THE FRANKLIN MINING DISTRICT
The minerals of the pegmatite contact zones form a complex group that includes more than half of the species peculiar to the district, as listed below.
Minerals found only at Franklin or Sterling Hill, New Jersey (Species found at Franklin, F; at Sterling Hill, S. Margarosanite and nasonite, first found at Franklin, are known elsewhere only at Langban, Sweden) |
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There is a lack of detailed observations in most of the mines to support conclusions based on the chemical nature of the minerals, but in the Trotter mine the relations displayed were definite and conclusive. Plainly the great heat of the intrusive pegmatite not only caused recrystallization of the primary minerals of the ore but also set up a vigorous interaction between those minerals and constituents of the pegmatite magma near the contact, and the magma was rich in volatile constituents, differing from place to place, which combined with the ore minerals in a variety of new compounds. These reactions seem to have continued through a long period of falling temperature to merge at length with purely hydrothermal processes, whose products are classed in another group.
The group of contact zone minerals comprises two overlapping subgroups. One, to which the convenient Swedish mining term "skarn" is applied, includes silicates and oxides, which are conspicuous, especially at Franklin, for their abundance and brilliant colors. Most of them are direct reaction products between ore minerals and the silica and alumina of the pegmatite. The other subgroup is termed pneumatolytic because it includes mostly minerals that contain volatile elements or metals clearly derived from the pegmatite magma.
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© by Herb Yeates 1997-2006.
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page created: August 12, 2006 6:37 PM
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