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Cr-Ca Symplectite Lamellea in an Olivinen Grain from the Luna-24 Regolith

Introduction: Cr-Ca symplectites in olivine are known as spinel-pyroxene vermicular intergrowths usually observed as irregular shaped blebs along grain boundaries [1-3] or rarely as oriented needles [1, 2, 4] in olivine grains from terrestrial [3, 4 ] as well as lunar rocks [1, 2]. The origin of the Cr-Ca symplectites is debated yet [1-4] and suggested to be resulted from either exsolution process during cooling of Al-and Cr-rich olivine [4], or combination of processes including either diffusion, discontinuous precipitation, and metamorphism [3], or a breakdown of garnet [2], or crystallization of a trapped late-stage melt [1, 2], or diffusion of Ca, Al and Cr out of olivine [2] and a reaction between olivine and plagioclase [1, 2]. The Cr-Ca symplectites in terrestrial and lunar olivines are very similar in their morphology, texture and phase composition in spite of Cr content in lunar and terrestrial olivines as well as the f(O2) conditions on the Moon and on the Earth are different [5]. Here we describe lamellae of Cr-Ca symplectites in an olivine grain from a Luna-24 regolith sample firstly reported as Cr-Ca lamellar inclusions by [6]. The olivine grain was investigated by EMPA and ASEM. Results: Cr,Ca-rich lamellae of 1-3 mkm in width cross the olivine grain parallel to the (100) crystallographic plane (Fig. 1). BSE image show that lamellae have a vermicular texture that is typical for symplectites (Fig. 2). White and grey worm-like bands are in about 1:1 volume ratio and alternated approximately normal to the lamellae/olivine host interface. The adjacent host olivine matrix has a chemical composition of Mg 1..53 Fe 0.47 SiO 4. The lamellae are too thin to be analyzed precisely by traditional EMPA and ASEM methods. Just the same, our measurements demonstrate that (i) Ca/Cr atomic ratio is about 1.2 in the lamellae (Fig. 3); (ii) Mg/Fe atomic ratio is 3.25 in the olivine host and decreases in the lamellae being in relation to both the Mg substitution for Ca + Cr in lamellae and the Fe extraction due to Fe + 2Cr 2+ = vFe + 2Cr 3+ oxidation reaction; (iii) O/Si atomic ratio is <4 in the lamellae; (iv) A(tot)/Si ratio is less than 2 in the lamellae (where A(tot) is a total number of octahedral cations, A(tot) = Mg + Fe + Ca + Cr + Al). Thus, the chemical composition of the lamellae can be considered as Ca,Cr-olivine with a some …