Aspidolite is a rare sodium-dominant member of the trioctahedral mica group, chemically analogous to phlogopite. It typically occurs as small, transparent, platy crystals in metamorphosed dolomitic marbles, often difficult to distinguish from common phlogopite without chemical analysis.
Is this aspidolite?
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 aspidolite with a known reference. Aspidolite sits at Mohs 2-3, 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: pearly.
- 4Match the color rangeCompare against the expected color range: light brown, white, greenish, red.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: platy.
Often confused with
Aspidolite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Luster reads pearly on Aspidolite and splendent to submetallic, vitreous, pearly on cleavage on Biotite.
How to tell apart: Luster reads pearly on Aspidolite and vitreous to pearly or silky on Muscovite.
Often found alongside aspidolite
Minerals reported to co-occur with aspidolite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- NaMg₃(AlSi₃O₁₀)(OH)₂
- Mohs hardness
- 2-3
- Density
- 2.885 g/cm³
- Streak
- White
- Luster
- Pearly
- Transparency
- Translucent
- Crystal system
- Monoclinic
- Crystal habit
- Platy
- Cleavage
- Perfect on {001}
- Rarity
- Rare
- Uses
- Collector, Scientific research
- Host rock
- In thermally metamorphosed rocks (greenschist to amphibolite facies) in the contact aureole of a granite; in metamorphosed evaporite rocks (Algeria); from norite xenoliths (Chile)
Where rockhounds find aspidolite
Classic worldwide localities
- Derrag, Tell Atlas, Algeria and Volcán San Pedro, Andes Mountains, Chile
- Dumps of Kawai pit, Kasuga-mura, Gifu Prefecture, central Japan
- Zillertal, Tyrol, Austria
- North Qilian Mountains, Qinghai Province, northwestern China
Field-hunting tip
Where it forms: In thermally metamorphosed rocks (greenschist to amphibolite facies) in the contact aureole of a granite; in metamorphosed evaporite rocks (Algeria); from norite xenoliths (Chile). If you start seeing phlogopite, pargasite, magnesiosadanagaite in float, you are in the right ground. Typical habit: platy. Train your eye for that shape before scanning the outcrop.
Common questions
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Find aspidolite on the map
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