Colorado’s mining history is a rich one, filled with tales of gold and glory along with the more gruesome stories about those who met their demise on the quest for wealth. Most of the gold in Colorado, along with other valuable minerals and ores, was formed during the uplift of the modern Rocky Mountains, which occurred only about 60 million years ago. But there’s another mineral mined locally with a rich history and purpose all its own. If you haven’t guessed, I’m talking about gypsum, the mineral with more facets to it than the dull, ashen gray hillsides would indicate.
A Unique Substance
Gypsum is a unique substance. First, it appears in many different forms, including the lackluster gray powdery rocks that we see on the hillsides near the town of Gypsum and at the base of Beaver Creek. But if you venture out, you can find beautifully formed gypsum crystals. These crystals can take many forms, but the most common are white to clear with the appearance of scratched glass. These crystals are very soft, ranking a two on Moh’s scale of hardness, which means that they can be easily scratched with your fingernail! Larger crystals are even flexible, and they will bend (or break) easily. Finally, anomalies sometimes create the most beautiful specimens, and gypsum crystals occasionally form around grains of sand trapped in their crystal matrix. This forms a classic crystal known as a “desert rose,” a delicate rosette formed around an hourglass-shaped sand inclusion. Other unique crystal forms of gypsum go by names such as selenite, satin spar and alabaster.
The gypsum in our valley is believed to have formed much earlier than the Mineral Belt specimens, during the late Paleozoic Era, some 360 million years ago, when the large inland seas covering this area began to dry up. As the waters evaporated, dissolved minerals were left behind, including large amounts of the mineral gypsum, known chemically as hydrous calcium sulfate. And while the chemical name itself isn’t completely relevant, it is important that gypsum includes water in its structure.
When beds of gypsum are heavily compressed (as when other rock layers are deposited above them), the water is “squeezed out,” changing gypsum into the similar mineral, anhydrite. The process also works in reverse, and some gypsum crystals show evidence that they have been bent and fractured from the addition of water into their structure. In some rare cases, bubbles of water get trapped within the gypsum structure. These “enhydros” are extremely rare, and considered valuable by collectors.
Get with the flow
Gypsum’s ability to absorb or expel water results in another unusual property. Unlike most rocks, which have a less compromising crystal structure, gypsum flows. So imagine these huge beds of gypsum, some thousands of feet deep, being slowly covered by sediment over time. The Paleozoic Era ends and Mesozoic sediments are now piling up, adding their immense bulk to the pressure. Eventually, imperceptibly, the gypsum starts to flow, oozing through any cracks or weak spots in the above layers, to form the beds that we see exposed today. This flowing property creates two problems — only one of which affects the average citizen. First, for geologists, gypsum’s tendency to flow means that these deposits are out of sequence, making them difficult to age or determine their origin. The average person is more concerned with the potential for gypsum to continue to flow, resulting in sinkholes or landslides (or both).
Only Scratched the Surface
Finally, any local discussion of gypsum needs to include some mention of its importance to the local economy and to the construction industry. As many of you know, gypsum is the main ingredient in sheetrock, largely owing to its inflammability. But it is also used in cement, fertilizers, and Plaster-of-Paris. The gypsum mine, located just north of the town, has gone through several owners and deposits over the years, and the current owner is American Gypsum LLC, which has been operating under its current permit since 1996.
Believe it or not, we have just scratched the surface of what this common mineral has to offer. And while you may not be able to sell those gypsum crystals for the price you would get for silver or gold, remember that beauty is in the eye of the beholder and these crystals clearly have much beauty to behold.
Jaymee Squires is the director of graduate programs at Walking Mountains Science Center. She is a great fan of geology and she loves to uncover the stories in stone that surround us.
Anomalies sometimes create the most beautiful specimens, and gypsum crystals occasionally form around grains of sand trapped in their crystal matrix.