Episode 378: Pegmatites

If you’ve listened to this series for any length of time, you know that geologists, like most scientists, are fond of jargon. They use specialized words as shortcuts for particular meanings, like ‘gabbro,’ which is easier to say than “a coarse-grained igneous rock with considerable iron and magnesium, often dominated by the minerals pyroxene and plagioclase.” I try to explain jargon terms in this podcast when I use them, since my goal really is to help understanding.

Today’s jargon word is pegmatite. It’s from a Greek word meaning “to stitch together,” and the crystals in pegmatites often do show a complex interlocking texture. But the key thing about pegmatites is that the crystals in them are big. Sometimes really big.

There isn’t really a legal definition in geology for “big,” but pegmatites usually have crystals bigger than one or two centimeters, and some would use 2.5 centimeters, one inch, as an arbitrary cut-off. But it isn’t just size that matters.

Pegmatites often, but not always, have a more or less granitic composition, so they often contain large crystals of quartz and especially feldspar. But they often form late in the history of an igneous granite body, and because those late-stage magmas contain superheated water, they can mobilize and concentrate elements that solidify into minerals at relatively low temperatures as the molten granite cools. Minerals containing lithium, boron, tantalum, and rare earth elements are pretty common in pegmatites.

The crystals grow to large sizes because they have longer times to grow – that’s the basic difference between rocks like granite and rhyolite, which have essentially the same composition, but granite cools slowly and has large crystals while rhyolite cools more quickly and has a very fine-grained texture. Think of pegmatites as the slowest of all, slow enough that in some cases crystals tens of feet long can form. At the Etta Pegmatite in the Black Hills of South Dakota, spodumene, a lithium silicate, grew into huge, log-like crystals up to 42 feet long. The mine there was an important source of lithium for years.

Tourmaline is another fairly common mineral, actually a group of minerals, found in pegmatites. It’s a complex boron silicate that often makes long, black rod-like crystals, but it’s sometimes beautiful green, pink, and other colors, even sometimes zoned from inside to outside like a watermelon. Tourmaline group minerals are hard, around seven on the Mohs harness scale, so the gemmy colored varieties are sometimes made into jewelry.

Besides their value as sources for large, collectible mineral specimens, pegmatites in some places are valuable economic resources, like the spodumene containing lithium I mentioned a minute ago. Rare earths, beryllium, and tantalum are often found in pegmatites, ultimately finding their way into things like cell phones, automobile brake shoes, and capacitors in computers.

In addition to containing large crystals, pegmatites can be big themselves. Some of the pegmatites in South Dakota are more than a mile long, but just last fall I visited a little one in the hills east of Butte, Montana, where I live. That one was no more than a meter, three feet, across, but it did have pretty cool feldspar crystals in it more than six inches long.

—Richard I. Gibson

Pegmatites of the Black Hills