lazurite

n.
A relatively rare blue, violet-blue, or greenish-blue translucent mineral, Na₄-₅Al₃Si₃O₁₂S, the chief component of lapis lazuli.

[Medieval Latin lāzur, lapis lazuli (from Arabic lāzaward; see lapis lazuli) + -ITE¹.]

The chief mineral constituent in the ornamental stone lapis lazuli. Lazurite is a feldspathoid. It crystallizes in the isometric system, but well-formed crystals, usually dodecahedral, are rare. Most commonly, it is granular or in compact masses. The hardness is 5–5.5 on Mohs scale, and the specific gravity is 2.4–2.5. There is vitreous luster and the color is a deep azure, more rarely a greenish-blue. Lazurite is a tectosilicate, the composition of which is expressed by the formula Na₄Al₃Si₃O₁₂S.

Lapis lazuli is a mixture of lazurite with other silicates and calcite and usually contains disseminated pyrite. It has long been valued as an ornamental material. Localities of occurrence are in Afghanistan; Lake Baikal, Siberia; Chile; and San Bernardino County, California. See also Feldspathoid; Silicate minerals.

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(NaCa)
Cubic -- hexoctahedral

Environment

A mineral of metamorphosed limestones.

Crystal description

Dodecahedral crystals to 1 in. (2.5 cm) in size, but rare. Usually granular, massive, disseminated in limestone.

Physical properties

Blue, violet-blue, or greenish blue. Luster glassy; hardness 5-5Ɖ; specific gravity 2.4-2.5; streak sky blue; fracture uneven; cleavage poor dodecahedral. Brittle; translucent to opaque.

Composition

Sodium aluminum silicate, with sulfur and some admixture of related minerals such as sodalite (approximately 23.1% Na ₂ O, 30.7% Al ₂ O ₃ , 39.3% SiO ₂ , 8.4% S).

Tests

Retains color even after heating to incandescence. Fuses with difficulty.

Distinguishing characteristics

Almost invariably associated with pyrite and so distinguishable from the similar blue minerals lazulite and sodalite. On heating does not swell as lazulite does; usually is deeper in color, more opaque, and finer-grained than sodalite. Commonly associated with calcite, which dissolves with effervescence in the hydrochloric acid. Not associated with copper minerals; the hue is attributed to sulfur, not iron, cobalt or copper.

Occurrence

A rather rare mineral; significant amounts in the U.S. found only in Colorado, where it occurs as small grains in a dark rock. The best occurrence of fairly large rich masses and scattered crystals is in Afghanistan; the dull-surfaced crystals are embedded in white marble and range to 2 in. (5 cm) or more in diameter. Found in disseminated grains near Lake Baikal, Siberia, and high in the Andes at Ovalle, Chile.

Lazurite forms a decorative and jewelry stone known as lapis lazuli; it is the "sapphire" of the ancients. Selected crushed grains of lapis lazuli formed the "ultramarine" pigment of the old masters. A synthetic lazurite has now replaced it in paint.

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