11. The Crystals of Crown Butte

This piece is about 6 inches (15 cm) across with crystals as large as 1 cm.

The butte is made up of a very dark rock called "Shokinite", named after the town of Shonkin (east of Great Falls) where it was first discovered. Shonkinite is an igneous rock that forms through the cooling of magma that originated beneath the Earth's crust, probably from a depth of over 25 miles.

Floating image and dynamic text: right A closer look (right) reveals the black "augite" crystals. These crystals formed before the magma moved into the lacolith while the magma was cooling at a slower rate (time for the molecules to arrange themselves into crystals). Since augite has a higher freezing point than the other minerals in the magma, the crystals were surrounded by magma for a time, allowing them plenty of space to grow. Then the magma moved ("pulsed") from the magma chamber into the laccolith, where it began to cool at a faster rate. So, as the other minerals froze they didn't get as much time to form nice big crystals. Such a rock is called a "porphyry".

porphyry: (POR fah ree)
rocks containing crystals surrounded by a fine-grained mass (or matrix)

Check out this photo album from a hike onto Crown Butte with the Helena High Outdoors Club in the fall of 2015.


One last look
Start of the Crown Butte Virtual Field Trip

Below: In some places on the butte, the rock has weathered so that black crystals of augite can be collected.

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