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PbZnSiO4
Orthorhombic, Pna21, a = 8.244, b = 18.963,
c = 5.06 Å, Z = 8.
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| Figure 15-62. Crystal drawings of two individual larsenite crystals from Franklin. Drawings are from Palache (1935) who provided crystallographic data. | ||
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Larsenite was originally described by Palache et al. (1928a, 1928b) from the Franklin Mine. The formula and symmetry had suggested a relation to the olivines, but this was disproven by Layman (1957), who provided accurate cell parameters. Larsenite was synthesized by Ito and Frondel (1967) and Ito (1968). It is among the rarest of the lead silicates from Franklin.
The crystal structure was described by Prewitt et al. (1966, 1967); their unit-cell is given above. The structure is composed of a network of corner-linked silicon tetrahedra and zinc tetrahedra and three-sided and distorted four-sided lead pyramids.
Larsenite occurs as colorless, lustrous, commonly acicular prismatic crystals up to several mm in length; the morphology was given in detail by Palache (1935) (Figures 15-62 and 15-63), and the hemimorphic habit was described by Yeates (1991).
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| Figure 15-63. Crystal drawings of two individual larsenite crystals from Franklin. Drawings are from Palache (1935) who provided crystallographic data. | ||
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Thin, platy crystals were observed in one anomalous association with ganomalite and barysilite.
Larsenite aggregates may be white; cleavage is good; the luster is vitreous to adamantine; and the density is 5.90 g/cm3. Optically, larsenite is biaxial, negative, 2V = 80o, with a = 1.92, b = 1.95, and g = 1.96; dispersion is perceptible, r > v. There is no discernible fluorescence in ultraviolet. Larsenite is distinguished easily from willemite by its crystal habit and lack of fluorescence; the association with altered esperite is helpful as well.
Larsenite is a zinc lead silicate mineral. The extant analyses (Dunn, 1985b) conform closely to the theoretical composition. A representative analysis is given in Table 8. There is little solid solution of other cations in larsenite.
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| Figure 15-64. Larsenite crystals from Franklin, exhibiting parallel growth. Field of view is 0.3 mm in maximum dimension. | Figure 15-65. Superb euhedral crystal of larsenite from Franklin. Field of view is 0.3 mm in maximum dimension. | |||
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| Figure 15-66. Stout, prismatic larsenite crystals in parallel growth from Franklin. Field of view is 1 mm in maximum dimension. | ||
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Larsenite was found in superb crystals (Figures 15-64, 15-65, and 15-66), but only in a few secondary vein occurrences in the Franklin mine. Although first found in 1928 on the picking table, the one documented occurrence was 20 feet above the 400 level, in top slice 1080 (Palache, 1935). Larsenite is restricted to vein occurrences and forms by direct alteration of primary esperite, which is associated with willemite, franklinite, hardystonite, and calcite. The secondary alteration minerals of this assemblage include larsenite, clinohedrite, hodgkinsonite, and rarely a light-blue-fluorescing mixture of willemite and hodgkinsonite. Ganomalite and barysilite were observed on but one specimen. Valid specimens are rare and prized.
Larsenite was named in honor of Professor Esper S. Larsen of Harvard University.
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| Copyright © 1995 by Pete J. Dunn |
Website
by Herb Yeates
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Link
to homepage
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This
page created: January 11, 2001 |
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