New Mexico Mineral Symposium — Abstracts

Datolite, CaBSiO₄(OH), is typically colorless to very light green at most localities worldwide. The same mineral from Michigan copper country displays colors ranging from gray to white, red, red-brown, blue-green, green, and yellow. These colors unusual unusual for a mineral that generally lacks significant substitution of chromophore elements. An electron microprobe and optical study was undertaken to determine if the cause of color in the Michigan samples was due to mineral inclusions or chemical substitutions into the crystal structure.

Optical examination of Michigan datolites reveals a number of interesting color features. Many datolite nodules have a white rim that can vary from hard to chalky. Microprobe analysis of these white rims indicates no chemical differences exist from the main mass of the nodule to the edge; however, empty polygonal pits are more abundant in the white zones. Translucent gray nodules are typically devoid of inclusions and have a consistent chemical composition supported by published data. Some gray specimens do have very small color "spots" of red, usually less than 0.1 mm. When these spots are abundant, the datolite takes on a red coloration. A few microchemical analyses of these spots reveal high iron content, and the spots probably represent hematite inclusions. In some samples these red spots persist into the white rims. Many of these red spots seem invisible in the microprobe, although readily apparent in the microscope, suggesting many of these red spots are merely a residual iron oxide "dye," much like that described in agates. The most common red to tan variation in many Michigan datolites appears to be a function of the abundance of these color spots.

Copper was the most abundant mineral inclusion noted in the datolites. In some samples, the copper occupies the same polygonal spaces noted in the white rims. Copper included datolites generally appear tan to red-brown with the intensity of red-brown color proportional to the number of copper inclusions. Brown-yellow datolites also contain fairly abundant inclusions of copper.

The more exotically colored blue and green datolites contain inclusions of secondary copper minerals. The bright blue samples appear to contain inclusions of a Cu-Ca silicate, most likely kinoite, based on stoichiometry. Green hues appear to be due to copper carbonate (malachite) inclusions. Only in light-green specimens are elevated copper concentrations noted in the datolite chemistry. Yellow specimens have many inclusions of an Mg-Ca-Fe silicate, most likely clays. Black veins or rinds on the yellow samples are predominantly iron oxide inclusions.

Based on the consistent chemistry in samples of various colors, it appears the dominant coloring mechanisms in Michigan datolite are inclusions of various types. Copper, secondary iron and copper oxides, and empty space inclusions of variable density impart most of the color observed. Trace element substitution into datolite is very limited, similar to results noted by previous workers, and is unlikely to cause the variations in color observed in these samples.