AEGIRINE Chapter17. Inosilicates

AEGIRINE

Na(Fe³⁺,Mg,Mn,Zn,Al)Si₂O₆
Monoclinic, C2/c, a = 9.776, b = 8.939, c = 5.270 Å,
b = 106.20^o, Z = 4

The preponderance of extant data on Franklin aegirine-augite is that published by Frondel and Ito (1966). End-member aegirine is rare locally; the reported occurrences have all been from Franklin; and none has been reported from Sterling Hill.

Description

Most specimens of end-member aegirine are medium green, forming euhedral crystals which are elongate and prismatic (similar to amphibole) with good cleavages and vitreous luster.

Although colorless to light brown clinopyroxene in the orebodies is largely diopside, it is clear that some of the very dark brown and greenish material from Franklin is aegirine. Brownish specimens, the majority of such material, are properly termed augite. The aegirine described by Frondel and Ito (1966b) as aegirine-augite is massive and occurs in brown, dark brown, and dark green colors. Some specimens have a greasy luster, but most have vitreous luster; the density is 3.4-3.6 g/cm³, depending on composition; and parting is common. Its physical appearance otherwise is very similar to that of common diopside from the orebodies. Optically, this material is biaxial, positive, with a = 1.732, b = 1.742, and g = 1.763 (Frondel and Ito, 1966b). There is no discernible fluorescence in ultraviolet. Differentiation from augite requires chemical analysis in most cases.

Composition




	Table 10. Chemical analyses of pyroxenes and pyroxenoids.

Aegirine is a sodium ferric-iron silicate mineral of the pyroxene group. End-member material is known, but the preponderance of local material is in solid-solution with augite, thus containing much Ca and Mg, and it also as appreciable amounts of Mn and Zn. Three analyses are presented in Table 10.

Occurrence and paragenesis

The most notable occurrence for Franklin end-member aegirine is of dark-green prismatic crystals, several cm in length and fractured, which are associated with the type assemblage for baumite and franklinphilite. Aegirine also occurs as residual altered remnants in blue magnesioriebeckite. Aegirine also occurs as small radial 0.5 mm brown crystals in a sphalerite/bornite pod in gneissic franklinite/willemite ore and rarely as a dark brown reaction-product between altered mica (caswellite) and a nasonite/andradite mixure, both from Franklin. Much of the brown to green pyroxene in the matrix for some Franklin titanite specimens is likewise aegirine; such material is Ti-free. Other occurrences of near-end-member material are sparse.

The aegirines reported by Frondel and Ito (1966b) as aegirine-augite occur in the calcium silicate units of the Franklin orebody in assemblages similar to those of diopside. Two important assemblages were reported by Frondel and Ito (1966b); the compositions are given in Table 10. The material with SiO₂ = 46.38 wt. % is associated with andradite, rhodonite, bustamite, feldspar, apatite, franklinite, willemite, and calcite. The material with SiO₂ = 48.64 wt. % is dark brown with numerous, evenly-distributed rounded grains of brightly fluorescent willemite and calcite. Because of its very pretty red-and-green spotty fluorescence in a non-fluorescent groundmass, the composite assemblage is well-known to some local collectors as “Christmas-tree rock.”

Although Frondel and Ito (1966b) referred to their material as jeffersonite, their use of this now-obsolete name was applied solely to the dark-colored Franklin pyroxene, and should not be confused with the more abundant jeffersonite crystals from Sterling Hill.

FOOTER LBI




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