Plazolite Identification Guide
A field guide to plazolite, a hydrous calcium-aluminum hydrogrossular garnet, covering its pale color, dodecahedral habit, hardness, and how to separate it from grossular and vesuvianite.
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What Plazolite Looks Like
Plazolite is a hydrous member of the hydrogrossular garnet group (a calcium-aluminum garnet with hydroxyl replacing some silica). It is essentially a variety name now grouped with katoite/hydrogrossular. Specimens are typically colorless, white, pale gray, or faintly greenish or pinkish, with a vitreous to slightly resinous luster and translucent to transparent appearance. It commonly forms small dodecahedral or trapezohedral garnet crystals, or massive, granular aggregates resembling jade-like material.
Step-by-Step Field ID Checklist
- Recognize the garnet form. Look for equant, isometric crystals — rounded dodecahedra or trapezohedra without cleavage.
- Note the pale color. Unlike typical red garnets, plazolite is colorless to pale, an immediate hint toward the hydrogrossular group.
- Check the geologic setting. Plazolite forms in contact-metamorphosed (skarn) limestones and altered calc-silicate rocks; setting strongly supports the ID.
- Test hardness. Scratches glass easily but is slightly softer than common garnet.
- Confirm no cleavage. Garnets fracture conchoidally rather than cleaving.
Key Diagnostic Tests
- Mohs hardness: About 6 to 7, somewhat lower than anhydrous garnet due to water content.
- Specific gravity: Roughly 3.1 to 3.4, lower than dense red garnets (~3.6–4.3).
- Cleavage: None; conchoidal to uneven fracture.
- Streak: White.
- Acid: No fizz (distinguishes from the calcite matrix it often sits in).
- Optics: Often isotropic to weakly anomalously birefringent.
Common Look-Alikes and How to Tell Them Apart
- Grossular garnet: Chemically related; true grossular is denser and harder, while plazolite's lower SG and hydrous chemistry distinguish it (lab testing is often needed).
- Vesuvianite (idocrase): Forms in the same skarns but is tetragonal with prismatic crystals and shows weak birefringence, versus plazolite's isometric crystals.
- Jadeite / hydrogrossular "jade": Massive plazolite can mimic jade; jadeite is tougher and shows a fibrous interlocking texture.
- Calcite: Softer (3) and fizzes in acid; plazolite does not react.
Where Plazolite Is Found
Plazolite was first described from Crestmore, Riverside County, California, in contact-metamorphic skarn at the limestone-igneous boundary. Similar hydrogrossular garnets occur in skarns and rodingites worldwide, including South Africa (Transvaal "jade"), Italy, and Russia.
Frequently asked questions
What is plazolite?
Plazolite is a hydrous calcium-aluminum garnet of the hydrogrossular group, now generally regarded as a variety related to katoite and hydrogrossular. It forms pale, glassy garnet crystals in skarns.
How can you tell if it's real plazolite?
Look for pale, equant garnet crystals with no cleavage, hardness near 6 to 7, lower density than red garnet, no acid reaction, and a skarn or contact-metamorphic setting. Definitive confirmation usually needs lab analysis.
Plazolite vs grossular garnet: what is the difference?
Both are calcium-aluminum garnets, but plazolite is hydrous, with hydroxyl replacing some silica, giving it lower density and slightly lower hardness than anhydrous grossular.
Where is plazolite found?
It was first described at Crestmore, California, in contact-metamorphic skarn, and similar hydrogrossular garnets occur in skarns and rodingites worldwide.