Large mica crystal in pegmatitic granite in the Devonian of New Hampshire, USA.

Spectacular mineral collecting can be had at the Ruggles Pegmatite Mine near Grafton, New Hampshire, USA. The Ruggles Mine started off in the early 1800s as a muscovite mica mine, but it's now a tourist site. 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, potassium 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.

One of the most visually intriguing aspects of the Ruggles Pegmatite is the huge masses of both biotite mica and muscovite mica. The example shown above is called a mica book. Upon weathering, large mica crystals have the appearance of pages in a book. This is a consequence of mica having one plane of perfect cleavage. Hard, nonweathered mica crystals can be split and peeled into ultrathin, flexible sheets.