Crocoite: The Toxic Beauty

Crocoite: The Beautiful, Toxic Mineral

Crocoite is a lead chromate mineral with a brilliant orange-red color. Crocoite is a highly toxic mineral and can be harmful if ingested or inhaled. It is also a fire hazard and can ignite spontaneously when exposed to heat. It is a rare mineral that is typically found in hydrothermal veins and oxidized lead deposits.

Crocoite provides a nice color to the mineral kingdom and is well known for its distinctive orange-red color. Its main source of quality specimens comes from the Dundas District of Tasmania, Australia.

Crocoite is a rare lead chromate, known in the oxidation zone of lead deposits, especially those located near ultra-basic chromium-rich rocks.

It was discovered at the Berezovskoe Au Deposit (Berezovsk Mines) near Ekaterinburg in the Urals in 1766; and named crocoise by F. S. Beudant in 1832, from the Greek κρόκος (krokos), saffron, in allusion to its color, a name first altered to crocoisite and afterwards to crocoite.

Crocoite from the Dundas extended mine, Dundas, Tasmania, Australia
Photo: Henk Smeets/Tomeik Minerals.

The relative rarity of crocoite is connected with the specific conditions required for its formation: an oxidation zone of lead ore bed and presence of ultramafic rocks serving as the source of chromium (in chromite). Oxidation of Cr³⁺ into CrO₄²⁻ (from chromite) and decomposition of galena (or other primary lead minerals) are required for crocoite formation. These conditions are relatively unusual.

 

As crocoite is composed of lead(II) chromate, it is toxic, containing both lead and hexavalent chromium.

Properties of Crocoite:

Chemical Formula: PbCrO₄
Color is bright orange-red to yellowish-red.
Luster is adamantine to greasy.
Transparency: Crystals are translucent to transparent.
Crystal System is monoclinic; 2/m
Crystal Habits include prismatic crystals somewhat square in cross-sections, often striated lengthwise. Terminations are usually poorly developed and appear uneven or pitted. Also, granular and columnar.
Cleavage is distinct in two directions lengthwise, prismatic.
Fracture is conchoidal and uneven.
Hardness is 2.5 - 3.
Specific Gravity is approximately 6.0+ (very heavy for translucent minerals)
Streak is orange-yellow.
Other Characteristics: Index of refraction is a very high 2.36
Type Material: Natural History Museum, Paris, France.
Associated Minerals are wulfenite, pyromorphite, cerussite, limonite and vanadanite.
Notable Occurrences include Dundas District, Tasmania, Australia; Ural Mountains, Russia and Inyo and Riverside Counties in California, USA.
Best Field Indicators are crystal habit, locality, density, high luster and color.

Crocoite  crystals from the Red Lead Mine, Dundas, Tasmania, Australia
Photo: Eric Hunt/Flickr

Crocoite  crystals from the Red Lead Mine, Dundas, Tasmania, Australia
Photo: Ingo Lange