FAYALITE Chapter15. Nesosilicates

FAYALITE

Fe₂SiO₄
Orthorhombic, Pbnm, a = 4.798, b = 10.387, c = 6.055 Å, Z = 4.

A number of local fayalite specimens were studied and named before the olivine series was well understood. Accordingly, there are some superfluous names in the local literature. These are:

Knebelite: The first local use of this name is obscure; it is probably fayalite.

Iron-manganese-zinc chrysolite: Although first used by Roepper (1870), this term did not attain much recognition as a mineral name.

Roepperite: This name was assigned by Brush (1872b) to fayalite from Sterling Hill which had been previously described by Roepper (1870). It was also reported at Franklin by Brush (1871) who found it associated with cubic gahnite in the tunnel from the Buckwheat Mine to the Wallkill River.

Stirlingite or Sterlingite: This is fayalite. The name was applied by Kenngott (Brush, 1872a) to the material now known as roepperite. The name was previously applied to zincite by Alger (1844).

Hortonolite: The first local use of this name was by Frondel (1965) for a magnesian fayalite (HU #113637) from Franklin. This sample was also the subject of four high-temperature crystal-structure analyses (up to 900^o C) by Smyth and Hazen (1973).

Description

Franklin fayalite is massive, has fair cleavages, slighty greasy luster, and is black to yellowish dark-green. Hand-sized specimens are common. The color of some massive black Sterling Hill fayalite has been attributed to inclusions of franklinite (Metsger et al., 1958). The density is 3.95-4.08 g/cm³. Well-formed crystals from Sterling Hill (roepperite) are black. A morphological description of these crystals was given by Palache (1935); some are tabular and rectangular, and others are flattened and pseudohexagonal. Optically, fayalite is biaxial, negative, with 2V = 77(±5)^o, a = 1.758, b = 1.786, and g = 1.804 (Larsen, 1921); dispersion is strong, r > v. There is no discernible fluorescence in ultraviolet. Fayalite resembles tephroite, from which it is best differentiated by chemical analysis.

Composition

Fayalite is an iron silicate mineral of the olivine group. There has been little analytical work done since Palache (1935); an analysis of a Franklin hortonolite is given in Table 1, together with one of Sterling Hill fayalite. No end-member fayalite is known locally; local fayalite contains much Mn, Mg, and Zn in substitution for Fe, and a complete series to tephroite may exist. The superb euhedral crystals, locally known as roepperite, from the Noble Mine at Sterling Hill, have average composition SiO₂ 30, FeO 41, MgO 6, ZnO 6, MnO 17, total = 100 wt. %. Correlative studies of fayalite-tephroite solid solutions by infrared spectrometry were given by Burns and Huggins (1972), and they provided an analysis of Franklin fayalite with Fe:Mn nearly 1:1. Aside from these investigations, fayalite from Franklin and Sterling Hill has been little studied.

Occurrence and paragenesis

The local occurrences of fayalite might have been very few in number. The best-described Franklin occurrence was noted by Brush (1871) associated with cubic gahnite. There are no other known Franklin occurrences except the hortonolite reported by Frondel (1965).

At Sterling Hill, there are two notable assemblages. One is the roepperite crystal assemblage noted by Palache (1935) in which the fayalite crystals are associated with franklinite, augite (jeffersonite), and gahnite. Palache quotes Canfield as saying this was from a large deposit high on the east limb of the orebody and that over 100 tons of it was shipped as iron ore. This area might have provided many of the best specimens.

The other Sterling Hill occurrence is of black massive fayalite, associated with black willemite, sphalerite, abundant loellingite, and other minerals from the black-willemite zone at Sterling Hill (Figures 12-12 and 21-29). An analysis of one of Lawson Bauer’s samples from this occurrence is given by Burns and Huggins (1972), as cited by Francis (1980). Although massive and slightly difficult to distinguish from black willemite, there is a wealth of specimen material, and much of it has been preserved.

FOOTER LBI




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