Ferromerrillite, Ca9NaFe2+(PO4)7, a new mineral from the Martian meteorites, and some insights into merrillite–tuite transformation in shergottites

Abstract

Ferromerrillite, ideally Ca 9 NaFe 2+ (PO 4 ) 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 R 3 c ; 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) A; V 3467(3) and 3464.1(1) A 3 ; Z = 6. The calculated density D calc . is 3.11 g/cm 3 based on the empirical formula Ca 9.00 (Na 0.60 Ca 0.07 ) ∑0.67 (Fe 2+ 0.53 Mg 0.40 ) ∑0.93 P 7.08 O 28 (Shergotty) and 3.14 g/cm 3 for Ca 9.00 (Na 0.49 Ca 0.15 ) ∑0.64 (Fe 2+ 0.78 Mg 0.23 ) ∑1.02 P 7.03 O 28 (Los Angeles). The crystal structure of ferromerrillite from the Los Angeles meteorite was solved and refined to R 1 = 0.066 on the basis of 1518 independent reflections with I > 2 σ ( 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 ( 3 (PO 4 ) 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 2 V up to (−)20°. Refractive indices are: ω 1.623(1) and 1.624(1), e 1.621(1) and 1.621(1) for Shergotty and Los Angeles material, respectively. The mineral is named as Fe-dominant analogue of merrillite.