Mariposa (butterfly) Calcite, Santa Eulalia, Chihuahua, Mexico
This specimen of Mariposa calcite shows bright orange fluorescence under short wave UV on the underside of the base of the piece. Note the fluorescent zoning. This fluorescent response is typical for Mariposa calcite. This specimen is likely from either the Potosi Mine or the Buena Tierra Mine, and measures 101 x 79 x 32 mm and weighs 272 grams.
Mariposa calcite showing bright orange fluorescence under short wave UV (254 nm) on the underside of the base of the piece. Note the fluorescent zoning. This fluorescent response is typical for this material.
Not much to look at under normal lightting, the above photograph shows the same surface as shown in the fluorescent image above.
Top side of same specimen showing the 2-generation crystal growth that is characteristic of mariposa calcite. The initial growth phase results in the formation of classic scalenohedral calcite crystals which are typically coated with a thin layer of iron oxide, giving them a red/brown-colored appearance. During the second phase of growth, the initial scalenohedral phase is overgrown with yellow/brown rhombohedral calcite, resulting in caramel-colored dogtooth crystals containing dark brown internal phantoms. Of interest to the fluorescent mineral collector, the first phase of scalenohedral calcite growth often shows a bright orange fluorescent response under short wave UV.