brookite

(brʊk'īt')
n.
A mineral form of titanium dioxide, TiO₂, having characteristic orthorhombic crystals and a red-brown to black color.

[After Henry James Brooke (1771-1857), British mineralogist.]

TiO
Orthorhombic -- Rhombic bipyramidal

Environment

Silica-bearing veins deposited by hot solutions.

Crystal description

Always crystallized; usually thinly tabular parallel to a side pinacoid and then elongated and striated vertically. More equidimensional at Magnet Cove, Arkansas.

Physical properties

Red-brown to black. Luster adamantine to submetallic; hardness 5Ɖ-6; specific gravity 3.9-4.1; streak white to gray or yellowish; fracture subconchoidal to uneven; cleavage poor prismatic and basal. Brittle; translucent to opaque.

Composition

Titanium oxide (60.0% Ti, 40.0% O).

Tests

Same as for rutile (p. 151).

Distinguishing characteristics

The crystals--brown elongated flat plates often variegated in color with black corners--are very typical in their association with quartz in veins. There are not many minerals with which it can be confused, and none of those will give a titanium test.

Occurrence

Brookite is another of the titanium oxide group (with rutile and anatase) forming under special conditions at relatively low temperatures. Found also as detrital grains in sandy sediments--grains reported apparently to have grown larger after their deposition in sand, presumably fed by cool solutions percolating through the rocks.

The Swiss occurrences are among the best, yielding very thin crystals, almost an inch (better than 2 cm) long and clearly showing the red-brown color. A well-known English occurrence is the long-exhausted quartz vein with embedded typical brookite plates at Tremadoc, Wales.

It is not uncommon in similar environments in the U.S., but the outstanding American occurrence is in atypical crystals in the quartzite at Magnet Cove, Arkansas. Abundant, black, more or less equidimensional crystals--1 in. (2 cm) or so across at their best--dot a corroded, rusty quartz at this locality. The more typical thin plates have been found with the quartz at Ellenville, New York, associated with chalcopyrite. Brookite is likely to be encountered in any anatase occurrence, as in Somerville, Massachusetts. Small crystals were in the "sand" in the bottom of an amethyst pocket near Butte, Montana.

---

This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)