Definition of Three Newly-approved Clinopyroxenes:

Grossmanite (CaTi³⁺AlSiO₆), Davisite (CaScAlSiO₆), Kushiroite (CaAlAlSiO₆)

American Mineralogist, 94, 1491-1494 (2009):

“Grossmanite, Ca(Ti³⁺,Mg, Ti⁴⁺)AlSiO₆ with an end-member formula CaTi³⁺AlSiO_6, is a new member of the Ca clinopyroxene group, where the trivalent cations are dominant in the M1 site with Ti³⁺ being the dominant trivalent cation.

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Grossmanite is a new member of the Ca clinopyroxenes (diopside group) with space group C2/c, the Ti³⁺-dominant analogue of esseneite (CaFe³⁺AlSiO₆) (Cosca and Peacor 1987), davisite (CaScAlSiO₆) (Ma and Rossman 2009b), kushiroite (CaTs pyroxene, CaAlAlSiO₆) (Kimura et al. 2009; Ma et al. 2009), where the trivalent cations are dominant in the M1 site with Al occupying about 50% of the tetrahedral Si site. As shown in Figure 5, there seems to be a solid-solution among diopside, grossmanite, davisite and kushiroite based on this study and on Ma and Grossman (2009b) and Ma et al. (2009).

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Term “fassaite” refers to Ti-bearing and Al-rich Ca clinoproxene in refractory inclusions from chondrites (Dowty and Clark 1973; Brearley and Jones 1998), which is well-known and widely-used in the meteoritics community, although the name was discredited officially by the IMA-CNMNC (Morimoto et al. 1988). By definition, Ti-rich “fassaite” pyroxenes are grossmanite if trivalent cations are dominant in the M1 site with Ti³⁺ being the dominant trivalent cation (Fig. 6). If trivalent cations dominate the M1 site with Sc³⁺ the dominant trivalent cation, such “fassaite” is davisite. When trivalent cations dominate the M1 site with Al the dominant trivalent cation, the “fassaite” is kushiroite. If Mg²⁺ is dominant in the M1 site, the “fassaite” is diopside.”

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Figure 5. (a) Molar ternary diagram of three cation groups in the M1 site from the Allende clinopyroxenes of this study, Ma and Grossman (2009b) and Ma et al. (2009); (b) normalized ternary diagram of Ti³⁺-Sc³⁺-Al³⁺ from the same clinopyroxenes where the trivalent cations are dominant in the M1 site. Square: grossmanite (USNM 7562, 7555, 7554); circle: davisite (USNM 7555); triangle: kushiroite (USNM 7554, ALH1); diamond: Ti-rich diopside (USNM 7554, TS32).

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2009 Chi Ma, Caltech