Tridymite is a high-temperature polymorph of silica found primarily in acidic volcanic rocks like rhyolite and andesite. It typically occurs as small, thin, tabular, pseudo-hexagonal crystals often found within cavities or vesicles in volcanic matrices.
Is this tridymite?
5-step field checkRun through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.
- 1Test the hardnessTry to scratch tridymite with a known reference. Tridymite sits at Mohs 7, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: white.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous, may be pearly on {0001}.
- 4Match the color rangeCompare against the expected color range: colorless, white, yellowish white, gray.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: platy, spherical.
Often confused with
Tridymite vs. its common look-alikes, and how to tell them apart in the field.
Often found alongside tridymite
Minerals reported to co-occur with tridymite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- SiO₂
- Mohs hardness
- 7
- Density
- 2.25-2.28 g/cm³
- Streak
- White
- Luster
- Vitreous, may be pearly on {0001}
- Transparency
- Transparent
- Crystal system
- Orthorhombic
- Crystal habit
- Platy, spherical
- Cleavage
- Prismatic, poor
- Fluorescence
- Dark cherry red under SW UV
- Rarity
- Uncommon
- Uses
- Collector, Scientific research
- Host rock
- Deposited from hot gases in vesicles and lithophysae; as phenocrysts in felsic volcanic rocks, and less commonly, in basalts; in contact metamorphosed sandstone
Where rockhounds find tridymite
Classic worldwide localities
- Portola, Plumas Co., Pomona, Los Angeles Co., and at Bumpass Hill, Lassen National Park, Shasta Co., California, USA
- Mule Springs, 160 km northeast of Lakeview, Harney Co., Oregon, USA
- San Juan Mountains, San Juan Co., Colorado, USA
- Thomas Range, Juab Co., Utah, USA
- Cerro San Cristóbal, Pachuca, Hidalgo, Mexico
- Bellerberg volcano, two km north of Mayen, Eifel district, Germany
Field-hunting tip
Where it forms: Deposited from hot gases in vesicles and lithophysae; as phenocrysts in felsic volcanic rocks, and less commonly, in basalts; in contact metamorphosed sandstone. If you start seeing cristobalite, sanidine, quartz in float, you are in the right ground. Typical habit: platy, spherical. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify tridymite?+
Where is tridymite found?+
What rocks look like tridymite?+
What minerals are found with tridymite?+
What kind of rock does tridymite form in?+
What is tridymite used for?+
Find tridymite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
