New Mexico Mineral Symposium
November 12-13, 1994
New Mexico garnets: mineralogical beauty and economic potential
Virgil W. Lueth
Six species of garnets are reported in New Mexico in a wide range of geologic environments. Garnets are most commonly found in metamorphic rocks as almandine, andradite, or grossular. Igneous-formed garnets, as the species pyrope, almandine, and spessartine, are also found in the state. Clastic sedimentary rocks also contain garnets as detrital grains, but these garnets are not found in any significant concentration. Mineralogical and geological descriptions of individual garnet species are presented along with specific collecting locations.
Almandine [Fe2+2Al2(SiO4)3]—Almandine garnets are most commonly found in mica schists and gneisses in the Proterozoic rocks in the northern part of the state. Crystals are typically isolated, euhedral, and embedded in mica schist. Most often the crystals contain many inclusions of mica, quartz, and feldspar that detract from their appearance. Tedious physical removal of the host rock by scraping is often required to expose the grains. The best collecting localities for almandine include the schists of Mora and San Miguel Counties. Fine crystals are found in the Picuris Range of Taos County. Igneous occurrences of almandine include the Capitan Mountains of Lincoln County and the pegmatites of the Petaca district in Rio Arriba County. Igneous-formed garnets are rarely of specimen grade because of the difficulty in removing host rock material and the poor condition of the material. The dispersed nature of the garnets along with their small size prevents mining of these types of garnets in New Mexico.
Spessartine [Mn3+2Al2(SiO4)3]—Spessartine garnet ranging from yellow to black is most often found in igneous rocks. Topaz rhyolites of the East Grants Ridge district host very fine red to brown microspecimens along with topaz. Yellow to red spessartine garnets are found in the Petaca district as large masses in pegmatite and as individual grains in the host schists immediately adjacent to the pegmatites. Euhedral crystals up to a centimeter diameter are found in the bordering schists. Spessartine exhibits a complete solid solution with other garnets and is found as a significant component in the andradite garnets associated with the zinc skarns in the Central district of Grant County. The rarity of this species precludes any practical utilization of this mineral.
Pyrope [Mg3Al2(SiO4)3]—Pyrope garnets are derived from mafic to ultramafic rocks and represent New Mexico's most famous gemstone other than turquoise. These stones have been collected on the Navajo Reservation of northwestern New Mexico since before the arrival of the Spaniards. Commonly known as "Arizona rubies," many fine stones have been found and are described in a number of gemstone publications. The garnets are found as loose grains, often on ant or scorpion mounds. The garnets are weathered from mafic breccia dikes and further concentrated by eolian processes in the desert. The concentrations are not great, and the stones are small and fractured. However, facetable material can still be found in McKinley and San Juan Counties. The volume of pyrope garnet in the state is very small.
Grossular [Ca3Al2(SiO4)3]—Contact metamorphic deposits are the most common host rock for grossular garnets. The most favorable protolith consists of argillaceous limestones or limy siltstones or marls. The best specimens of this garnet are most often found at the marble/garnet interface of contact metasomatic deposits. Grossular is one of the most common garnets of New Mexico, found in many skarn deposits of the southern part of the state. The mineral displays a range of colors from light yellow to brown. Near-gem-quality stones of essonite occur at Orogrande in Otero County and in the Hanover-Fierro district of Grant County. Grossular forms a complete solid solution with andradite. Accordingly large masses of mixed "grandite" occur in place and in tailing piles in many mining districts of the state. These large masses of garnet represent a potential resource, as yet untapped.
Andradite [Ca3Fe2+2(SiO4)3—Andradite is a contact metasomatic mineral common in large skarn deposits where it often overgrows earlier-formed grossular. It is the most abundant type of garnet in the state. It is usually found in massive layers, and crystal face development is limited. Green demantoid-like andradite is known from the Continental open pit at Fierro although it is badly fractured and often contains inclusions. Most andradite is red brown, massive, and fractured. Large masses of this variety of garnet, in place and on dumps, represent the largest garnet resource in the state. Unfortunately, andradite is often fractured, has overgrowths, and contains foreign mineral inclusions that affect its physical properties. Feasibility studies of individual deposits or dumps will be required to utilize this type of garnet.
Uvarovite [Ca3Cr2(SiO4)3]—Uvarovite is a bright-green variety of garnet that is only found in very small grains. A single report of uvarovite is known in the state. The locality is in the South Canyon district in Doria Ana County on the White Sands Missile Range. The uvarovite is reported to be in a limestone xenolith. The district is off limits to the public.
In the past, garnet was of interest only to the gem and mineral collector. Mining companies considered the mineral to be part of the gangue assemblage. Today, interest is being shown in using garnet in industrial applications. Garnet has been used as an abrasive, a common use for the mineral for years, and is finding application in newer hydroblasting technologies. A more recent use of the mineral is for granular filter media. Large-scale filter beds are used for sewage and water purification. The angular particles of garnet do not settle upon repeated episodes of filter backwashing. Thus the filter bed does not need to be replenished as often as more common sand filter beds. A large quantity of premilled garnet exists in many tailings piles, especially in the skarn deposits of the southwest part of the state. These tailing piles represent a potential resource, and their use could help defray the cost of cleanup in some mine dumps. Royalstar Industries is currently awaiting approval to mine garnet at San Pedro in Santa Fe County for abrasive grit and sand-blasting material.
15th Annual New Mexico Mineral Symposium
November 12-13, 1994, Socorro, NM