Andradite



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."
Investigation of andradites intimately associated with zincite, by Lawson Bauer of pinkish material, and by [Dunn] of yellow-orange material 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.
Andradite is the most abundant garnet at Franklin, and it is the major host for ferric iron in the silicate assemblages. 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.
Franklin andradite occurs in fine crystals up to 15 cm in size, predominantly dodecahedral, with some trapezohedral modifications. 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. The index of refraction is approximately 1.87 for near-end-member material. There is no discernible fluorescence in ultraviolet.
The occurrences of Franklin andradite are extremely varied. The bulk of Franklin andradite occurs in the calcium-silicate units, in association with numerous species, most commonly with rhodonite, calcite, microcline, 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.
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. Superb microcrystals 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. (Dunn, 1995)


 Location Found: Franklin and Ogdensburg
     
 
 Year Discovered: 1868
     
 Formula: Ca3Fe23+(SiO4)3
 Essential Elements: Calcium, Iron, Oxygen, Silicon
 All Elements in Formula: Calcium, Iron, Oxygen, Silicon
     
 IMA Status: Valid - first described prior to 1959 (pre-IMA) - "Grandfathered"
     
     
 To find out more about this mineral at minDat's website, follow this link   Andradite

     
 References:
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.357

Frondel, Clifford (1972). The minerals of Franklin and Sterling Hill, a checklist. NY.: John Willey & Sons. p.41


The Picking Table References
 PT Issue and PageDescription / Comment
View IssueV. 30, No. 1 - Spring 1989, pg. 8The Epidote-Pyroxene-Fluorapophyllite Assemblage in the Franklin Mine at Franklin, New Jersey, Philip P. Betancourt, Andradite (small description)
     
Images

     
Andradite, franklinite, grape willemite and calcite, from the Buckwheat Dump, Franklin NJ.Andradite, franklinite, grape willemite and calcite, from the Buckwheat Dump, Franklin NJ. under shortwave UV Light
Andradite (yellow-brown to gemmy brown), franklinite (black), grape willemite (red-violet) and calcite (white), from the Buckwheat Dump, Franklin NJ. Photo by WP.
Andradite, franklinite, grape willemite and calcite, from the Buckwheat Dump, Franklin NJ under shortwave UV light. The calcite fluoresces red-orange and the willemite green, andradite and franklinite are non-fluorescent. Photo by WP.


Andradite garnet crystals, calcite and franklinite from Franklin, NJ
Andradite garnet crystals (reddish brown to brown), calcite (gray-blue) and franklinite (black) from Franklin, NJ. From the collection of, and photo by Robert A. Boymistruk.


Garnet crystals from Ball's Hill, Franklin, NJ.
Garnet crystals from Ball's Hill, Franklin, NJ. 2" x 1 1/8". From the collection of, and photo by Robert A. Boymistruk.







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