Cut slab of lilac-colored spurrite with an unknown accompanying green mineral. Width ~3.3 cm.
I doubt if many people know about a semi-gemstone called spurrite. I certainly did not until I was rummaging through the cases down at the local rock shop (Ackleys on Stone Street here in Colorado Springs) and noticed this “kind of pretty” blue slab with some green streaking. The label was “spurrite .” The price was right so I was off to explore this latest acquisition.
It turns out that I really can’t find out much information about spurrite! It is a calc-silicate [Ca5(SiO4)2(CO3)] that usually forms in high-temperature, contact metamorphic zones where a mafic magma (high magnesium and iron content, low silica content) intrudes into limestone or other carbonate rocks, as in a skarn. Spurrite has a hardness of ~5 (Mohs), and if crystalline is transparent to translucent with a vitreous luster; however, most specimens seem massive. In the U. S. the best-known specimens are from the Crestmore Quarry in Riverside, California. The type locality for the mineral is the Temeras Mine, Velardena District, Durango, Mexico. The specimen I have is a lilac colored mass with some sort of a greenish mineral also present (although it may be a variety of spurrite as it has a similar hardness but it looks sort of like "serpentine") from the Negra Mine, Marconi, Queretaro, Mexico. A thin calcite rim is also present.
There are a number of web sites that sell spurrite beads and cabochons with many of them labeled as strombolite. The latter name is listed as an alternative for spurrite but is not recognized by the mineralogical world. Some sites specifically cite strombolite as being mined at South Sister Peak in Luna County, New Mexico. Other than sites like this I was unable to locate nice crystals for sale.
What I did find though was perhaps more interesting---spurrite can form in cement kilns! Carbonate or spurrite rings (detrimental to cement production) are formed through CO2 desorption into the freshly formed free lime, or even through belite recarbonation. Belite is an industrial mineral thata is not recognized by mineralogists and is a dicalcium silicate, Ca2SiO4. Spurrite is a form of carbonated belite. When the carbonate in the spurrite is replaced with sulfur the new mineral is called sulfated spurrite (CemNem.com). I don't have the slightest idea about the color of this type of spurrite!