THE MINERAL MIMETITE

Chemistry: Pb5(AsO4)3Cl, Lead Chloroarsenate
Class: Phosphates
Group: Apatite
Uses: As a minor ore of lead and mineral specimens.
Specimens

Mimetite shares the same structure with apatite and occasionally crystals of the two will have similar shapes. Mimetite also forms a chemical series with two other minerals; Pyromorphite (Pb5(PO4)3Cl) and Vanadinite (Pb5(VO4)3Cl). This series is a little different than most chemical series which involve substitution of cations such as calcium for magnesium. Instead, this series substitutes its basic chemical units the anion groups; Phosphate (PO4), Arsenate (AsO4) and Vanadate (VO4). Green Mimetite or yellow Pyromorphite can make identification between the two difficult, but usually pyromorphite is green and mimetite is yellow. Vanadinite is usually red. Fortunately for identification purposes, Mimetite does not form well shaped crystals often and especially of the green color. It usually is found as a botryoidal crust, a sparkling cauliflower aggregation or as minute spike-like crystals.

PHYSICAL CHARACTERISTICS:

Color is typically yellow, orange or brown, less commonly green, colorless or gray.
Luster is resinous to adamantine.
Transparency: Crystals are transparent to translucent.
Crystal System is hexagonal; 6/m
Crystal Habits include the barrel shaped hexagonal prism with the hexagonal pyramid and/or a pinacoid as a termination. more commonly found as classic botryoidal crusts, sparkling aggregations that resemble cauliflower, small attached spherical masses or as minute spike-like crystals.
Cleavage is rarely noticed.
Fracture is subconchoidal.
Hardness is 3.5 - 4.
Specific Gravity is approximately 7.1+ (very heavy for translucent minerals)
Streak is off white.
Associated Minerals are wulfenite, limonite, calcite, barite, galena and secondary lead deposit minerals.
Other Characteristics: Index of refraction is 2.13 (typically high for lead minerals) and crystal terminations can be hollowed out or pitted.
Notable Occurrences include Mapimi, Mexico; Arizona, USA and Tsumeb, Africa.
Best Field Indicators are crystal habit, high luster, color, and density.