New Mexico Mineral Symposium — Abstracts

Red-tinted, lithium-bearing micas are prominent and readily visible mineral components of pegmatites in some of the pegmatite districts in Precambrian rocks of New Mexico and Colorado. Although pink or red colors are often characteristic of lithium-bearing micas, the presence of the lithium ion itself (Li⁺) has no direct effect on the color, which is due to manganese as the weakly colored Mn2+ ion or the more intensely colored Mn³⁺. Aside from pegmatites, certain manganese-rich metamorphosed sedimentary rocks also contain pink- or violet-colored micas.

The following table of lithium-bearing and related mica species shows the ideal chemical formulas written to distinguish the octahedrally coordinated ions (Li⁺,Mg⁺²,Mn⁺², Fe²⁺, Al⁺³) from
the tetrahedrally coordinated ions (Al³⁺, Si⁴⁺.). Muscovite, lepidolite, biotite, and zinnwaldite occur in granitic pegmatites and in granites (zinnwaldite does not appear to have been report¬ed from New Mexico). In contrast, the higher-lithium mica, polylithionite, is found in alkaline igneous rocks (such as the nepheline syenite of Mont St-Hilaire, Quebec, and the phonolite at Point of Rocks, Colfax County, New Mexico). Taeniolite (again not known from New Mexico) is likewise a mineral occurring in alkali syenites rather than granitic pegmatites. The rare, manganese-rich lithium mica, masutomilite, is known from only four localities worldwide (two in Japan, associated with topaz, schorl, and in one case, cassiterite, plus Czechoslovakia and Idaho); no micas from New Mexico pegmatites are rich enough in manganese to be classed as masutomilite. The lithium-bearing micas commonly show high-fluorine contents, in contrast to the low-fluorine contents of common muscovite and biotite in most igneous and metamorphic rocks.

Species name Ideal Formula wt% Red-tinted, lithium-bearing micas are prominent and readily visible mineral components of pegmatites in some of the pegmatite districts in Precambrian rocks of New Mexico and Colorado. Although pink or red colors are often characteristic of lithium-bearing micas, the presence of the lithium ion itself (Li+) has no direct effect on the color, which is due to manganese as the weakly colored Mn2+ ion or the more intensely colored Mn3+. Aside from pegmatites, certain manganese-rich metamorphosed sedimentary rocks also contain pink- or violet-colored micas.
The following table of lithium-bearing and related mica species shows the ideal chemical formulas written to distinguish the octahedrally coordinated ions (Li⁺,Mg⁺²,Mn⁺², Fe²⁺, Al⁺³) from
the tetrahedrally coordinated ions (Al³⁺, Si⁴⁺.). Muscovite, lepidolite, biotite, and zinnwaldite occur in granitic pegmatites and in granites (zinnwaldite does not appear to have been report¬ed from New Mexico). In contrast, the higher-lithium mica, polylithionite, is found in alkaline igneous rocks (such as the nepheline syenite of Mont St-Hilaire, Quebec, and the phonolite at Point of Rocks, Colfax County, New Mexico). Taeniolite (again not known from New Mexico) is likewise a mineral occurring in alkali syenites rather than granitic pegmatites. The rare, manganese-rich lithium mica, masutomilite, is known from only four localities worldwide (two in Japan, associated with topaz, schorl, and in one case, cassiterite, plus Czechoslovakia and Idaho); no micas from New Mexico pegmatites are rich enough in manganese to be classed as masutomilite. The lithium-bearing micas commonly show high-fluorine contents, in contrast to the low-fluorine contents of common muscovite and biotite in most igneous and metamorphic rocks.

In New Mexico, the Harding pegmatite near Dixon in Taos County and the Pidlite pegmatite in the Rociada district, Mora County are the outstanding examples of deposits that contain a variety of colored Li-bearing micas, ranging from reddish pink to lilac and violet. At the Harding pegmatite mine, true lepidolite ranges from lilac to wine red and purple and typically contains approximately 3.5-4.5 wt% Li₂O and approximately 1 wt% MnO. In contrast, lithian muscovite ranges from rose to lilac to gray and pale green and may contain from about 0.2 to several weight percent Li₂O, and typically only a few tenths of a percent MnO. Some reddish-purple, relatively hard, massive mineral material is often mistakenly assumed by visitors to the mine to be lepido¬lite, but it is actually microcline feldspar showing an incipient alteration to lepidolite; the bulk of the material is still feldspar, as evidenced by its cleavage and hardness. Lithium-bearing micas in pegmatites are not usually of primary magmatic origin, and hence are not found in "simple" pegmatites, but occur in zones that were subject to hydrothermal alteration during crystallization and cooling in "complex" zoned pegmatites. The hydrothermal, postmagmatic formation of Li-micas is evidence of the increasing concentration and chemical activity of Li, F, Mn, and other chemical components in the evolving hydrothermal fluids.

Other occurrences of reddish-colored micas in New Mexico include sparse reported occurrences of rose muscovite in the Petaca pegmatite district, Rio Arriba County. Pale-violet to lilac-colored muscovite near Pilar, Taos County, is associated with piemontite or with piemontite + thulite + vesuvianite + grossular in manganese-rich layers in schist of the Picuris Range. This metamorphic mica contains approximately 0.2-0.8 wt% MnO and little or no lithium.

The most prominent occurrence of lepidolite in Colorado is the Brown Derby pegmatite in Gunnison County. However, lepidolite is also known from the Meyers pegmatite near Royal Gorge, Fremont County; the Chief Lithia pegmatite near Texas Creek, also in Fremont County; the Bald Mountain pegmatite east of Mount Evans in Clear Creek County; and the Kings Kanyon pegmatite in the Crystal Mountain district, Larimer County. There are a few other minor or unverified reports of lepidolite in Colorado. In addition to the "pink" micas referred to in the title of this paper, the brown, lithium- and iron-bearing mica, zinnwaldite, is common in the miarolitic pegmatites of the Pikes Peak batholith. Zinnwaldite resembles biotite, but is a lighter, reddish-brown color, especially in the interior of color-zoned crystals.