Description:
<jats:title>Abstract</jats:title><jats:p>Nickolayite, FeMoP, is a new terrestrial phosphide structurally related to allabogdanite (high-pressure modification of (Fe,Ni)<jats:sub>2</jats:sub>P), and the meteoritic phosphides florenskyite, FeTiP and andreyivanovite, FeCrP. From the point of view of chemical composition, nickolayite is an Fe-analogue of monipite, MoNiP. The mineral was discovered in the Daba-Siwaqa complex, Central Jordan, a part of the pyrometamorphic Hatrurim Formation (the Mottled Zone), whose outcrops encompass a 150 × 200 km area around the Dead Sea in the Middle East. Nickolayite appears as an accessory phase in the fused clinopyroxene–plagioclase rocks texturally resembling gabbro–dolerite. The irregularly shaped grains of the mineral, up to 80 μm in size are associated with baryte, tridymite, chromite, hematite, pyrrhotite, fluorapatite, titanite and powellite. Macroscopically, nickolayite grains possess light-grey to greyish-white colour and metallic lustre. The mineral is ductile. The mean VHN hardness (50 g load) is 538 kg mm<jats:sup>–2</jats:sup>. The calculated density based on the empirical formula and the unit-cell parameters is 7.819 g cm<jats:sup>–1</jats:sup>. In reflected light, nickolayite has a white colour, with no bireflectance or pleochroism. The COM approved reflectance values [<jats:italic>R</jats:italic><jats:sub>max</jats:sub>/<jats:italic>R</jats:italic><jats:sub>min</jats:sub>(%), λ(nm)] are: 48.5/46.5 (470), 50.5/48.5 (546), 51.8/49.9 (589) and 53.9/52.0 (650). The chemical composition of the holotype crystal is (electron microprobe, average of 4 analyses, wt.%): Fe 32.21, Mo 47.06, Ni 3.69, Co 0.13, P 17.45, total 100.54, that corresponds to the empirical formula Fe<jats:sub>1.00</jats:sub>(Mo<jats:sub>0.87</jats:sub>Ni<jats:sub>0.11</jats:sub>Fe<jats:sub>0.02</jats:sub>)<jats:sub>Σ1.00</jats:sub>P<jats:sub>1.00</jats:sub>and an ideal formula of FeMoP. Nickolayite is orthorhombic, space group<jats:italic>Pnma</jats:italic>, unit-cell parameters of holotype material are:<jats:italic>a</jats:italic>= 5.9519(5),<jats:italic>b</jats:italic>= 3.7070(3),<jats:italic>c</jats:italic>= 6.8465(6) Å,<jats:italic>V</jats:italic>= 151.06(2) Å<jats:sup>3</jats:sup>and<jats:italic>Z</jats:italic>= 4. The crystal structure of holotype material was solved and refined to<jats:italic>R</jats:italic><jats:sub>1</jats:sub>= 0.0174 based on 251 unique observed reflections. The origin of the mineral is probably connected to the processes of co-reduction of molybdenum- and phosphorus-bearing minerals during high-temperature pyrometamorphic processes.</jats:p>