Abstract
Seventeen new analyses of lepidolites are presented. These analyses seem to be in agreement with the structural requirements of micas, as revealed by the x-ray studies of other investigators. The analyses indicate that the rare alkalies, rubidium and cesium, are normal constituents of lepidolites.
The interpretation of a new analysis of polylithionite, an end member of the lepidolite series, shows it to approximate closely the formula [K · Li2Al · Si4O10F2. Columbium and titanium seem to be normal constituents of lepidolites near polylithionite in composition.
The composition of the seventeen lepidolites closely approximates that of isomorphous mixtures of polylithionite [K · Li2Al · Si4O10F2] with biotite [K · R″3 · AlSi3O10(OH, F)2], lithium muscovite [K4 · Li6Al6·Al4Si12O40(OH, F)3], and muscovite [K · Al2 · AlSi3O10(OH, F)2], The selection of biotite as an end member is arbitrary and other end members may be taken to account for the bivalent ions present.
Evidence is presented indicating discontinuous isomorphism. Theoretical consideration of this evidence suggests the following formulas for lepidolites: ideal formula A, K · Li2Al · Si4O10F2, the formula of polylithionite; ideal formula B, K4 · Li6Al5 · AlSi15O40(F, OH)8; ideal formula C, K4 · Li7Al5 · Al2Si14O40(F, OH)8; ideal formula D, K4Li5Al6 · Al3Si13O40(OH, F)8; and ideal formula E, K4 · Li6Al6 · Al4Si14O40(OH, F)8. The analyses may be divided into the types, A, B, C, D, and E with limits of composition expressed by the corresponding ideal formulas. One of the samples analyzed is close to polylithionite (type A) in composition; eight samples are of type C, and five of the samples are of type D. Three samples do not correspond to any of the above types because of their bivalent ion content.
