RUGGLES PEGMATITE
Spectacular mineral
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
Acadian Orogeny.
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.