Augite

Pyroxene Group, Clinopyroxene Subgroup. In the Ca-Fe-Mg (diopside-hedenbergite-augite) quadrilateral, the ideal structural formula is M2M1[Si₂O₆], where M2 is a distorted octahedral site which preferentially accepts large cations such as alkaline earth/alkali atoms (Ca, but may accept transition metals) and where M1 is a smaller regular octahedral site preferentially accepting transition metals, etc. (most commonly Mg, Fe2+). The substitutions containing no changes in valence within M2 (2+), Mi (2+), and tetrahedral sites, T (4+), is the normal scheme. A variety of additional substitution schemes, which require coupling of substituents, have been identified in these pyroxenes. Often these substitutions result in a change of species designation when they are dominant.

Substitution 1 has Na substitution in M2 and trivalent ions of Al, Fe, Cr, and Sc substituting in M1.

Substitution 2 involves Na substitution in M2 and mixed valance transition metals such as [Fe²⁺_0.5Ti⁴⁺_0.5] substituting in M1.

Substitution 3, also called Tschermak's Component, involves coupled M1 octahedral and tetrahedral substitutions, such as CaAl[AlSiO₆] or CaTi³⁺[AlSiO₆].

Substitution 4 is a variant of Substitution 2 and has dominant Ca in M2 with mixed valence substitution in M1 involving divalent and tetravalent Ti and Al substitution in tetrahedral sites, e.g. Ca[(Mg, Fe²⁺_0.5Ti⁴⁺_0.5)][AlSiO₆]. In highly substituted Ca-Fe-Mg pyroxenes, several substitutional schemes may be identified.

Substitution schemes 2 and 4 have not yet yielded named mineral species.

Augite could be considered to be a sub-calcic member of the diopside-hedenbergite series but, as defined by Morimoto et al. (1988) may have Fe>Mg or Mg>Fe without a change in name. Because of the way augite has been defined (Morimoto et al, 1988), there is the implication that in rare cases, a variety of anions may be dominant in various the various sites, including M2, M1, and T. These extreme compositions are rare in the large majority of augites. The clinopyroxenes with Wo_5-20 are classed as pigeonite, another mineral that could be thought of as slightly alkalic clinoenstatite-clinoferrosilite.

However, there is a structural discontinuity between these pyroxenes at low-medium temperatures, and detailed chemical analysis and even structural analysis should be obtained in order to properly classify these unusual compositions. Similarly, extensive substitution of Na, for example, as in scheme 1 (Morimoto et al., 1988) could lead to Aegirine-augite or aegirine compositions depending on the other elements involved.