Micro-World Gems & Gemology, Fall 2016, Vol. 52, No. 3

Quarterly Crystal: Quartz with Axinite

Quartz with ferroaxinite inclusions, from Calaveras County, California
Figure 1. Two examples of quartz with inclusions of ferroaxinite from the New Melones Dam in Calaveras County, California. The larger specimen measures 65.28 mm tall. Photo by Kevin Schumacher.

In their September-October 1982 Mineralogical Record article, D. Pohl et al. described a discovery of ferroaxinite from New Melones Dam in Calaveras County, California. There was a very brief mention of ferroaxinite inclusions in quartz, but no illustrations of the inclusions were provided. Recent exploration of the Calaveras County locality by one of the authors (JM) led to the discovery of a large pocket that produced some beautiful axinite and quartz specimens (figure 1) containing bladed inclusions of ferroaxinite.
This new mineralized pocket was situated on the edge of a ledge and had a different geological appearance than the surrounding area. Successive layers within the pocket, 6–12 cm in thickness, were composed of metagabbro and crushed coarse rock crystal quartz that contained well-developed single quartz crystals measuring up to 9 cm in length and 6 cm in diameter. These crystals displayed both long and short columnar habits. Also found within them were crushed coarse crystalline axinite and mats of small epidote needles. Cracks in the crystals allowed quartz-forming fluids to seep through and crystallize on the surface of the metagabbro, often surrounding both the epidote matting and a mixed layer consisting of chlorite/axinite/epidote.
The large pocket, measuring approximately 7 feet deep, 4 feet wide, and 6 feet long, produced almost 2,000 pounds of quartz. The specimens ranged from translucent to extremely clear and varied in size from 1 to 18 cm. Some of them contained axinite or chlorite phantoms as well as full and partial inclusions of bladed axinite floater crystals. This huge pocket also produced several spectacular plates of axinite, with individual crystals measuring up to 8 cm long, along with floater crystals of similar dimensions.
The axinite inclusions themselves displayed the typical bladed habit expected for this mineral. When examined with polarized light, they also showed pronounced pleochroism, from a light brown (figure 2, left) to a more intense pinkish purple (figure 2, right). This represents the first visual record of axinite inclusions in quartz published in the gemological and mineralogical literature.

Pleochroic axinite inclusions in quartz
Figure 2. With plane-polarized light, the axinite inclusions in quartz show very distinct pleochroism. Both of these images were taken with the polarizer rotated at approximately 90 degrees from each other. Photomicrographs by John I. Koivula; field of view 9.0 mm. 

John Miatech lives in Kelseyville, California. John Koivula is analytical microscopist at GIA in Carlsbad, California.