Esperite

Esperite
240px Esperite under ultraviolet light
General
Category	Silicate mineral
Formula (repeating unit)	PbCa3Zn4(SiO4)4
Identification
Color	White
Crystal habit	Typically massive
Crystal system	Monoclinic - Prismatic (2/m)
Cleavage	Distinct on [010] and [100] - poor on [101]
Fracture	Conchoidal, brittle
Mohs scale hardness	5 - 5.5
Luster	Vitreous
Streak	White
Diaphaneity	Subtranslucent to opaque
Specific gravity	4.28 - 4.42
Optical properties	Biaxial (-), 2V measured: 5° to 40°
Refractive index	nα = 1.762 nβ = 1.770 nγ = 1.774
Birefringence	Max δ = 0.012
Other characteristics	Brilliant yellow fluorescence under SW UV; kelly green cathodoluminescence.
References

Esperite is a rare complex calcium lead zinc silicate (PbCa₃Zn₄(SiO₄)₄) related to beryllonite and trimerite that used to be called calcium larsenite. It was named in honor of Esper F. Larsen Jr. (1879–1961), petrologist of Harvard University.

Esperite has a white, greasy appearance in daylight and is much prized for its brilliant yellow green fluorescence under shortwave ultraviolet light. It is found in association with calcite, franklinite, willemite, hardystonite and clinohedrite. It has also been found as prismatic crystals up to 1 mm in length at the El Dragon Mine, Potosi, Bolivia in association with allophane, chalcomenite, clinochalcomenite and barite.

References

• Mineral galleries
• Pete Dunn, Franklin and Sterling Hill, New Jersey: the world's most magnificent mineral deposits, part 3 p. 368 (1995)
• G. Grundmann, et al.: The El Dragon Mine, Potosi Bolivia, Mineralogical Record v.21 #2 p. 142 (1990)
• Anthony et al., Handbook of Mineralogy, Vol. 2 (silicates) part 1 p. 225 (1995)

Robbins, Manuel: Fluorescence Gems and Minerals under Ultraviolet Light Geoscience Press pp 50–51,243 (1994)