GROSSULAR Chapter15. Nesosilicates

GROSSULAR

Ca₃Al₂(SiO₄)₃
Cubic, Ia3d, a = 11.814 Å, Z = 8.




	Figure 15-34. Franklin grossular-andradite in a fine-grained impure mixture which has replaced a mica, preserving the micaceous habit of the precursor. This is the mineral mixture referred to as “caswellite.” The gray granular mineral at the upper right is andradite. Specimen is 9 cm in maximum dimension. Smithsonian Institution, #C3691-1. Photo by the author.

Grossular is a moderately common garnet at Franklin, but has been little noted and barely studied. It was first noted by French (1953). Frondel and Ito (1965b) described a manganoan grossular analyzed by Lawson Bauer. Several grossulars were studied by Titus (1986) and Hansen (1986); others have been found at Sterling Hill by the writer.

Description

Grossular from Franklin and Sterling Hill occurs as both dodecahedral crystals and massive coarse-grained to extremely fine-grained material. The color varies substantially from cinnamon-brown to light brown, pink, white, and colorless. The luster is vitreous, and there is no cleavage; few physical measurements have been made. The pink material described by Frondel and Ito (1965b) has the unit-cell value given above and an index of refraction of 1.742. There is no discernible fluorescence in ultraviolet.

Composition

Grossular is a calcium aluminum silicate mineral of the garnet group. Few analyses of local material exist; one of impure material is cited in Table 2, and others were given by Titus (1986) and Johnson (1990). Locally, grossular occurs with up to 89 mole % of the end-member. There is much solid solution with andradite at Franklin and with spessartine at Sterling Hill. Vanadian grossular is discussed under goldmanite. Hansen (1986) reported minor amounts of Cr and V in a specimen from Franklin, and she reported Ba, Zn, Cd, Br, As, Co, and other elements at the trace-amount level.

Occurrence and paragenesis

Grossular occurs at Franklin in varied assemblages; many are recrystallized, and it is highly likely that many more occurrences exist unrecognized. It occurs as light pink to colorless massive material associated with wollastonite, minehillite, margarosanite, prehnite, and other species in a well-preserved assemblage discussed under minehillite. From another assemblage, Titus (1986) described 6 mm dodecahedral crystals on a matrix of massive grossular.

Grossular also occurs as massive, very fine-grained, commonly pink to pinkish gray, turbid to opaque material. It occurs as replacements of other minerals, and at the interface between caswellite (discussed below) and unaltered minerals, especially microcline (Figure 12-31), and in other associations. The specimen analyzed by Bauer in Frondel and Ito (1965b) may be of this type; Bauer found only 0.24 wt. % H₂O (at 110^o C). This material physically resembles some hydrogrossular from other locations worldwide.

Grossular is also known from Sterling Hill as light brown massive material, associated with diopside, vesuvianite, and calcite on the 1680 level (Hansen, 1986), and iron-bearing material occurs on the 340 level (Johnson, 1990). At Sterling Hill, as at Franklin, grossular may be more widespread than the few recognized occurrences indicate. See figures 12-24 and 12-31.

Much of the Franklin material called caswellite, named for John H. Caswell by Chester (1894a, 1894b, 1896, 1910), is an impure mixture of minerals replacing a mica which was likely phlogopite or hendricksite. This replacement grades, in different specimens, from barely altered material to that which has been completely replaced (Figures 12-23, 12-31, 15-34, 18-1, and 18-5). Relict textures of the pre-existing mica are commonly preserved, but much shearing has taken place locally, and such features are not always evident. The chief component of most caswellite is grossular, together with vesuvianite, chlorite, and other phases. The proportions of these components vary substantially, perhaps explaining in part the substantial difference between Chester’s 1894 and 1910 analyses, although this does not account for the 15.95 wt. % Mn₂O₃ reported in his earlier analysis. Additionally, as noted by Frondel (1972), some andradite may be present in solid solution; the known analyses of caswellite are open to varying interpretations. Much vesuvianite can be present; in other assemblages, small isolated previously-existing mica crystals may be wholly replaced by green, blue, or colorless vesuvianite. In the common “caswellite” assemblage, grossular is associated with numerous other minerals, including but not limited to willemite, mica, franklinite, andradite, prehnite, manganaxinite, margarosanite, bustamite, microcline, vesuvianite, and many of the species known locally as “Parker-Shaft” minerals. The name caswellite has no mineralogic significance.

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