Tridymite

Although Tridymite crystallizes in the triclinic system, it is always a pseudomorph. It will either be a pseudomorph after Beta Tridymite, which crystallizes in the hexagonal system, or a pseudomorph of Quartz, which also crystallizes in the hexagonal system. Thus Tridymite occurs in hexagonally shaped crystals.

Click here for a detailed explanation on the crystal structure of Tridymite and other forms of silica.

Crystals are usually small, and appear as thin tabular plates. They commonly form intergrowths of two or three thin individuals, forming unusual and distinctive twins or trillings. Tridymite also occurs as small grains and as complex icicle-like formations.

Some of the finest examples are from the Euganean Hills (Zovon and San Pietro in Montagna), Italy. Upon analysis, crystals from these localities were determined to be Quartz pseudomorphs after Tridymite. Other localities include Lyttleton Harbor, New Zealand; Dinas Head, Padstow, Cornwall, England; Vechec, Northern Slanske Mountains, Slovakia; and Cerro San Cristobal, Pachua, Mexico.

In the U.S., Tridymite occurs in the Thomas Range, Juab Co., Utah; Beidel Creek, in the San Juan Mountains in Colorado; Obsidian Cliff in Yellowstone National Park, Wyoming; Highway 78 roadcut, Big Lue Mountains, Greenlee Co., Arizona; and Mt. Lassen in Lassen Volcanic National Park, California.