SPESSARTINE Chapter15. Nesosilicates

SPESSARTINE

Mn₃Al₂(SiO₄)₃
Cubic, Ia3d, a = 11.697 Å, Z = 8.

Spessartine was first reported from Franklin by French (1953) and was described in more detail by Frondel and Ito (1965b) and Titus (1986). It is known from Franklin, but is much more abundant at Sterling Hill.

Description

Spessartine from Franklin and Sterling Hill occurs in massive material and in dodecahedral crystals; modifying forms are common. Generally, as elsewhere, spessartines have an orange-red color, vitreous luster, and no cleavage. The density of a spessartine (Sp₆₇) studied by Titus (1986) is 3.87 g/cm³; that of the material studied by Frondel and Ito (1965b) is 4.01 g/cm³. Spessartine is isotropic; measured values for the index of refraction vary from 1.789 to 1.818. There is no discernible fluoresence in ultraviolet.

Composition

Spessartine is a manganese aluminum silicate mineral of the garnet group. Most specimens from Franklin and Sterling Hill are in solid solution with andradite and grossular; the dominant substituents are Fe³⁺ and Ca. A calcian spessartine was described by Frondel and Ito (1965b). Amthauer et al. (1976) provided analytical and Mössbauer data for a Franklin spessartine and called attention to the data of Whipple (1973, not seen). Microprobe analyses of some local spessartines are given in Table 2 and are representative of local material.

Of particular note is the report of highly hydrated spessartines noted by Wilkins and Sabine (1973). These were reported to contain 2.50 and 2.57 wt. % H₂O in 5-cm dodecahedral calcian crystals (Sp₆₂An₂₁Gr₁₇) from Sterling Hill. A similar value was reported by Frondel (1972). A reexamination of this garnet (HU-85448) by this writer yielded 2.0 wt. % H₂O by the Penfield method. The composition, determined by microprobe: SiO₂ 36.8, Al₂O₃ 16.2, FeO (total Fe as FeO) 9.4, MgO 0.3, CaO 11.1, TiO₂ 0.1, MnO 26.9, total = 100.8 wt. %, for an average of three closely agreeing analyses, shows it to be a calcian spessartine. Additional unpublished studies at Harvard University found 0.027 and 0.05 wt. % H₂O in Sterling Hill spessartines. There have been no other detailed studies of the hydration of local garnets.

Occurrence and paragenesis

Spessartine from Franklin occurs in few known assemblages, but might have been more abundant than the sparse record indicates. The few studies done have been on atypical specimens, and common-grade material is largely unstudied. The material described by Frondel and Ito (1965b) occured as thin orange-red veinlets cutting massive andradite. Spessartine (Sp₈₇) is also associated with nelenite, tirodite, and willemite. Spessartine also occurs as tiny 1-2 mm crystals on spinel octahedra associated with calcite.

At Sterling Hill, spessartine was also found in isolated assemblages, such as with tirodite, calcite, and pyroxmangite on the 700 level, and also with galena, bornite, chalcopyrite, and calcite. Assuredly pre-eminent among Sterling Hill occurrences is that of large spessartine crystals associated with fayalite (roepperite) crystals, augite (jeffersonite), spinel, and other minerals. Some of these crystals have been freed from associated minerals as a result of weathering. Many of these have been preserved in systematic collections, and are the best of local spessartines (see analysis #C2412 in Table 2). Reilly (1983) reported an anomalous green spessartine rimming pyroxene and franklinite.

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