MARGARITE

CaAl₂(Al₂Si₂)O₁₀(OH)₂ 
Monoclinic, C2/c, a = 5.10, b = 8.84, c = 19.05 Å,
b = 100^o, Z = 4

	Figure 18-6. Pseudo-hexagonal crystals of margarite in parallel growth from Sterling Hill. Crystals appear slightly etched, perhaps naturally, but possibly from dissolution by acids used in mineral separations. Field of view is 0.9 mm in maximum dimension.

Margarite was described by Cook (1973) and Dunn and Frondel (1990) from an occurrence in the marble very near the Sterling Hill orebody, but was known locally before then; some specimens were mislabeled as muscovite, mariposite, or kyanite. An early report in the local journal, The Picking Table, referred to a fuchsite from Franklin; this may have been margarite as well. Another occurrence of margarite was investigated by Yau et al. (1984).

Description

Margarite from the principal occurrences given herein occurs in crystals up to 3 cm; they are pseudohexagonal in habit (Figure 18-6). Sterling Hill margarite is light bluish-green to light greenish-blue; colorless material is known and is white in the aggregate. It has a pearly luster, which is sometimes distinctive; perfect {001} cleavage; and a density of 3.03 g/cm³; physically deformed material is common. Optically, it is biaxial, negative, with moderate 2V, a = 1.625, b = 1.640, and g = 1.641; the orientation is Z = b. The fluorescence is pale yellow in both shortwave and longwave ultraviolet. It is distinguished from muscovite by its brittleness.

Composition

Margarite is a calcium aluminum silicate hydroxide mineral of the brittle mica group. Analyses of local material were given by Dunn and Frondel (1990) and Yau et al. (1984) and are presented in Table 13.

Occurrence and paragenesis

Although margarite is not known to be common locally, colorless masses may have been overlooked. Margarite was found in the Franklin Marble, immediately adjacent to the Sterling Hill orebody, and specimens were brought out through Sterling Hill mining adits. Hence, specimens are commonly labeled “Sterling Hill,” this implying an incorrect orebody occurrence. The occurrence was approximately 12 meters from the orebody, on the 340 and 430 levels, and was locally abundant. This bluish-green margarite is in a bright-white calcite matrix, associated with 1-2 cm crystals of red corundum. The combination is very esthetic, and this fact may have ensured the preservation of much material. A host of other minerals are present, including but not limited to anorthite, gahnite, hercynite, arsenopyrite, graphite, titanite, phlogopite, clinochore, rutile, zircon, and very rare minerals such as goldmanite and thortveitite (Dunn and Frondel, 1990).

Another notable occurrence was found about a kilometer distant in the Franklin Quarry in Franklin, in 1982, and was studied by Yau et al. (1984). The assemblage is simpler, having no corundum and none of the rarest minerals. Here, margarite is associated with calcite, dolomite, titanite, rutile, muscovite, phlogopite, and clinochlore. This is interpreted as a retrograde assemblage; the parent rock is assumed to have been composed of feldspars and/or scapolites. The data indicate an assemblage in which phlogopite is replaced by chlorite at 370-450^o C at an assumed pressure of 2 kilobars. 

A specimen from Franklin in the Smithsonian Institution consists of massive, white-to-colorless margarite enclosing 1-2 cm uvite crystals. Additional occurrences have been found in the Franklin Marble between the Franklin and Sterling Hill deposits.

 

Copyright © 1995 by Pete J. Dunn			Website by Herb Yeates
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