Mimetite Composition,
Crystallization & Structure
Composition. Pb4(PbCl)(AsO4)3 = Arsenic pentoxide 23.2, lead protoxide 74.9, chlorine 2.4. Phosphorus replaces the arsenic in part and calcium, the lead. Endlichite is a variety intermediate between mimetite and vanadite.
Crystallization. Hexagonal, tri-pyramidal. Crystals prismatic, showing basal plane and at times pyramids. Usually in rounded barrel-to globular-shaped forms.
Structure. In roundel crystals, mammillary crusts.
Physical Properties. H. = 3.5. G. = 7-7.2. Resinous luster. Colorless, yellow, orange, brown. Subtransparent to almost opaque.
Test. Easily fusible (1.5). Gives globule of lead when fused with sodium carbonate on charcoal. A fragment placed in C. T. and heated in contact with a splinter of char coal gives deposit of charcoal gives deposit of metallic arsenic on walls of tube.
Occurrence. A comparatively rare minerals of secondary origin, occurring in the upper, oxidized portion of lead veins. Notable localities for its occurrence are in Cornwall and Cumberland, England; Johanngeorgenstadt, Saxone; Nerchinsk, Siberia; Phoenixville, Pennsylvania; Eureka, Utah, etc.
Name. Derived from the Greek for imitator in allusion to its resemblance to pyromorphite.
Use. A minor ore of lead.
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