lepidolite

n.
A lilac or pink to gray mineral of the mica group, K(Li,Al)₃(Si,Al)₄O₁₀(F,OH)₂, used as lithium ore and in ceramic production.

[LEPIDO- + -LITE.]

A mineral of variable composition which is also known as lithium mica and lithionite, K₂(Li,Al)₅₋₆(Si₆₋₇, Al₂₋₁)O₂₀₋₂₁(F,OH)₃₋₄. Rubidium (Rb) and cesium (Cs) may replace potassium (K); small amounts of Mn, Mg, Fe(II), and Fe(III) normally are present; and the OH/F ratio varies considerably. Polithionite is a silicon- and lithium-rich, and thus aluminum-poor, variety of lepidolite.

Lepidolite usually forms small scales or fine-grained aggregates. Its colors, pink, lilac, and gray, are a function of the Mn/Fe ratio. Hardness is 2.5–4.0 on Mohs scale; specific gravity is 2.8–3.0. See also Mica; Silicate minerals.

Lepidolite is uncommon, occurring almost exclusively in structurally complex granitic pegmatites, commonly in replacement units. Common associates are quartz, cleavelandite, alkali beryl,and alkali tourmaline. It is a commercial source of lithium, commonly used directly in lithium glasses and other ceramic products.

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K
Monoclinic -- domatic

Environment

A mica mineral of the core area of lithium-bearing pegmatites.

Crystal description

Commonly occurs in medium- to fine-grained aggregates; well-developed crystals of sharp hexagonal outline less common. Sometimes in late growths bordering ordinary muscovite mica, but the cleavage flakes show a slight marginal break and are not quite continuous. Crystals 1-2 in. (2.5-5 cm), tapering down to a slender point and more prismatic than tabular, occur at some localities. Elsewhere often in curving convex hemispheres embedded in feldspar.

Physical properties

Lilac, graygreen, pale yellow. Luster pearly and vitreous; hardness directional variable 2Ɖ and 4; specific gravity 2.8-3.3; cleavage perfect basal (micaceous), making elastic plates. Micaceous; translucent to transparent; can be gorgeous amethystine hue through the side, reminiscent of kunzite spodumene.

Composition

Alkaline fluosilicate of lithium, potassium, and aluminum (about 5% Li ₂ O, 12% K ₂ O, 26% Al ₂ O ₃ , 51% SiO ₂ , 1.5% H ₂ O, and 4.5% F).

Tests

Fuses easily to a bubbly fluorescent glass (blue and pinkish fluorescence). Colors the flame red.

Distinguishing characteristics

Since lepidolite colors can be confusing, a melting and flame test is desirable if there is any reason (such as the presence of elbaite tourmalines or of other lithium minerals) to suspect a mica is lepidolite, rather than the more common muscovite. The fluorescence and flame tests will also distinguish it from the more intensely colored chromium chlorites, dumortierite, and similar hydrous silicates.

Occurrence

Once, before alkali lake brines proved better, an important ore of lithium. Relatively rare, since it is only found in the complex pegmatites that have a history of a long series of replacements by successive elements; always an associate with lithium minerals. Only found in regions where dikes of this type are exposed, as in New England, particularly in Maine, and at Portland, Connecticut, and San Diego Co., California.

Well-formed crystals up to an inch (2.5 cm) or more across are found at Auburn, Maine. Fine-grained aggregates are common in many Maine localities, and are associated with microlite at Dixon, New Mexico. The coarsest crystals form bladed aggregates at Ohio City, Colorado. Foreign localities include Afghanistan and Pakistan; Minas Gerais, Brazil; Madagascar; Varutrask, Sweden; Namibia; Western Australia (where there are sheets as much as 6 in. [15 cm] across in the Londonderry pegmatite); Russia; and Germany. The Alto Ligonha pegmatite in Mozambique had abundant notable lepidolite knobs to 12 in. high and 6 in. across (30 × 15 cm).

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Note: click on a word meaning below to see its connections and related words.

The noun has one meaning:

Meaning #1: a mineral of the mica group; an important source of lithium

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