BARYSILITE Chapter16. Sorosilicates and cyclosilicates

BARYSILITE

Pb₈Mn(Si₂O₇)₃
Hexagonal, R3c, a = 9.80, c = 38.3 Å, Z = 6.




	Figure 16-4. Curved platy barysilite crystals from late-stage deposition at Franklin. Field of view is 0.1 mm in maximum dimension.

Barysilite was first reported from Franklin by Shannon and Berman (1926) using a very sparse sample from the picking table at Franklin. Additional data were provided by Bauer and Berman (1930). Glasser (1964) investigated Franklin barysilite using single-crystal methods; he gave a = 8.46 (in error?) and c = 38.3 Å. This study was later cited by Harnik (1972), who gave Glasser’s values as they appear in the heading above. Harnik gave no explanation for the change, but implied that the given values were derived by refinement of Glasser’s powder data. The value of a given in the heading has been validated by all subsequent studies. Barysilite was synthesized by Frondel and Ito (1967) and Ito (1968). The literature on synthesis studies is extensive; a review of barysilite-type compounds is given by Schmidt et al. (1983).

Crystal structure

Lajzérowicz (1964, 1965) showed that Mn is essential to barysilite and published the description of the structure which consists of non-linear Si₂O₇ groups, with each Pb atom surrounded by three oxygen atoms.

Description




	Figure 16-5. Platy barysilite (light gray) with franklinite (black, top left-and-right center) and andradite (dark gray). Specimen is 12 cm in maximum dimension. Smithsonian Institution, #C6390. Photo by the author.

Barysilite occurs as multi-centimeter-sized lamellar aggregates, platy crystals (Figure 16-5), gray botryoids, and 1.0 mm white opaque spherules; platy aggregates are the common habit. Druse habits are not uncommon, and epitactic intergrowths with ganomalite (Figure 16-8) were illustrated by Yeates (1991). Barysilite is white, gray, light violet, or commonly light pinkish gray. The luster is vitreous to adamantine to pearly, and the cleavage is perfect on {0001}. Optically, barysilite is unixial, negative, with w = 2.033, and e = 2.015. There is no discernible fluorescence in ultraviolet. Its visual appearance, lack of fluorescence, and mineral associations distinguish it from most other local minerals.

Composition

Barysilite is a lead manganese silicate. The composition is uniform from specimen to specimen, and solid solution of other cations is limited; Ca and small amounts of Mn may substitute for Pb (Dunn, 1985b). Yeates (1991) noted Ca-enrichment in the latest formed barysilite in a secondary druse. A representative analysis is given in Table 8.

Occurrence and paragenesis

Although sparse when first discovered on the picking table, barysilite has since been found in relative abundance in the restricted assemblage of lead silicates. Barysilite occurs in recrystallized assemblages as platy masses associated with hardystonite, clinohedrite, andradite, manganaxinite, willemite, grossular, and nasonite, among other species. Although commonly occurring with nasonite as the only other lead silicate present, minor amounts of margarosanite or hancockite may be associated. Barysilite commonly occurs as a late-stage mineral, forming druses, breccia-cements, and crack-fillers in other species (johannsenite for example), and as fresh material on previously formed fractures and slickensides of other minerals. Among the most prized specimens are those of a barysilite- dominant breccia enclosing fragments of andradite and crystals of bright green willemite. An association with larsenite and ganomalite was observed in one specimen.

FOOTER LBI




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