New Mexico Mineral Symposium — Abstracts

The Picuris Range of the Sangre de Cristo Mountains has long been known for producing bright red piemontite crystals. Piemontite is a major constituent of the upper portion of the Vadito Formation, a Precambrian muscovite-quartz schist and quartzite that is exposed extensively on the west slopes of the range near Pilar, New Mexico. The piemontite in stratified layers within the quartzite occurs as individual crystals in quartz stringers and as eyes in the quartzite. While the quartzite in this area is generally compact, separation along seams in the layers has allowed, in some cases, the growth of free-standing crystals.

One such seam that is exposed on the west slope of the Picuris Range approximately 300 ft above NM-68 and roughly one-half mi south of Pilar has revealed the occurrence of cyprine, the rare blue variety of idocrase, in direct association with free-standing crystals of zoisite (thulite), piemontite, and grossular. The seam is restricted laterally to approximately 3 m and is from 1 cm to 5 cm thick. This seam is composed of disintegrated rock that is predominantly zoisite (thulite), piemontite, and grossular in the core whereas the idocrase is restricted to the surface. Most of the idocrase is opaque, but a small percentage of the crystals are transparent and sapphire blue. Crystals range in size from 2 to 8 mm and are heavily striated. Many crystals are zoned with the blue color abruptly changing to gray. Microprobe analyses reveal almost identical chemical compositions in the blue and the gray zones. There are only subtle variations in the minor-element contents; the blue zone is richer in Cu and Ti whereas the gray is richer in Mn and Fe. Weathering along the seam has destroyed the integrity of most crystals so that freestanding, terminated crystals on matrix are scarce. Minor seams located stratigraphically above and below the described seam show minor evidence of idocrase, but the thinness of the seams has precluded the growth of free-standing crystals.

In addition to the occurrence of attractive specimens of cyprine, thulite, and piemontite, several other minerals of interest can be found as float in the arroyos and on the talus slopes. Dravite has been identified as the brown tourmaline that is found quite commonly in the schistose rocks of the Vadito Formation. Andalusite and its green Mn-rich variety viridine are also common. The other aluminosilicate polymorphs, kyanite and sillimanite, occur, although not in noteworthy specimens. Finally, a muscovite of unusual appearance has been found in a canyon directly south of the idocrase area. It is found in a seam similar to that in which the idocrase occurs although the maximum seam width is only 4-5 mm. The muscovite at this location occurs in distinct but somewhat rounded crystal aggregates about 1 mm in size with a distinctive lilac to purple color. They are found in association with free-standing crystals of piemontite and, to a minor extent, grossular.

We feel that this assemblage of unusual minerals and mineral varieties in such a restricted area is noteworthy but not unique. We expect that diligent searching of the appropriate stratigraphic horizons will reveal other occurrences of these minerals in the metamorphic rocks of northern New Mexico.