Eastonite is a rare magnesium-rich mica belonging to the biotite group, often appearing as platy or foliated dark-colored crystals. It is predominantly found in contact-metamorphosed limestone or marble environments. Collectors prize it for its specific chemical composition which distinguishes it from more common phlogopite specimens.
Is this eastonite?
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 eastonite with a known reference. Eastonite sits at Mohs 2.5-3, 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: dark brown, black, greenish-brown.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: platy crystals, foliated masses, micaceous aggregates.
Often confused with
Eastonite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Luster reads pearly on Eastonite and splendent to submetallic, vitreous, pearly on cleavage on Biotite.
How to tell apart: Streak differs, so Eastonite leaves white, Annite leaves brownish white; luster reads pearly on Eastonite and vitreous on Annite.
Often found alongside eastonite
Minerals reported to co-occur with eastonite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- KAlMg₂(Si₂Al₂)O₁₀(OH)₂
- Mohs hardness
- 2.5-3
- Density
- 2.9-3.2 g/cm³
- Streak
- White
- Luster
- Pearly
- Transparency
- Translucent
- Crystal system
- Monoclinic
- Crystal habit
- Platy crystals, foliated masses, micaceous aggregates
- Cleavage
- Perfect basal
- Rarity
- Rare
- Uses
- Collector, Research
- Host rock
- Forms a series with siderophyllite and other biotite group micas
Where rockhounds find eastonite
Classic worldwide localities
- Easton, Pennsylvania, USA (type locality)
Field-hunting tip
Where it forms: Forms a series with siderophyllite and other biotite group micas. If you start seeing calcite, diopside, spinel in float, you are in the right ground. Typical habit: platy crystals, foliated masses, micaceous aggregates. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify eastonite?+
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Find eastonite on the map
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