Herkimer Diamond quartz crystals are found in abundance within a few kilometres of Herkimer, New York State. The area is close enough to southern Ontario to be suitable for a weekend field trip.
The host rock for the crystals (Little Falls Dolomite) was formed in a warm, shallow salt sea which lapped against the basement Precambrian rocks now forming the Adirondack Mountains. Limy sediments slowly accumulated, including algal stromatolites which form beds resembling small, flattened cabbages. This material gradually compacted under the weight of thousands of metres of additional sediments. While the rock was still beneath the sea, magnesium (Mg2+) was added to the calcite of the limestone to form dolomite and produce the holes in which the quartz crystals grew.
Water-clear doubly terminated quartz crystals microscopic to 10 centimetres in length occur in the Cambrian Little Falls Dolomite. The crystals are interesting viewed with a low power binocular microscope, or hand-lens. Some crystals have holes containing liquid and solid inclusions. Sometimes the holes take the form of negative crystals – a hole in the shape of the crystal. The hole-filling liquids, solids and gas form a "time capsule" of materials available at the time of crystal formation. The possible depth of the rocks beneath the surface during crystal formation may be derived from a study of fluid inclusions.
There are two types of inclusions found in Herkimer quartz. Primary inclusions formed during the initial growth of the crystal. Secondary inclusions formed when fluids entered a fracture in a crystal and were trapped there when the crystal healed over. Healed fractures are found in about 75 percent of Herkimer quartz crystals. Some inclusions contain a black hydrocarbon called anthraxolite. It is usually seen as black, dull or shiny fragments or smears within a crystal, or inclusion. Occasionally the hydrocarbon was deposited as a thin film on crystal faces as the crystal grew giving the crystal a smoky colour and forming multiple phantom crystals similar to the one illustrated above. The hydrocarbon was derived from the life forms living during the time of the deposition of the sedimentary rocks. The silica is probably also biogenic in origin – sponge spicules are made of silica. Sponge spicules form chert in many limestone regions. The hydrocarbons were concentrated at the same time as the quartz crystals grew. Study of fluid inclusions shows that the crystals formed at 51oC. Using an average temperature gradient of 1oC for 33 metres and an average surface temperature of 21oC, it has been estimated that the crystals formed at a depth of 1,000 metres, about 400 to 500 million years ago.
Herkimer Diamond crystals and stromatolite specimens may be collected at a number of fee-paying sites in the Herkimer area. The best collecting is at the Herkimer Diamond Development Corporation site, near Middleville. The largest specimens are collected by using energetic digging with crow bar, sledge hammer, chisel and a few day's work digging into a pocket of crystals. Most visitors find crystals by turning over the rocks and looking for a sparkle in the sunshine. If you plan to visit be sure to sue safety glasses. Stromatolite specimens are easily collected at the top of the quarry, the best ones have been weathered, bring out the layered structure. Holes containing large crystals are found under the stromatolite layer, and at the bottom of the quarry.
Visitors to the area may also visit the Petrified Creatures Museum, east of Richfield Springs to collect Devonian fossils and see life sized restorations of dinosaurs.
W. ULRICH, The Quartz Crystals of Herkimer County and its Environs. Rocks and Minerals, Volume 64, Number 2, March/April 1989.
D.E. JENSEN, 1978. Minerals of New York State. New York: Ward Press.
Best Western Little Falls Motor Inn, for reservations call 1-800-528-1234.
KOA Campground at the Herkimer Diamond Mines, Box 510, Herkimer, NY 13350, (315) 891-7355.