WOLLASTONITE Chapter17. Inosilicates

WOLLASTONITE

CaSiO₃
Triclinic




	Figure 17-36. White lath-like crystals of wollastonite in sub-parallel alignment in bands alternating with bands of andradite-grossular (dark gray) from Franklin. Specimen is 9 cm in maximum dimension. Smithsonian Institution, #C6152-2. Photo by the author.

Wollastonite is known from both Franklin and from Sterling Hill, where it occurs abundantly. Franklin wollastonite was described in coexistence with bustamite by Mason (1975), and the relation was further discussed by Ohashi and Finger (1978). Many data on the occurrence of local wollastonites were given by Baum (1972).

Description

Wollastonite from Franklin and Sterling Hill occurs as white rock-locked idiomorphic crystals, varying in size up to 7 cm; the luster is vitreous, and the cleavages are perfect, resulting in splintery breakage. Local material has not been examined optically. The fluorescence in ultraviolet varies in both hue and intensity, perhaps due to Mn content. In general, the fluorescence is strong in shortwave and weak in longwave. The best Franklin specimens fluoresce with a vivid orange color and occur in assemblages (calcite, willemite, hardystonite) which provide spectacular fluorescent specimens. Sterling Hill wollastonite occurs in idiomorphic crystals, and thus also provides very esthetic fluorescent specimens; the fluorescent color varies substantially, from orange to light yellow. Wollastonite is sometimes visually confused with pectolite or prehnite, both of which can be white and fibrous.

Composition

Wollastonite is a calcium silicate mineral of the pyroxenoid group. Local material has been little studied. Mason (1975) reported a wollastonite with up to 8.09 % MnO with exsolved bustamite; zinc was absent. Baum (1972) reported MnO contents of 6.9 and 2.6 wt. %, respectively, for the initial Franklin occurrence and the two subsequent ones, as noted below.

Occurrence and paragenesis

Generally, most wollastonite from Franklin and Sterling Hill is associated with calcite, unanalyzed garnet, and a clinopyroxene, probably diopside. The best statements on the occurrences of Franklin and Sterling Hill wollastonite are those of Baum (1972). At Franklin, wollastonite was first found in 1944 in grains up to 7 cm, 70 feet below the 600 level, in disseminated lean ore 12 feet from a pegmatite on the footwall. Additional occurrences were on the 900 level near the Palmer Shaft pillar and on the 1050 level, both associated with barite and tremolite. Some Franklin specimens are very rich, with wollastonite forming between hardystonite and calcite, such as occurred in pillar 290 on the 1000 level. Another notable Franklin occurrence is of mostly fine-grained or fibrous wollastonite, associated with microcline, grossular, vesuvianite, allanite, margarosanite, and minehillite.

At Sterling Hill, wollastonite was first found in a drill-core and subsequently on the 900 level, 70 feet west of the west limb of the orebody. Baum (1972) noted that this material is similar to that from Willsboro, Essex County, New York, and that several tons of the New York material had been brought to the Franklin Mill for experimental purposes; thus, local collections may be tainted in part. Much Sterling Hill wollastonite has been found between the 340 and 1600 levels, near the footwall of the east limb; these finds provided the best fluorescent specimens, and substantial quantities have been preserved. Johnson (1990) reported nearly pure end-member wollastonite, associated with titanite from beneath the east limb, on the 1400 level, at the 1060 crosscut. Additionally, wollastonite has been found in other scattered occurrences.

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