Halamishite, Ni5P4, a new terrestrial phosphide in the Ni–P system

Abstract

Halamishite, Ni5P4, is a new natural phosphide discovered in pyrometamorphic assemblages of the Hatrurim Formation (Mottled Zone), Southern Levant. The mineral occurs as minute grains up to 20 μm in size intimately intergrown with zuktamrurite, FeP2, and transjordanite, (Ni,Fe)2P, which are disseminated in fine-grained diopside microbreccia. Chemical composition of the holotype material (electron microprobe, wt%): Ni 69.23, Fe 1.80, P 29.59, total 100.62, was recalculated to (Ni4.90Fe0.13)5.03P3.97 on the basis of nine atoms per formula unit. The ideal formula of halamishite is Ni5P4. In reflected light, the mineral is white with beige tint, non-pleochroic. It is moderately anisotropic and bireflectant (ΔR589 = 7.2%). Reflectance values for the four wavelengths recommended by the IMA Commission on Ore Microscopy are [Rmax/Rmin (%), λ (nm)]: 44.3/36.6, 470; 49.2/42.1, 546; 51.3/44.1, 589; 53.3/46.1, 650. Crystal structure of halamishite was solved and refined to R1 = 0.031 based on 425 unique observed [I ≥ 2σ(I)] reflections. The mineral is hexagonal, space group P63mc, a 6.8184(4), c 11.0288(8) A, V 444.04(6) A3, Z = 4. Dx calculated for an empirical formula is 6.249(1) g cm−3. Halamishite is named for its type locality, the Halamish wadi in the Hatrurim basin, Negev Desert, Israel. It is a naturally occurring analogue of synthetic Ni5P4, the compound widely used in electro- and photocatalytic applications. Due to chemical proximity to the Ni5P4 end-member, halamishite can be used as a geothermometer indicating that formation of phosphide assemblages had occurred at a temperature below 870 °C.