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(Ca,Ce)2(Al,Fe3+)3(SiO4)3(OH)
Monoclinic
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| Figure 16-1. Crystal drawing of allanite from Franklin. Drawing is from Palache (1935) who provided crystallographic data. | ||
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Allanite-(Ce) was first noted at Franklin by Jackson (1850a) and later reported by Hunt (1861). Subsequent studies by Eakle (1894a, 1894b) provided much of the extant data, as reviewed by Palache (1935). It has not been reported from Sterling Hill.
Allanite-(Ce) occurs as euhedral crystals, up to 25 mm in length, and tabular or bladed (Figures 16-1 and 16-2). The color is invariably black; the cleavages are imperfect to good; the luster is vitreous; and the density is 3.84 g/cm3. The crystals are extremely brittle, and few survive attempts to extricate them from matrix. Optically, it is nearly opaque, brown in thin fragments, and strongly pleochroic, with a mean index of refraction of 1.74.
Allanite-(Ce), a calcium cerium aluminum iron silicate hydroxide mineral of the epidote group, has been little studied. The analysis of Hunt (1861) reported by Palache (1935) and the partial analysis given by Frondel (1964) both show cerium (Ce) to be the dominant REE element in Franklin material. Several semi-quantitative analyses by the writer confirm this. All of these data are for the large euhedral allanites described below.
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| Figure 16-2. Typical allanite crystal in microcline from Franklin. Field of view is 6.5 cm in maximum dimension. Smithsonian Institution, #R3831. Photo by the author. | ||
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Most Franklin allanite occurs as black, euhedral, bladed crystals embedded in green to whitish-green microcline; much of this material was found on the Trotter Dump, and inferior specimens are still abundant (Figure 16-2). The preponderance of the known specimens suggests a simple assemblage of microcline, allanite-(Ce), quartz, and minor andradite. The local abundance of this material indicates that the solutions from which allanite and microcline formed were relatively rich in Ce. There are few rare-earth minerals at these deposits; most of the rare-earth element concentration at Franklin is in allanite-(Ce).
Allanite, unstudied as to REE-dominance, also occurs in an altered assemblage reported by Dunn et al. (1984a) as altered 1-3 mm crystals with burn-haloes in the surrounding minerals and associated with wollastonite, microcline, minehillite, margarosanite, and other minerals.
Palache (1935) mentioned the occurrence of allanite on the dumps of the local nearby iron mines; this material has not been studied by the writer.
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| Copyright © 1995 by Pete J. Dunn |
Website
by Herb Yeates
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This
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