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Dr.C. did two talks at the Herkimer Diamond Festival (July)which are in a Youtube movie. The research on the Herkimer District is in need of samples and photos.
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Dolomite: The most common mineral associated with Herkimer diamonds
Dolomite is the most common mineral associated with Herkimer diamonds. It is the first mineral to appear in crystal form. It is at every deposit, although not exactly the same in appearance. There is evidence to support at least two phases of dolomite, and early and then a late. There may have also been a third iron rich phase, but how this presents itself throughout the district is not yet know. In many places the dolomite is heavily weathered and thus distinguishing second phase from third is difficult. The three phases are described below.
Early dolomite - phase 1
Second dolomite - phase 2
Late dolomite: Iron rich - phase 3
Above is a specimen from St. Johnsville donated by Bob (2008) shows the lusterous clear little rhombs of the first phase dolomite when they are not weathered. The larger white rhombs are second phase dolomite. Photo Dr.C., 2008, 2 cm top to bottom.
The people over at Diamond Acres (Fonda, NY) complain that they don't have any nice dolomite (which is because their large second phase dolomite is quite etched). But they do have the best first phase dolomite in the district, and it is not hard to find, and can be seen with a simple hand lense. The picture above is about 2 cm across. There is some black hydrocarbon material near the top right. The first phase dolomite occurs as a coating of tiny, mostly clear, rhombs. They almost look like a sprinkle of salt. There is a close up of this on the hydrocarbon page. Collected 2008, photo Dr.C. When they are weathered they will turn to a dull tan - most of the time.
Second phase saddle dolomite
Space for Hickory Hill first phase "stacked" dolomite photo
The photo on the left is a different specimen from the same location as above (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 is tan and is also highly etched, as seen in the photo below.
The best second phase dolomite in the Herkimer district comes from the Hanson/Benchmark Quarry in St. Johnsville NY. The photo (by Dr.C.) is from a specimen purchased in 2008 and is approximately 2 cm across. It shows both phases, with the first phase as very small clear crystals (just barely visible under the white dolomite), and the second as large white crystal groups that show curved "saddle shapes" as the step pattern of growth stretches out the rhombs, but not as well developed as the saddle form in the photo at the top of this page (a rare find from TCR). Compared to the dolomite on the right, this is bright and shiny and shows almost no etching. This white dolomite is seen in many photographs on this website.
It is worth noting that the first phase dolomite is very tiny in most locations outside of Fonda, and you need some magification to see it clearly. In Middleville it is even smaller than in the above photos. To complicate the matter further, some of the mineral phases that follow after the first phase dolomite will either totally coat over these tiny crystals or disolve them away. Nice second phase dolomite is fairly easy to find in Middleville and it has been seen at other sites outside of Fonda (where is is hard to find good dolomite crystals).
The best third phase dolomite from the district is from Palatine Bridge, NY. This specimen photo (2 cm across) bottom left, 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, NYSM, 2009) 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, or late in the formation of the second phase dolomite.
On the right is pocket dolomite, with a slight peach tint, from HDM (2008, 4 cm across). 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 left, is perhaps what third phase dolomite might look like if it was not weathered to a peach-rust color. It might be black! Photo, Dr.C. (2008).
Space for Middleville saddle phase 2 dolomite photo
On the upper left is black dolomite. 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. It is proposed here that at one time there was much more black dolomite present throughout the district. 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.
Upper right 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 Dr.C. - 3 cm across, 2009.
Space for CG dark dolomite photos and
W. David Hoisington, Ph.D.