collecting is to be had at the Ruggles Pegmatite Mine near Grafton, southern
Grafton County, western New Hampshire, USA (see
map). The Ruggles Mine started off in the early 1800s as a muscovite
mica mine, but it's now a tourist mine. Its walls have beautiful
exposures of a mid-Paleozoic granite pegmatite, having unbelievably large
crystals. Well over 100 minerals have been reported from this pegmatite,
but the most common rock-forming minerals here are quartz, K-feldspar, biotite
mica, muscovite mica, and schorl tourmaline. The Devonian-aged pegmatite
at Ruggles Mine is one of several in the Grafton Pegmatite Field. These
intrusions are part of the New Hampshire Plutonic Series, emplaced during the
Granite pegmatite at Ruggles Mine, Grafton Pegmatite Field, New Hampshire Plutonic Series, Devonian.
Intrusive contact between granite pegmatite (left, covered with moss) and muscovite schists of the Devonian-aged Littleton Formation (right).
Huge single crystal of K-feldspar. Pegmatites are characterized by being very coarsely-crystalline (all crystals are >1 cm). Pegmatites tend to form in the margins of cooling batholiths, during the final stages of a crystallization. After most of the magma has crystallized, the residual magma is rich in gas & water & silica & incompatible ions (atoms too large or too small to fit in “normal” minerals that formed earlier). Cooling of such residual magma gets you pegmatites. The water-rich nature of this residual magma allows rapid ion transport, resulting in very large crystals. The incompatible ions go into forming unusual minerals and “garbage can” minerals (e.g., beryl, chrysoberyl, columbite/tantalite, uraninite, cryolite, monazite, apatite, lepidolite, spodumene, zoisite, topaz, zircon, molybdenite, etc.).
Huge single crystals of whitish quartz (left) and light tan-colored K-feldspar (right).
Graphic granite is not uncommon in the pegmatite rocks at Ruggles Mine. Graphic granite is characterized by having interpenetrating K-feldspar & quartz (see example from Russia). The dark gray crystals are quartz. The creamy colored material is K-feldspar.
Mica is what makes the Ruggles Mine pegmatite especially interesting. Large masses of both muscovite & biotite mica are exposed in the walls of the mine.
Mica book - most of the mica masses in the pegmatite have weathered so that they look like pages in a book. Mica books look this way as a consequence of mica's one perfect cleavage. Hard mica crystals can be split & peeled into ultrathin, flexible sheets.
Muscovite mica & biotite mica litter the ground at Ruggles Mine. Numerous, large quality specimens can be collected in a few minutes.
Diabase dike - a nice vertical dike has intruded the pegmatite at Ruggles Mine. Unlike the coarsely-crystalline granitic host rock, the dike itself is composed of finely-crystalline, mafic rock. If this had erupted as lava from a volcano, it would be called basalt. As this material is intrusive, rather than extrusive, it's not called basalt. Instead, this dark rock is called diabase, although it has the same general mineral content as basalt.
Mt. Cardigan, as seen from Ruggles Mine. The rocks at its bare summit are porphyritic quartz monzonites of the Cardigan Pluton (Early Devonian, ~411 million years), emplaced during the Acadian Orogeny.