Ferromerrillite, Ca9NaFe2+(PO4)7, a new mineral from the Martian meteorites, and some insights into merrillite–tuite transformation in shergottites
Britvin, Sergey N.; Krivovichev, Sergey V.; Armbruster, Thomas
European Journal of Mineralogy Volume 28 Number 1 (2016), p. 125 - 136
published: Mar 1, 2016
ArtNo. ESP147052801011, Price: 29.00 €
Ferromerrillite, ideally Ca9 NaFe2+ (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) Å 3 Z = 6. The calculated density D calc. is 3.11 g/cm3 based on the empirical formula Ca9.00 (Na0.60 Ca0.07 )Σ0.67(Fe2+ 0.53Mg0.40)Σ0.93P7.08O 28(Shergotty) and 3.14 g/cm3 for Ca9.00 (Na0.49 Ca0.15)Σ0.64 (Fe2+ 0.78Mg0.23)Σ1.02P 7.03O28(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 (< 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: ϖ 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.