Weathering (and Iron Oxides, like rust) in the Mines and Deposits within the Herkimer Mining District

Weathering, the effect of water and ice, on the mines and deposits of the Herkimer Mining District is always present - somewhere in the mine.  This is particularly true in the deposits that are very close to the surface, like Hickory Hill. At times collectors can encounter small cavities (up to 2 ft) that have totally smooth walls - no crystals of any kind, not even the very early dolomite.  In many areas the second dolomite, and the first calcite, show heavy etching from weathering. Often the sulfides have been changed to rust coatings, or lumps of iron oxide (likely a mixture of limonite and goethite).  In the slide show there will be side - by - side photos, non-weathered and weathered.

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Weathering and Herkimer Diamonds

Weathering has had a dramatic effect on Herkimer diamonds. It is quite difficult to find a crystal larger than 3 cm long that is without flaws (although a few 4 cm ones have been reported to this author).  Smaller clear crystals, less than a cm, are very common.  The dominant process causing these fractured in the crystals is the weathering - the forces of water freezing and causing the crystal to crack. Larger crystals have more surface area and thus receive more force from the water freezing.  Larger crystals also tend to have internal structural weak points created during the growing process out of a hypersaturated solution (see the hopper - skeletal page).

There is some other evidence to support this linking of flawed crystals to ice pressures.  Herkimer diamonds are found totally encased within calcite.  This calcite has served as a protective shield against the ice pressures.  The Herkimer diamonds within calcite are almost always without fracture flaws. This is shown in the slide show, along with a series of “size versus flaws” photos.