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+AlSiO6, 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.

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).

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.

 

(a)

 

(b)

 

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