Esperite

Esperite
Esperite under ultraviolet light
General
Category	Silicate mineral
Formula (repeating unit)	PbCa3Zn4(SiO4)4
IMA symbol	Epr[1]
Strunz classification	9.AB.15
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	P21/m
Identification
Color	White
Crystal habit	Typically massive
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	[2][3][4]

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.^{[clarification needed]}

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.