Rhyolite Garnet Areas - By Bill Shelton
Almandine
We as collectors may have visited a few areas where the garnets are found in rhyolites. One that we may find of interest is the Aquarius Mountains in Arizona. According to some sources, the garnets might be either almandine or spessartine. While we should not necessarily rule out the possibility of both species occurring here, it may an error in terms of identification. The lack of good evidence leaves me wondering what the correct answer is. In Garnet (2008), the author says the garnets from here may be either spessartine or almandine.
Ash Creek, near Hayden, Arizona is an area where the garnets are usually labeled as spessartine; I have not been able to find conclusive evidence regarding them so will accept the species as given for the time being. Dana’s New Mineralogy says the garnets form here are spessartine.
The last two areas are perhaps the most widely known and collected for garnets form rhyolites. Garnet Hill, Ely, Nevada is a rich source for specimens and very well-known to us. The material from here has been sold as spessartine and almandine. Chemical study shows the correct label should say these garnets are almandine. It is unfortunate that the Sinkankas text (1976) claimed they were spessartine. Dana’s New Mineralogy, for example indicates Alm 73 Spess 27 for these garnets. Pabst (1938) shows 69.3 % Alm and 28.5 Spess.
Ruby Mountain, Nathrop, Colorado has always been considered to be a location for spessartine and the chemical data confirms this to be true. Early studies and recent work corroborate this answer. Yet, Dana’s New Mineralogy (1999) says they are almandine. Lauf, (2012) claims the specimens are spessartine. So, we see two relatively recent sources that do not agree. An older source, Pabst (1938) shows 65.4 % Spess and 31.3 Alm.
Spessartine
Perhaps one lesson we may learn from this is apparent: sources may not agree on the proper species for a garnet from rhyolites. It is clear, at least to me, that both can occur but whether almandine and spessartine will be found at a specific location is a different problem. I think the consensus now is spessartine is correct for Ruby Mountain and almandine is correct fro Garnet Hill.
I hope to visit some other less popular areas and see if I can get analysis done on similar garnets. Perhaps we can provide the results to you in the future.