The best website on "Herkimer Diamonds"
The host rock for Herkimer diamond deposits is called the "Little Falls Dolostone (or dolomite)" or sometimes the "Little Falls Formation". It is a sequence of sandy limestones interfingering with limestones that have been altered to dolostone - a dolomite rich rock. How that happened is something that geologists are still debating, but it is likely that dolomite crystals were the first crystals formed inside the "cavities" that were part of the original limestone (see the host rock page for more information). All the mines contain well formed dolomite crystals. It also appears that there is evidence for two, and possibly three, dolomite episodes in the Herkimer district. Thin section photography (from personal communication Taury Smith, NYSM, 2008) shows a third iron rich episode (darker) making "rims" around an earlier (much lighter) dolomite. In the mines we do sometimes see a bright "peach" dolomite as different from the white and cream versions.
Other Minerals - Dolomite from Herkimer deposits
The photo below is a different specimen from the same location (DA) and about 2 cm across. You can see the well developed coating of nearly clear, small, first phase dolomite (arrow 1). But you can also see the larger crystals of the second phase dolomite (arrow 2), which are always opaque, and show some type of step-like growth pattern. The nature of this step pattern varies across the district. At Diamond Acres (DA) the second phase dolomite tan and is also highly etched, but it can be white and shiny in other mines.
The best third phase dolomite from the district is from Palatine Bridge, NY. This specimen photo (2 cm across) was donated by Brian Slater (NYSM) who did extensive research in a quarry there. The white crystals here are calcite and the dark peach crystals are dolomite with some showing excellent saddle textures. Thin section microscope work (personal communication Taury Smith, NTSM) shows that these darker dolomites have a lighter interior, and thus may represent an iron rich dolomite phase that happened after 2nd phase white saddle dolomite.
Additional field work suggests that this iron rich dolomite may be more prevelent througoout the district than was previously thought.
It is likely that the "peach" color common to most of the third phase dolomite found throughout the Herkimer Mining District is due to oxidation, or a rust forming from the original iron in the dolomite. But below is perhaps what third phase dolomite might look like if it was not weathered to a peach-rust color. It might be black!Photo, DrD. (2008).
Thanks to Bob for donating this unique St. Johnsville specimen (2 cm across). Dolomite this dark is an unusual find for the Herkimer District, and chemically might be ankerite. The clear crystals are calcite (a picture of them is on the calcite page).
It is proposed here that there was, at one time, much more black dolomite. What has happened is that the surface of the dolomite has been altered to colors ranging from cream, to tan, peach and brown-red. Peach dolomite is quite common in Middleville, and it has been seen at DA and TCR.
Below is a photo that shows the inner black core of the second/third phase dolomite altered on the outside to a light color. Specimen is from TCR, photo DrD, 2009.
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