Stibnite

Stibnite is antimony trisulfide and the most important ore of antimony. It crystallizes in the orthorhombic system, producing some of the mineral kingdom's most striking specimens: clusters and sprays of long, slender, prismatic to bladed crystals with a brilliant metallic, lead-gray to steel-gray luster.

The crystals are often vertically striated and bend or curve easily because the mineral is soft (hardness about 2) and the structure is built of weakly bonded chains. Surfaces frequently develop a colorful blue-black iridescent tarnish.

Spectacular stibnite from the Ichinokawa mine in Japan and from Romania set the standard for collectors, while Chinese deposits dominate world antimony supply.

Stibnite forms in low-temperature hydrothermal veins and hot-spring deposits, where antimony-bearing fluids precipitate the sulfide in open fractures, often together with quartz, cinnabar, realgar, orpiment, and gold.

It also occurs in replacement bodies in limestone and in some epithermal precious-metal systems. World-class crystal localities include Ichinokawa on Shikoku, Japan; Baia Sprie and Herja in Romania; and the giant Xikuangshan deposit in Hunan, China, which is the largest antimony deposit on Earth. Additional deposits are mined in Bolivia, Tajikistan, and the western United States.

Stibnite is easy to recognize from its long, slender, bladed silvery-gray crystals with strong vertical striations and a bright metallic luster, often showing an iridescent blue-black tarnish. It is soft (hardness 2, scratched by a fingernail or copper coin) and gives a lead-gray streak.

It is notably heavy and the crystals are flexible, bending rather than snapping. Look-alikes include galena (which is cubic, cleaves into cubes, and is much denser) and bismuthinite (very similar but distinguished by chemistry). A confirming clue is its low melting point — stibnite fuses readily in a match flame, unlike most sulfides.

Stibnite is the principal ore of antimony, a metal used chiefly in flame retardants, in lead-acid battery alloys, in bearing metals and solders, and in some semiconductors and pigments. Antimony has been used since antiquity; powdered stibnite served as the black eye cosmetic kohl in the ancient world.

Fine crystallized specimens command high prices among collectors and museums. The mineral has no jewelry use due to its softness and toxicity, and antimony compounds should be handled with care.