New Mexico Mineral Symposium — Abstracts

"Onegite" is a now-little-used varietal term that refers to amethyst containing abundant, acicular inclusions of goethite. It was first used in 1802 by C. C. Andre (see Clark, 1993) to describe material from Wolf Island, Lake Onega, western Russia. The term was formerly used as a varietal name for goethite, and in fact, its use predates the name goethite itself, not coined until 1806 by J. G. Lenz (see Dana, 1892). "Onegite" can be considered to be a type of sagenitic quartz (Dake et al., 1938), though "sagenite" is also used in a more restrictive sense to refer only to rutilated quartz or reticulated, acicular rutile (Clark, 1993).

Mineral intergrowths fitting the description of onegite are one of the distinctive but somewhat uncommon mineral associations found in the miarolitic pegmatite cavities of the Pikes Peak batholith, Colorado. Onegite has been known from the Lake George intrusive center (Florissant-Lake George-Crystal Peak area) of the Pikes Peak batholith since the late 19th century and is still found by field collectors today. It is also reported from other areas in the batholith, including Wigwam Creek and Stove Mountain. The Pikes Peak batholith, a 1.06-1.09-billion-year-old anorogenic granite batholith covering some 3,000 km2 in central Colorado (see Hutchinson, 1976; Wobus and Hutchinson, 1988; Foord, 1995; Unruh et al., 1995), is known for its pegmatites, which commonly contain smoky quartz, microcline (amazonite), albite, fluorite, topaz, and numerous other minerals. The mineralogy of pegmatite "pockets" in the Pikes Peak Granite has been described by Odiorne (1978), Foord and Martin (1979), and Muntyan and Muntyan (1985). The batholith was intruded to a relatively shallow level in the crust, as evidenced by the common occurrence of what were initially fluid-filled pockets (now miarolitic cavities) in parts of the batholith; one phase of granite in the batholith (fayalite granite) has been estimated to have crystallized at about 700°C and at a pressure of about 1.5 kb, = 5 km depth (Barker and others, 1975; Wobus, 1986).

Goethite is a common accessory mineral in the pegmatite "pockets", occurring as late-stage growth of dark-brown to golden-brown, bladed to acicular crystalline masses, typically attached to and surrounding crystals of smoky quartz. Amethyst, however, is relatively uncommon and is locally observed as small (<1 cm, rarely to 2.5 cm) crystals, characteristically associated and sometimes heavily included with goethite blades and needles. Swindle (1982) described goethite and "onegite" occurrences in the pegmatites, as did Berry (1994) in an as yet unpublished manuscript. The color of the goethite-included quartz crystals ranges from brown to near black in densely included crystals, to nearly inclusion-free, medium-purple amethyst. Sector zoning of the amethyst color is common. Habit of the goethite-included amethyst ranges from short prismatic, often flattened, to pseudo-bipyramidal; some crystals are pagoda-shaped, skeletal, or in radiating, pineapple-shaped clusters attached to larger smoky quartz crystals. This represents essentially the only mode of occurrence of amethyst in rocks of the Pikes Peak batholith (see Michalski, 1984, for a description of other amethyst occurrences in Colorado). R. R. Berry provided specimens, which have been examined in the present study, from pegmatites associated with aplitic granite near the outer margin of the Lake George ring complex about 0.5 mi northeast of Lake George.

The amethyst appears to be restricted to pegmatite cavities in which there was a somewhat unusual, late stage of quartz growth, contemporaneous with and post-dating the goethite. A typically observed paragenetic sequence of minerals is (1) near-colorless or pale smoky quartz surrounded by white or tan microcline in massive or graphic-textured pegmatite; (2) euhedral smoky quartz, ±microcline (some amazonite), albite, fluorite, etc.; (3) goethite, alone or included in colorless to amethystine or smoky quartz; and (4) inclusion-free amethyst. Late overgrowths of colorless or milky quartz sometimes coat smoky quartz crystals, and in at least one case were observed to form a mold around a quartz-goethite crystal group (Berry, 1994). In most pegmatite cavities in the Pikes Peak Granite, the sequence only extends to (2) or (3) with goethite alone as the latest-stage mineral. Those cavities in which both goethite and quartz continued to grow, produced the "onegite" association. Amethyst is known to owe its color to ferric iron in the quartz; thus, the restricted occurrence to an intimate association with goethite is not surprising. Specifically, the purple amethyst color has been related to quartz in which the content of Fe³⁺ exceeds Fe²⁺, and in which exposure to ionizing radiation further oxidizes Fe³⁺ ions to Fe⁴⁺ (Cohen, 1985, 1989). Quartz with Fe³⁺>Fe²⁺ is reported to be characteristic of shallow-depth environments; hence, amethyst is normally found in mineral veins and cavities that formed at relatively shallow depth (Cohen, 1985).

Few studies have been made of the temperatures of crystallization of quartz and associated minerals in miarolitic-cavity-bearing pegmatites of the Pikes Peak type, so only general estimates can be made based on published studies of other granite and pegmatite systems. Of the mineral associations found in the miarolitic-cavity pegmatites, only stage (1) above probably represents true magmatic pegmatite (water-saturated silicate melt coexisting with supercritical aqueous fluid), formed at temperatures on the order of 650-600°C. Stage (2), euhedral smoky quartz associated with amazonite and other well-crystallized "pocket" minerals, almost certainly formed in the hydrothermal regime, at temperatures from perhaps 550° downward to a few hundred ( 200?)°C. Goethite and the latest-stage amethystine quartz most likely formed at quite low temperatures, perhaps as low as 200-100°C(?). Fluid inclusion geothermometry studies of Pikes Peak pegmatite minerals would be a stimulating avenue of research and should produce some interesting results!

References:

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2. Berry, R. R., 1994, Goethite inclusions in quartz from the Pikes Peak Granite, unpublished manuscript, 8 p.
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