samarskite

n.
A velvet-black mineral that is a complex mixture of several rare-earth metals with niobium and tantalum oxide.

[After Col. M. von Samarski, 19th-century Russian mining official.]

(Y,Er,Ce,U, Ca,Fe,Pb,Th)(Nb,Ta,Ti,Sn)
Orthorhombic -- Rhombic bipyramidal

Environment

Tight pegmatite dikes with high concentrations of rare-earth elements, with smoky quartz, biotite, and redstained feldspars (no aquamarine, topaz, or elbaite).

Crystal description

Crystals usually embedded in rock and consequently difficult to see. Best obtained when weathered out. Usually prismatic, in quartz or feldspar, showing a rectangular cross section an inch or more (2-3 cm) across. Also massive and partly shattered.

Physical properties

Velvety black (on a fresh break). Luster vitreous to resinous; hardness 5-6; specific gravity 4.1-6.2; streak reddish brown to black; fracture conchoidal; cleavage 1. Brittle; thin edges translucent.

Composition

An extremely complex mixture of rare-earth elements with niobium and tantalum oxide. The last two account for about half of the weight.

Tests

In closed tube it rapidly crumbles to black powder. Splinter edges usually split away, but if preheated in closed tube they will fuse to a black glass on charcoal in the blowpipe flame. Makes a fluorescent bead with sodium fluoride.

Distinguishing characteristics

Rather difficult to tell from related and associated species, but the fracture, color, and gravity mark it as one of the rare-earth minerals. More specific identification requires tests not practical for collectors who do not have access to a mineralogy laboratory.

Occurrence

Like its rare-earth mineral associates, samarskite is exclusively a mineral of pegmatites, usually forming tightly held, roughly crystallized shapes with no free-growing faces. At the few localities where it is abundant, it has some economic value as a source of the rare-earth elements contained. Crumbles with weathering, so ordinarily not found in alluvial deposits.

Originally found in the Urals in a pegmatite rich in rare-earth elements; later it also turned up in Norway and Sweden. The most abundant specimens have come from a deeply weathered pegmatite very rich in rare-earth minerals at Divino de Ubá, Minas Gerais, Brazil, where it formed parallel growths of columbite associated with monazite and euxenite (another black lustrous mineral of about the same composition). Surfaces of these crystals are coated with a yellow-brown oxidation film. Samarskite is not common in the U.S. It is found in Mitchell Co., North Carolina, in large, poorly formed crystals, and in small quantities in Maine, Connecticut, and Colorado.

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The noun has one meaning:

Meaning #1: a complex black mineral occuring in pegmatites

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