Muscovite: The Glittering Ancient
by Marnie Craig
Broadcast 6.2021 and 6.12 & 6.15.2024

Muscovite schist. Photo by James St. John, CC 2.0.




Clusters of arrowleaf balsamroot and swathes of heart-leaved arnica splashed yellow beneath the towering ponderosa pines. They clamored for my attention, but the rocks won! I could only take a few steps before stopping to fondle and examine another shiny specimen. I came to photograph wildflowers and to escape the walls of my apartment after months of social distancing, but the sparkling rocks took my breath away. I had to touch each one. I even brought some home.

Mirror-like minerals in the creek and along the trail shimmered in the bright sun. I spent five hours exploring a five-mile section of the lower Bass Creek drainage in the Bitterroot Mountains. The rocks cast a spell on me. I couldn’t pass one without stopping for a closer look. I crouched and crawled in the dirt, marveling at the sparkles, textures and colors. Unlike flowers, each rock is different from the next—and they tell stories of the workings of the earth.

Once upon a time, the waters of the Pacific Ocean met the land of western Idaho. About 100 million years ago the North American tectonic plate moved west and collided with the rich layers of rock-forming minerals that made up the ocean floor. The collision pushed the ocean plate down into the earth’s hot mantle, into a cauldron of molten rock. The liquified rock rose and cooled to form the granite of the Idaho batholith, an igneous mass of quartz, quartzite, feldspar and mica extending from what is now central Idaho into southwestern Montana.

The plates continued their migration and the earth bulged upwards to form the Bitterroot Mountain range. The rock was mashed, folded and crumbled by heat and friction. It metamorphosed into quartzite and schist. After another 40 or 50 million years, the top layer of sediment slid off to the east to form the Sapphire Mountains and the Bitterroot Valley in the middle. About twelve million years ago, flowing glaciers carved, crumbled and eroded the bulging mountains in the west to create the drainages of today’s Bitterroot range, providing multiple access trails into the Selway-Bitterroot Wilderness, the largest expanse of wild land in the lower 48 states.

Thin layers of muscovite. Photo by James St. John, CC 2.0.

Two-dimensional sheets of transparent muscovite, a mineral in the mica family that once formed in layers on the ocean floor, adorn the cliffs and peaks of the mountains and the rocks of the Bass Creek drainage. Iron-stained schist, a coarse-grained rock made of potassium feldspar and quartz, delightedly flaunted the sheets of muscovite on its orange surface, as if they were diamonds. The sparkling schist I held in my hand had been crushed, pulverized and heated by the friction of moving earth and hot magma.

Transparent muscovite. Photo by James St. John, CC 2.0.

When humans first discovered that light passed through these thin transparent layers, muscovite was made into windows. Today it provides the sparkle for paints, glazes and cosmetics. Unfortunately, illegal mica mining and insufficient regulations allowed mines in India to collapse and lung disease to take the lives of many of India’s child laborers. In 2017, a global coalition established a fair and sustainable mica supply chain, creating policies that improve working conditions in the mines and eradicate child labor in India.

Beside me on the footpath, chatting women passed by on horseback. They were oblivious to the glittering. Sweating bicyclists and runners focused on their physical goals and challenges. Mothers called to wandering children and fathers to straying dogs. I waded among the glistening muscovite in the creek immersed in beauty and history. A crystalline window had opened. I heard the voice of a meek and silent mineral and I delved into the glittering ancient.


Every week since 1991, Field Notes has inquired about Montana’s natural history. Field Notes are written by naturalists, students, and listeners about the puzzle-tree bark, eagle talons, woolly aphids, and giant puffballs of Western, Central and Southwestern Montana and aired weekly on Montana Public Radio.

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