ANDRADITE Chapter15. Nesosilicates

ANDRADITE

Ca₃Fe³⁺₂(SiO₄)₃
Cubic, Ia3d, a = 11.996 Å, Z = 8.




	Figure 15-30. Dodecahedral crystals of andradite in calcite from Franklin. Specimen is 5 cm in maximum dimension. Smithsonian Institution, #C6778. Photo by the author.

Andradite is the most abundant garnet at Franklin, and it is the major host for ferric iron in the silicate assemblages (Figures 12-19, 12-20, 12-22, 12-23, 12-24, 12-28, and 12-29). Andradite is nearly ubiquitous in calcium-silicate assemblages at Franklin, where it is present as both a primary and secondary mineral, but it is possibly less common at Sterling Hill.

A number of obsolete and superfluous names have been assigned to Franklin andradite, among them are the polyadelphite of Thomson (1828, 1836), melanite of Seybert (1824b) and Seymour (1868), and also colophonite, topazolite, and titanmelanite, all of obscure origin locally. Andradite may form a part of the mixture known as caswellite and discussed herein under grossular.

Description

Franklin andradite occurs in fine crystals up to 15 cm in size, predominantly dodecahedral, with some trapezohedral modifications (Figures 15-30 through 15-33). Massive material is common, forming chiefly with the calcium and manganese silicates. Chevron-like intergrowths with calcite are known, but have not been analyzed; they may be andradite. The color of Franklin andradite varies; the preponderance of the material is brown or yellow-brown, and the color is affected by grain size, fine-grained material being paler. Andradite also occurs in black, green, reddish brown, and in many intermediate hues. The luster is vitreous; cleavage is absent; and the density varies with the chemical composition. Optically, it is isotropic; the index of refraction is approximately 1.87 for near-end-member material. There is no discernible fluorescence in ultraviolet.

Composition




	Table 2. Chemical analyses of minerals in the garnet group.

Andradite is a calcium ferric-iron silicate mineral of the garnet group. End-member material is uncommon, and most specimens have Mn and Al substitutions, providing much solid solution, principally toward spessartine and grossular. Franklin andradite was among the earliest minerals chemically described from here; it is mentioned by Nuttall (1822) who refers to an analysis by Vauquelin without citation, and it was known to early 1820’s writers simply as “garnet.”

Palache (1935) gives some analyses of Franklin andradite by Lewis and Bauer (1922) and Jenkins and Bauer and cites a few others from Rammelsberg (1860); Des Cloizeaux (1862) also published an analysis of Franklin andradite.




	Figure 15-31. Fine, dodecahedral crystals of andradite with calcite (white) from Franklin. Specimen is 10 cm in maximum dimension. Smithsonian Institution, #116738. Photo by the author.

Mackowsky (1939) provided unit-cell data and chemical analyses, reporting andradite with but 0.47 wt. % TiO₂ (Franklin) and 0.68 wt. % TiO₂ (Sterling Hill), but the correctness of the stated localities is uncertain.

Investigation of andradites intimately associated with zincite, by Lawson Bauer of pinkish material (#C6160), and by the writer of yellow-orange material (#R3468) found very little zinc (< 0.5 wt. % ZnO), as expected. Frondel (1970) found Franklin andradite to be a host for Sc in the orebody silicates. Wilkins and Sabine (1973) reported H₂O values of 0.33 and 0.67 wt. % in Franklin material, and Titus (1986) provided data for a number of local andradites. The number of analyses cited above and others, however, do not permit much in the way of general conclusions about the geochemistry or fine-detail of andradite at Franklin. Much more work remains to be done, particularly in petrography, trace-element, partitioning, and REE studies, among others. Several analyses are given in Table 2.

Occurrence and paragenesis




	Figure 15-32. Dodecahedral andradite crystals, with trapezohedral modifications, from Franklin. Field of view is 0.1 mm in maximum dimension.

The occurrences of Franklin andradite are extremely varied. This compilation is incomplete; only a very few occurrences are listed here.

The bulk of Franklin andradite occurs in the calcium-silicate units, in association with numerous species, most commonly with rhodonite (Figure 17-24), calcite, microcline (Figure 19-5), mica, and willemite. Andradite and rhodonite were commonly found bordering the pegmatites at Franklin, sometimes in large quantity; early reports suggest they were formed by direct reaction with the pegmatites and report much replacement of willemite by andradite (Ries and Bowen, 1922). Massive andradite occurs as fine-grained coronal structures surrounding franklinite or magnetite in the recrystallized calcium silicate units at Franklin. In this role it provides a ready host for Fe³⁺; in most cases this reaction precludes further iron reactions with silicate solutions, thereby possibly limiting the variety of Fe-bearing silicates at Franklin.




	Figure 15-33. Dodecahedral crystals of andradite from Franklin. Field of view is 0.6 mm in maximum dimension.

Fine crystals are found; they are brown, rarely black, and commonly in association with calcite. Unfortunately, many crystal specimens have no associated minerals, and the geologic context of such specimens is unclear. Many such crystal specimens have been preserved in systematic collections. Some faked specimens are known (Dunn, 1981a); they have a peculiar uniformity in having an atypical falsified “matrix” containing green shards of other minerals and much plaster! Superb microcrystals (Figures 15-32 and 15-33) are found lining vugs and seams and are commonly among the first minerals formed in such assemblages.

Green andradite, although relatively uncommon, occurs in fine crystals associated with magnetite, apatite, hematite, calcite, and mica; a different green-andradite assemblage is the matrix for jarosewichite (Dunn, 1982b). Reddish-brown andradite, wholly unlike that found in the Franklin orebody, is found with hedenbergite, magnetite, epidote, and sulfides at Balls Hill, specifically at the Gooseberry Mine, but also at many of the nearby magnetite mines.

Sterling Hill andradite is, in general, a bit less common in fine specimens. The preponderance of studied, superb specimens of garnet from here are spessartine; some minor grossular is known. Johnson, (1990), however, reported Sterling Hill garnets in the grossular-andradite series. The Sterling Hill garnet crystals referred to by Palache (1935) are here shown to be spessartine. Metsger et al. (1958) made reference to andradite, but provided no data. In the absence of careful detailed published studies, the extant data suggest that spessartine is a moderately common species at Sterling Hill, and andradite may be abundant in unstudied, common-grade material.

FOOTER LBI




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