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Ca3PbZn4(SiO4)4
Monoclinic, P21/n, a = 17.62, b = 8.270, c
= 30.52 Å, b = 90o,
Z = 12.
Esperite was originally described and named calcium larsenite by Palache et al. (1928a, 1928b), because it was assumed to be related to larsenite. Restudy by Moore and Ribbe (1965) established its independent status as a zincosilicate; they renamed it esperite. Esperite was synthesized by Ito (1968), and thermal decomposition data were given by Moore and Ribbe (1965). The crystal structure is unsolved.
Esperite is massive, forming fine-grained aggregates, some over 15 cm in diameter. The color is white to light yellow, but most specimens are of dingy appearance; the luster is variable, slightly greasy on fresh surfaces, but dull on exposed surfaces. Cleavage was not observed, but there is a conchoidal fracture, and some masses have a weak foliation, likely stress-induced. The density is 4.42 g/cm3. Optically, esperite is biaxial, negative, 2V = 40o, with a = 1.762, b = 1.770, and g = 1.774.
Optical anomalies reported by Palache (1935) have not been subsequently reinvestigated. Esperite fluoresces in ultraviolet, with a strong yellow-green color in shortwave; this is a diagnostic feature. The fluorescence is diminished at contacts with franklinite.
The duration of phosphorescence (33 hours) was noted by Millson and Millson (1950). Esperite has a pale greenish-white thermoluminescence, and its response is the most thermally unstable of all the minerals studied by Northup and Lee (1940). It attained maximum brilliance and then extinguished, at temperatures below those for any other studied mineral.
Esperite is a calcium lead zincosilicate mineral, related to beryllonite and trimerite. Ten microprobe analyses were given by Dunn (1985b); the average of these is given in Table 8. The known reliable analyses conform closely to Ca3PbZn4(SiO4)4, with some limited substitution of Mn and Mg for Zn, and Pb for Ca.
Esperite is a primary mineral and is moderately widespread at Franklin, but has not been found at Sterling Hill. It was originally found on the 400 foot level of the mine, approximately 1080 feet north of the north side of the Palmer Shaft pillar. It was subsequently found in other parts of the northern end of the orebody, but aside from one occurrence on the 1100 level with glaucochroite, specific locations were not noted.
The common occurrence is of somewhat nodular masses, up to 15 cm, associated with hardystonite, willemite, franklinite and, less commonly, calcite. Associated franklinite is rimmed with willemite in many specimens. Less common associated species are nondescript gray-to-brown glaucochroite and light pink hodgkinsonite, which commonly occurs in minor amounts at the feathery reaction zone where hardystonite is replaced by esperite. The replacement of hardystonite can be complete. An unnamed species occurs within this reaction zone as well (Dunn, 1985b). Ito (1968) noted a Ca-Zn-silicate mineral in this assemblage.
Esperite was named in honor of Professor Esper F. Larsen, Jr.
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| Copyright © 1995 by Pete J. Dunn |
Website
by Herb Yeates
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This
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