Mariposa Calcite, Santa Eulalia, Chihuahua, Mexico
Mariposa calcite specimens are produced by several mines in the West Camp area of the Santa Eulalia Mining District in the Mexican State of Chihuahua. This specimen is likely from either the Potosi Mine or the Buena Tierra Mine. Mariposa, or butterfly calcite is unique in that it forms in two separate and distinct phases of growth. 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 due to manganese content. As illustrated in the photographs below, the fluorescence typically shows a strongly zoned pattern, outlining the initial growth phase scalenohedral crystals. The second, rhombohedral phase of growth is not fluorescent. The specimen shown here measures 9 x 4.6 x 2.7 inches, and weighs 2.6 pounds (1183 grams).
Figure 1: The above photograph was taken under a combination of visible light and short wave UV (254 nm). Note the underlying layer of brightly fluorescent calcite that represents the initial phase of crystal growth. The second phase of crystal growth seen on the upper surface of the specimen is not fluorescent.
Figure 2: The photograph above shows the underside of the base of the specimen under short wave UV. Note the strongly zoned pattern of orange fluorescence outlining the initial phase of crystal growth, which is characteristic of mariposa calcite.
Figure 3: Visible light image taken from the same point of view as in Figure 1 above.
Figure 4: Visible light image showing the underside of the base of the specimen from the same point of view as in Figure 2 above.
Figure 5: Full view of the top side of the specimen under visible light. Note the dark internal phantoms typical of mariposa calcite.
Figure 6: The paired gallery images above show close-up views of the zoning on the underside base of the specimen, showing fluorescence under short wave UV (left column) and and the corresponding area under visible light (right column). Click on the individual images for a larger view.
Figure 7: Although the zoning is not as apparent, the second generation calcite is also fluorescent under other UV wavelengths. The gallery images above show fluorescence under short wave UV (upper left), mid wave UV (upper right), long wave UV (lower left), and visible light (lower right). Click on the individual images for a larger view.
Mariposa calcite has something to offer for both the collector of fine visible light display pieces, as well as the fluorescent mineral collector. The well-formed dogtooth calcite crystals with their dark internal phantoms make excellent additions to any mineral display case, while the bright, spectacular, zoned fluorescent response of the much less attractive (under visible light) cleaved base of these specimens make them extremely desireable to fluorescent mineral collectors.