New Mexico Mineral Symposium — Abstracts

A recent find of garnets from the famous grossular garnet locality near Lake Jaco, on the Chihuahua—Coahuila border is unique for the bright red color exhibited by the specimens. The geology of the area and petrology of the rock types involved are not exceptionally unique for skarn-type garnets of the grossular-andradite (grandite) series. These garnets typically exhibit colors ranging from pure white (grossular) to dark brown (andradite). Most grandite garnets are typically greenish in color and intermediate in composition. Some of the andradite garnets are black (a variety known as melanite) when they contain significant amounts of titanium. Red colors are typically observed in pyrope-almandine¬spessartine (pyralspite) series garnets and until now, never observed in the grandite series. Pyralspite series garnets are never observed in skarn environments.

A detailed geochemical study of the garnets was undertaken to determine the cause of the red coloration using petrography and electron microprobe microanalysis. The cores of the garnets are typically black and contain elevated concentrations of titanium (as much as 4.5 wt %) consistent with the andradite (Ad15-20) variety of melanite. The immediate layer adjacent to the black core is white grossular. Minor variations in calcium and iron indicate increasing amounts of andradite component outward from the core. The red coloration in the Lake Jaco garnets is due to elevated concentrations of manganese from 1.0 to 1.7 wt % (Sp 1.6-37) in the latest stage of garnet growth with a distinct change toward more grossular¬rich compositions. Geiger et al., 1999, determined the red coloration in these garnets is due to the presence of Mn3+ in the octahedrally coordinated silicate site using spectroscopic analysis. They postulate that the color is derived from a similar mechanism that causes the red color in the mineral piemontite of the epidote group.

References:

1. Geiger, C. A., Stahl, A., and Rossman, G. R., 1999, Raspberry red grossular from Sierra de Cruces Range, Coahuila, Mexico: European Journal of Mineralogy, v.11, pp. 1109-1113.