Angelite is a trade name given to a specific blue or lilac-blue variety of the mineral anhydrite, which is a calcium sulfate. The name Anhydrite literally means “without water,” a defining characteristic that distinguishes it from gypsum, its hydrated form. This mineral typically exhibits a pale blue-gray color, sometimes with white mottling, and is relatively soft, registering around 3.5 on the Mohs scale of hardness. Its physical properties include a vitreous to pearly luster and three perfect cleavage planes, making it delicate to handle and cut. A unique feature is Angelite’s inherent instability, as exposure to moisture causes it to slowly absorb water molecules and revert back into gypsum.
The Geological Conditions Required for Formation
Angelite’s formation is closely tied to the specific geological environment of evaporite deposits, which are mineral layers left behind after the evaporation of highly saline water. This process typically begins in large, enclosed basins or ancient seas where the influx of fresh water is exceeded by the rate of evaporation, concentrating the dissolved minerals. As the water volume decreases, minerals precipitate out of the solution in a specific sequence, with anhydrite forming under conditions of high salinity, elevated temperature, and pressure.
The lack of water in the anhydrite structure is maintained when the mineral forms at high temperatures. It is commonly found deep beneath the surface, where it is protected from circulating groundwater that would otherwise convert it into gypsum. In many instances, the anhydrite deposit originates from a pre-existing gypsum layer that loses its water content through burial and subsequent lithostatic pressure over millions of years. This transformation often occurs in association with other evaporite minerals like halite (rock salt), celestine, and various carbonates.
Major Global Sources of Commercial Angelite
The primary and most significant source of the gem-quality blue anhydrite, marketed as Angelite, is located in the Andes Mountains of Peru. This South American country is renowned for producing specimens with the vibrant, desirable sky-blue coloration that led to the stone’s commercial recognition in the late 1980s. The specific geological setting in Peru provides the perfect conditions for the formation of anhydrite with the trace elements necessary to impart the distinct blue hue.
While Peru is the most well-known locality for the blue gem variety, anhydrite deposits are geographically widespread. Mexico is a notable source, yielding fine semi-translucent to opaque specimens of blue anhydrite. Other global occurrences of blue anhydrite have been documented in locations such as Egypt, which sometimes produces material with distinctive white patterning. Poland and Germany are also recognized for having deposits of anhydrite, although the commercial quality of Angelite from these sources may vary compared to the prolific Peruvian mines.
Notable Occurrences in North America
In North America, the mineral anhydrite is geologically present across many regions, though the blue Angelite variety is much rarer. Anhydrite deposits are frequently encountered in the deep subsurface, often discovered during oil and gas exploration or mining operations. Within the United States, significant occurrences are found in the southwestern states, including Texas, Oklahoma, and New Mexico.
These deposits are commonly associated with the caprocks of salt domes and large sedimentary basins, such as those along the Gulf Coast and throughout the Permian Basin. For instance, deep drilling in Michigan has revealed substantial thicknesses of anhydrite in formations like the Salina Group. While anhydrite itself is a common component of these subsurface evaporite layers, the material typically lacks the trace elements and specific formation history required to produce the light blue color sought after by the gemstone market.