.23Mg_0.12Na_0.04 Sr_0.03 Ni_0.03K_0.01) _Σ1.00Mg_1.00(PO₄)_7.02. Karwowskiite is distinct from the known minerals of the merrillite subgroup with the general formula A₉XM[TO₃(Ø)]₇, where A = Ca, Na, Sr, and Y; X = Na, Ca, and □; M = Mg, Fe²⁺, Fe³⁺, and Mn; T = P; and Ø = O, in that the X site in it is occupied by Fe²⁺_0.5□_0.5. Karwowskiite is trigonal, space group R-3c with a = 10.3375 (2) Å, c = 37.1443 (9) Å, and V = 3437.60 (17) ų. Karwowskiite crystallizes at temperatures lower than 1100 °C in a thin layer of secondary melt forming on the walls of amygdules and gaseous channels in paralava as a result of contact with heated gases which are by-products of the combustion process.