Chladniite, Na2CaMg7(PO4)6: A new mineral from the Carlton (IIICD) iron meteorite

A new mineral, chladniite, Na₂CaMg₇(PO₄)₆, occurs as a single grain within a silicate-bearing inclusion in the Carlton (IIICD) iron meteorite. It is hexagonal, R3, a = 14.967, c = 42.595 Å. It is named for E.F.F. Chladni (1756–1827), who is widely regarded as the founder of the science of meteoritics. Chladniite is colorless and transparent when powdered. In polished section, the mineral is gray, dark, weakly bireflecting, and weakly anisotropic. Cleavage, rarely visible, is rhomboidal in plan. Polishing hardness is less than low-calcium orthopyroxene, but greater than Fe,Ni metal. Reflectance measurements at 589 nm give R₁ = 5.3%, R₂ = 5.6%. Assuming an absorption coefficient k = 0, the calculated refractive indices are n₁ = 1.60, n₂ = 1.62. Stronger reflections on Gandolfi X-ray film of chladniite are d = 3.694(0,1,11; 306)s, 3.558(0,2,10;0,0,12)m, 2.960(0,1,14)s, 2.753(1,3,10)s, 2.500(3,3,0)m, 2.126(2,4,10; 2,3,14; 0,2,19)m, and 1.851(701; 1,5,14; 6,0,12)m. Microprobe analysis gives, in weight percent, Na₂O 6.57, CaO 6.59, MgO 33.5, FeO 2.23, MnO 0.30, SiO₂ 0.59, P₂O₅ 49.9, total 99.68, leading to the empirical formula Na_1.77Ca_0.98Si_0.08(Mg_6.96Fe_0.26Mn_0.04)_Σ=7.26(P_0.98O₄)₆. Chladniite is the Mg analogue of fillowite and johnsomervilleite, which are Mn- and Fe-dominated end-members, respectively. Chladniite occurs with chlorapatite, olivine, orthopyroxene, plegioclase, schreibersite, Fe, Ni metal, and troilite.