Ferromerrillite, ideally Ca9NaFe2+(PO4)7, is a new mineral related to the whitlockite group. It occurs as a common accessory phase in several Martian meteorites classified as basaltic and olivine-phyric shergottites. The mineral described herein originates from the two meteorites Shergotty (the type occurrence) and Los Angeles. Ferromerrillite is trigonal, space group R3c; the unit-cell dimensions (single-crystal data) for material from Shergotty and Los Angeles are, respectively: a 10.372(2) and 10.3794(6); c 37.217(13) and 37.129(2) Å; V 3467(3) and 3464.1(1) Å3; Z = 6. The calculated density Dcalc. is 3.11 g/cm3 based on the empirical formula Ca9.00(Na0.60Ca0.07)∑0.67(Fe2+0.53Mg0.40)∑0.93P7.08O28 (Shergotty) and 3.14 g/cm3 for Ca9.00(Na0.49Ca0.15)∑0.64 (Fe2+0.78Mg0.23)∑1.02P7.03O28 (Los Angeles). The crystal structure of ferromerrillite from the Los Angeles meteorite was solved and refined to R1 = 0.066 on the basis of 1518 independent reflections with I > 2σ(I). Single-crystal studies reveal that ferromerrilite grains from the two studied shergottites are heavily deformed with an angular mosaicity reaching 7 degrees. The latter imply that the mineral grains experienced a shock event but the impact pressure was not high enough (<23 GPa) for the transformation to tuite, γ-Ca3(PO4)2, to occur. Ferromerrillite is colourless, no cleavage was observed, Mohs’ hardness is ~5. In the immersion liquids, the mineral is colourless and non-pleochroic. It is optically negative, uniaxial to anomalously biaxial with 2V up to (−)20°. Refractive indices are: ω 1.623(1) and 1.624(1), ε 1.621(1) and 1.621(1) for Shergotty and Los Angeles material, respectively. The mineral is named as Fe-dominant analogue of merrillite.

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