Grossular is a calcium aluminum silicate mineral of the garnet group. 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.
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 [Dunn].
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 an index of refraction of 1.742. There is no discernible fluorescence in ultraviolet.
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, and in other associations. 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.
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. 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. (Dunn, 1995)

 Location Found: Franklin and Ogdensburg
 Mineral Note: Locally nicknamed "caswellite"
 Formula: Ca3Al2(SiO4)3
 Essential Elements: Aluminum, Calcium, Oxygen, Silicon
 All Elements in Formula: Aluminum, Calcium, Oxygen, Silicon
 IMA Status: Approved
Fluorescent Mineral Properties

 Longwave UV light: Cherry red, very rare
 To find out more about this mineral at minDat's website, follow this link   Grossular

Dunn, Pete J. (1995). Franklin and Sterling Hill New Jersey: the world's most magnificent mineral deposits. Franklin, NJ.: The Franklin-Ogdensburg Mineralogical Society. p.360

Frondel, Clifford (1972). The minerals of Franklin and Sterling Hill, a checklist. NY.: John Willey & Sons. p.59

The Picking Table References
 PT Issue and PageDescription / Comment
View IssueV. 57, No. 2 - Fall 2016, pg. 19Fluorescent Minerals of Franklin and Sterling Hill, N.J., Part 1, Richard C. Bostwick - Grossular
View IssueV. 55, No. 1 - Spring 2014, pg. 19Fluorescent Grossular From Franklin, New Jersey - An Update
View IssueV. 52, No. 1 - Spring 2011, pg. 19Fluorescent Grossular From Franklin, New Jersey With Notes on Associated Gahnite
View IssueV. 6, No. 2 - August 1965, pg. 12Spessartite - Grossularite

Grossular (non-fluorescent) with willemite veins and franklinite Franklin, NJ.Grossular (non-fluorescent) with willemite veins and franklinite Franklin, NJ. under shortwave UV Light
Grossular (white-beige) with willemite veins and franklinite from Franklin, New Jersey. Photo by WP.
Grossular with willemite veins and franklinite from Franklin, New Jersey under Shortwave UV light. The grossular and franklinite is non-fluorescent, the willemite fluoresces green. Photo by WP.

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