Welcome to Thornton, Colorado! While the surface of the Denver Basin here is dominated by younger sedimentary layers, this specific location features a massive, displaced glacial or decorative White Granite Pegmatite Boulder. This boulder offers a window into the deep, ancient crystalline basement rocks of the Colorado Front Range.
By observing this boulder, you will learn how giant mineral crystals form from cooling magma deep within the earth and how to identify the massive white feldspar crystals trapped inside it.
The Geology of Pegmatites
A pegmatite is an igneous rock that forms during the final stages of a magma chamber’s crystallization. What makes a pegmatite unique is not its chemical composition—which is identical to ordinary granite—but its texture. Pegmatites contain exceptionally large crystals, often measuring several centimeters to several feet across!
How Do These Giant Crystals Form?
For a long time, geologists believed that large crystals only formed when magma cooled incredibly slowly over millions of years. While slow cooling plays a part, laboratory experiments have revealed a secret ingredient: Water.
As a granitic magma chamber cools and minerals like quartz and feldspar begin to harden, the remaining liquid melt becomes highly concentrated with water, carbon dioxide, and rare elements. This super-heated, water-rich fluid has a very low viscosity (it is highly fluid).
Because the fluid is so runny, atoms can travel through it and bond with growing crystals incredibly fast. Instead of creating millions of tiny crystals, the atoms pile onto just a few nucleation points, resulting in massive, mega-sized crystals over a relatively short period.
Mineral Composition of this Boulder
As you inspect this white granite pegmatite, you will notice three primary minerals:
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Feldspar (Microcline/Orthoclase): The star of this boulder. It appears as massive, blocky, opaque white to creamy-pink sections. When the light hits it just right, you will see flat, reflective surfaces known as cleavage planes.
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Quartz: This mineral looks like chunks of gray or smoky, fractured glass filling the spaces between the feldspar.
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Mica (Muscovite/Biotite): Look closely for small, shiny, metallic "books" or flakes that can be peeled apart with a fingernail like sheets of paper.
Logging Tasks (To Earn Your "Found It")
To log this EarthCache, please visit the coordinates and examine the white pegmatite boulder. Send your answers to the cache owner via the Geocaching message center. Do not post the answers in your public online log.
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Crystal Measurement: Find the largest single crystal of white feldspar on the surface of the boulder. Measure its length and width in centimeters.
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Texture Observation: Describe the boundary between the white feldspar and the gray quartz. Are the edges sharp and geometric, or are they completely blended together?
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Mica Hunt: Can you find any shiny flakes of mica embedded in the boulder? If so, what color are they (silver/clear muscovite, or dark black biotite)?
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Educational Synthesis: Based on the geological description above, what "secret ingredient" in the cooling magma allowed the feldspar in this boulder to grow so large?
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Photo (Optional but encouraged): Upload a photo of yourself or a personal item (like your GPS/phone) next to the boulder, without revealing the answers to the questions.
References & Further Reading
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Wake Tech Geologic Labs: Outdoor Geology Lab Tour: Granite Pegmatite – An excellent breakdown of pegmatite crystallization and mineral identification.
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United States Geological Survey (USGS): Pegmatites of the Crystal Mountain district, Larimer County, Colorado – A formal study detailing the composition and mineral structures of Proterozoic pegmatites found along the Colorado Front Range.
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City of Boulder Open Space & Mountain Parks: Geology on OSMP – Contextual regional history regarding the 1.7-billion-year-old igneous uplifts that formed the local crystalline basement rocks.
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Peter Barkmann colorado school of mines professer indenifyed the rock and told me about it
- a tad bit of gemani ai to make it look clean