The Topazes of San Luis Potosi
A look into the formation, location and types of topazes found in San Luis Potosi, Mexico
Where They're Found:
San Luis Potosi has a tertiary felsic volcanic belt that comprises three main areas along the eastern side of the Sierra Madre Occidental. The Cerro El Gato (DEG), Cerro Silva and Cerro El Lobo, all about 40km away and with an average altitude of 2,000 km.
Topaz Types
In San Luis Potosi the rhyolites commonly contain small topaz crystals but some rare crystals can reach up to 15 cm in length. Topaz found in San Luis Potosi ranges in color from golden yellow, orange, red, pink and brown. Some crystals display yellow to amber color in the center to colorless with red-black inclusions towards the edges.
Rhyolite Formation
The San Luis Potosi Volcanic Field is the result of partial melting from a residual granulitic continental crust source from the Precambrian era. The enrichment of fluorine by breakdown of biotite favored partial melting and the formation of these rhyolites, which are characterized by high contents of fluorine and post-magmatic Topaz. These host rocks for topaz are high-silica and from alkali rich lavas. These topaz rhyolites are geochemically similar to the ones from the Thomas Range and Spor Mountain, Utah.
Topaz formation
In Cerro El Lobo the vesicles of the topaz are small and isolated due to the lack of an abundance of fluid and no evidence of fluid renewal. These conditions limit the amount of crystal growth. In Cerro El Gato, since the fluid systems were open, the fluids were able to circulate and were concentrated in open structures toward the border of the dome, where topaz formed in relation to the folding of the flow foliation planes. The large crystal size and the possibility of the fluid renewal allowed for larger crystals. Studies argue that while colorless topaz crystallizes at a temperature above 500°C, Amber-colored topaz crystallizes below 500°C.
Why San Luis Potosi's Topazes?
Ever since my early childhood I would drive to my hometown in Mexico, San Luis Potosi. I can recall never being impressed with the dry mountainous desert on the way to my grandmothers house. When presented with this project I inquired about the possibility of researching any minerals found in San Luis Potosi, not expecting much content and mainly driven by my curiosity. I was pleasantly surprised to find out that such a precious gem was local to the area, and not only that but it was abundant. Knowing that, I set out to find information behind its formation and its specific locations. It's rewarding to know that behind the never ending desert landscape there was something interesting underneath it all.
Neida Villanueva, Northeastern University, 2021 - text
MGMH - photos
Refrences:
GeoScienceWorld. (2004). GeoScienceWorld. McLean, VA: GeoScienceWorld.
Rodríguez-Ríos, Rodolfo, Aguillón-Robles, Alfredo, & Leroy, Jacques L .. (2007). Petrological and geochemical evolution of a complex of topaciferous domes in the San Luis Potosí Volcanic Field (Mexico). Mexican Journal of Geological Sciences , 24 (3), 328-343. Retrieved on May 30, 2021, from http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S1026-87742007000300004&lng=es&tlng=es.
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