Out in the Woods

Pegmatite rock next to the beaver dam at Rail Trail Park shows crystals of quartz and feldspar.

Photos: Kevin McKeon

Don’t Just Take Our State Rock for Granite

By Kevin McKeon, Maine Master Naturalist

After many work sessions and readings in Augusta, Maine now has an official state rock – granitic pegmatite. So, I suppose we should know a bit about what this stuff is.

Let’s begin with pegmatite. It forms deep underground from molten, mushy rock called magma. Sometimes magma intrudes, or pushes into, cracks in Earth’s 3- to 18-mile-thick crust, cooling underground. (When magma spews from volcanoes it’s called lava.) But to form pegmatite, the intruded magma usually contains about 20% fluids. As the magma slowly cools into rock, the fluids form super-heated pockets.

Pegmatite vein at Flag Rock along Mousam Way Trail North, across from Holdsworth park. Flag paint partially covers the vein.

These pockets are rich in ions, which are electrically charged atoms and molecules that rapidly fuse, allowing the growth of relatively large crystals in the pockets. Some pockets are quite large, allowing many large crystals to form — like the famous tourmaline deposits and lithium-containing spodumene crystals found in Newry, Maine. (Tourmaline is our Maine state mineral.). The resultant rock cooled within these pockets contains mineral crystals that are mostly about one-eighth inch or larger, and it’s this relatively rough texture that defines the rock as pegmatite — not the mineral composition.

Granite is rock formed much like pegmatite, and is defined as containing mostly quartz, feldspar, and mica. Also, the fluid content — and ion content — present during formation is much lower, usually less than 10%. So, any fluid pockets formed are much smaller, resulting in lower ion action by fewer ions and faster cooling, thus hindering large crystal development. This rock holds crystals the size of grains of sand, whereas pegmatite crystals can be pebble-size to over 40 feet long! Describing granitic pegmatite as large-grained granite could be an accurate description of our Maine state rock: “Granitic” describes the mostly quartz/feldspar/mica composition of the rock, and “pegmatite” describes the texture.

Magma intrusions can be huge, measured in square miles, called batholiths. As it cools and granite forms, pegmatite can form from within fractures of the batholith and along the edges of the cooling granite, where the fluid-rich magma gets pushed into the super-heated pools. The larger, linear pegmatite formations thus formed are called dikes, and the smaller ones veins. A vein can be seen at the Flag Rock across the Mousam River at Holdsworth Park, running next to the flag’s star field.

Pegmatite rocks are now exposed at the surface of the earth in many locations around Maine. Some contain rare minerals attracting collectors and rock hounds. Maine also has several world-class locations for mineral and gem hunting thanks to our many pegmatite occurrences. The Maine Department of Agriculture, Conservation, and Forestry oversees the Maine Geological Survey, which maintains information for mineral collecting hobbyists.

Lithium deposit: https://www.mainepublic.org/2021-10-25/a-1-5-billion-lithium-deposit-has-been-discovered-in-western-maine-but-mining-it-could-be-hard

Mineral collecting: https://www.maine.gov/dacf/mgs/explore/minerals/faq.htm

Editor’s note: Did you see something unusual last time you were out in the woods? Were you puzzled or surprised by something you saw? Ask our “Out in the Woods” columnist Kevin McKeon. He’ll be happy to investigate and try to answer your questions. Email him directly at: kpm@metrocast.net