Zinnwaldite

zinnwaldite [′tsin‚väl‚dīt]

(mineralogy)

K₂(Li,Fe,Al)₆(Si,Al)₈O₂₀(OH,F)₄A pale-violet, yellowish, brown, or dark-gray mica mineral; an iron-bearing variety of lepidolite; the characteristic mica of greisens.

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Zinnwaldite 

(after the deposit at Zinnwald, now Cínovec, Czechoslovak Socialist Republic), a mineral; a phyllosilicate belonging to the mica group. Its chemical composition is KLiFe²⁺ Al [AlSi₃O₁₀](OH, F)₂. Zinnwaldite is an intermediate member of the biotite-lepidolite isomorphic series. Typical impurities are Rb₂O (up to 1 percent), Cs₂O (up to 0.2 percent), Ga, and Sc. The amount of fluorine ranges from 2.5 to 5.0 percent. Zinnwaldite crystallizes in the monoclinic system, forming small pseudohexagonal plates, blades, and tabular colloform crystals with clearly expressed zonal structure. Colors range from dark green to brown. The physical properties of zinnwaldite are similar to those of other micas. The density is 2,990 kg/m³.

Zinnwaldite is found in the cavities of granitic pegmatites and lithium fluoride-type tantalum-bearing granites, in which zinnwaldite develops after biotite and protolithionite. It is also found in tin-tungsten greisen and quartz-vein formations. It is associated with topaz. Where significant quantities of zinnwaldite occur as a by-product, they may serve as an ore of lithium and rubidium.