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Dwarves' Earth Treasures Online Museum:
Rainbow of Datolites
How the datolites got their colors
 What is so special about datolites nodules of Michigan is that they come in quite a rainbow of colors,

and the question is: How did the datolites get such color variety?

  The white color is most common color of any datolites and that color results from lack of impurities needed to produce any colors. The white color can also result from bleaching of colored datolites by water. It's possible to notice tiny red dots in any white datolites and the dots maybe due to iron "dyes" such as hematite and reddish chalcotrichite.
   The pink color result from the faint distribution of chalcotrichite, a copper mineral in form of aggregates which act like color dyes. The aggregates can be seen as "dots" (actually balls) when viewed under the lens. Well-known examples of pink datolites are of those from Nassau(Flintsteel) & Delaware mine.
   More dense the chalcotrichite aggregates ("red dots" when viewed under lens) becomes, the red color will be stronger. The Delaware Mine produced most reddish datolites of any mines so far, and in some cases, the distribution of chalcotrichite aggregates varies within a single datolite, resulting in color zoning. There are datolites such as those from Caledonia Mine coming in strong brownish reddish colors that are due to very fine copper inclusions.
  Chalcotrichite impurities also provided the orange colors, and the difference from the red aggregates is that the aggregates are larger, hence,  more orangish. That may account for the fact that the orange "dots" tend to be easier to be seen than those of red "dots". Clark Swamp Adit, and Mesnard Mine are well known for their orange datolites
   How the datolites got their yellow colors are not well understood. The optical examinations revealed that the yellow color may be due to the clay mineral inclusions. Mesnard, Quincy, and Keweenaw Point are the best known sources of yellow datolites. Some are found at Phoenix Mine, Delaware Mine, Drexal Mine and Hill Fissure in the area of Copper Falls.
  Many opaque datolite nodules tend to look big greenish which is due to absorbency of all colors but green from any sources of light, as if green light was cast away as "unwanted item". The faint green hue is also typical of many datolite crystals from other locations around the world.
   The true green colors of datolites got their colors from malachite and paratacamite, the micro-minerals that were formed by reactions between native copper inclusions and ancient salt brines. Centennial #2 and Isle Royale Mines are best known for greenish datolites. Some green datolites grade into teal (bluish-greenish).
   The true sky-blue to chryscolla-blue colors (without greenish tint) appear to be rarest of any colors except purple. The bluish colors may be due to kinoite inclusions as according to Tom Rosemary's article on optical examination results of blue datolites.
  The datolite from Mass Mine is showing what may be faint purple color, but I have yet to see any datolites with true purple color and I'm thinking of faint amethyst purple without deceptive grayish tint. It's tricky to tell which is more purplish or more grayish.
   The gray color tend to be a result of very fine copper and iron inclusions that have been tarnished and possibly silver. This datolite has an unusual black rim where dark inclusions are concentrated by the growth of datolite itself. The dark iron oxides are also responsible for black veins in many other datolite nodules.
  There are also some reports of black datolites. Very few had been observed/reported such as one is in Seaman Mineralogical museum, and others found at Laurium Mine and Hill Fissure (Copper Falls area). The black color may be a result of silver or iron oxide inclusions.
  Many datolite nodules contain high concentration of copper inclusions, making them look reddish brown as in case of the brownish-red datolites from Caledonia Mine and Laurium Mine.  Some datolites from Centennial and Ojibway mines show the zoning of bronze copper inclusions.
    It is also common for colored datolites to sport white rims which result from bleaching by water (swamps, rivers, puddles, etc..). The white rims tend to make colored datolites look more attractive. The bleaching of this datolite to the left may have taken about 100 to 150 years and if given more time, this colored datolite would become completely white.
    Who knows what sort of other strange colors and forms the datolites of Michigan can come in? For example, Some Iron City/Empire datolites come with unusual orange geometric pattern which couldn't be explained easily.


    All Rocks & Minerals articles on the minerals of Keweenaw Peninsula, Michigan and datolites done by Tom Rosemary.