Fluorite From Mexico
This article originally appeared in the January 2021 Backbender’s Gazette
Mexico is the third largest worldwide producer of fluorite for the steel making and chemical industry. However, the largest deposit, in San Luis Potosi, has produced few specimens for the collecting market because the fluorite is generally fine grained and disseminated in the host rock. The second largest deposit of fluorite also produces the majority of specimens. This is the Musquiz Mining District in the state of Coahuila. Purple cubic zoned fluorite crystals with distinct stepped faces are common and can resemble fluorite from Southern Illinois. The distinctive identifying characteristic is a strong rotten egg odor when scratched or broken, caused by numerous fluid inclusions containing petroleum.
Fluorite occurs in many other localities in Mexico, commonly as a gangue mineral (worthless minerals in ore deposits) in primary sulfide ore bodies or in the oxidation zones of mines. Despite its widespread occurrence throughout Mexican mines, collector grade specimens usually come from a relatively small number of mines. This article is designed to make collectors aware of the different localities they might encounter in looking through dealer stocks, and how to use Mindat.org to check on the accuracy of dealer labels. Many Mexican fluorite specimens show distinctive fluorescence, and this will be discussed also.
Collector fluorite specimens primarily come from the states of Chihuahua, Durango, Zacatecas and Coahuila. In Chihuahua the largest number of specimens come from the Naica Mine, a primary sulfide deposit where fluorite is a common gangue mineral. Crystals from this locality can be cubic, cubes modified by the octahedron and dodecahedron, octahedrons and octahedrons modified by the cube. Other more complex crystal forms are known but are less common. Spinel law twins are the rarest form of fluorite and command high prices when found. Many crystals are “stepped” like stairs due to the conflict between the cubic and octahedral forms as the crystal was forming. Color ranges from colorless to white to various shades of green, including an emerald green. Octahedral specimens are often a light purple. Associated minerals are anhydrite, chalcopyrite, calcite, sphalerite, bournonite, pyrrhotite and quartz. The stepped octahedrons modified by the cube commonly contain small suspended crystals of chalcopyrite. Fluorescence in Naica fluorite is usually a weak purple. An exception is the bright emerald green modified octahedrons found in the 2000s. These fluoresce a strong blue/purple under long and midwave UV light.
Santa Eulalia is a major mining district in Chihuahua, with a group of mines that are still producing. Fluorite is common in this district and can be seen in attractive combination specimens. The San Antonio Mine on the east side of the district is known for producing purple cubes up to 12 cm on a side. Late fine-grained crystalline purple fluorite can form crusts on quartz and calcite and even forms epimorphs after calcite. On the west side of the district the Potosi Mine has produced distinctive specimens where water-clear micro-crystals are found in association with rhodochrosite. Many crystal habits are seen in fluorite from this mine, ranging from simple cubes and octahedrons to complex forms resembling an insect eye.
The state of Durango provides the collector market with several unusual fluorite localities. The Navidad Mine produces attractive plates of pink to purple octahedral crystals that are etched out of quartz with hydrofluoric acid. These specimens show a strong blue fluorescence under long and midwave UV lights. The Ojuela Mine occasionally produces purple cubic crystals to 1.5 cm on a limonite gossan (oxides that cap an ore deposit) matrix. These crystals are strongly desired in the collector market for their rarity and a strong red fluorescence under long and midwave UV light. Nearby the El Filo Vein and the Santa Gertrudis Vein are part of a group of small veins that host small mining operations. Specimens from these mines are sometimes labeled as coming from the Ojuela Mine but have a distinctive spiral growth habit and siliceous matrix. They also fluoresce a strong blue color under long and midwave UV light.
The state of Coahuila has a major producer of fluorite specimens in the El Tule and other mines in the Musquiz Mining District. These crystals are always cubic with stepped faces. The most famous fluorite specimens from this locality are light purple cubes to 3 cm perched on colorless to white and light blue elongated twinned Celestine crystals to 20 cm.
In the state of Zacatecas, the San Martin District exploits a skarn system that is one of the largest in the world. Skarns are metamorphic rocks altered by heat and chemically active fluids from nearby magma bodies. Sometimes fluorite formed at the same time as the ore minerals, and the best specimens are those with inclusions of jamesonite, chalcopyrite, bournonite, galena, pyrite, arsenopyrite and scheelite. Color ranges from purple over green, blue-yellow, brown, colorless and even pink. The most common crystals are cubic, followed by octahedral and dodecahedral. Many are “stepped” with octahedral crystals showing cube faces modified by the dodecahedron. Fluorite formed at several stages during the ore deposition and thus can be found in a wide range of associations.
A selection of specimen photos from my collection are available for viewing on flickr.com:
https://www.flickr.com/photos/singingstone48/albums/72157717636749797
If you go to www.mindat.org and search for fluorite from the Mexican states listed above you will be able to see the localities and photos associated with each.