Rectorite is a rare regular mixed-layer clay mineral composed of alternating layers of mica and smectite. It typically occurs as soft, pearly, micaceous masses or coatings and is most famously associated with hydrothermal environments and altered igneous rocks.
Is this rectorite?
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 rectorite with a known reference. Rectorite sits at Mohs <1, 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 to greasy when massive.
- 4Match the color rangeCompare against the expected color range: white, yellowish white.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: earthy.
Often confused with
Rectorite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Muscovite is the harder of the two (Mohs 2.5-4 vs. <1); luster reads pearly to greasy when massive on Rectorite and vitreous to pearly or silky on Muscovite.
How to tell apart: Luster reads pearly to greasy when massive on Rectorite and earthy on Illite.
How to tell apart: Luster reads pearly to greasy when massive on Rectorite and dull, earthy on Montmorillonite.
Often found alongside rectorite
Minerals reported to co-occur with rectorite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- (Na,Ca)Al₄(Si,Al)₈O₂₀(OH)₄·2H₂O
- Mohs hardness
- <1
- Density
- 2.34 g/cm³
- Colors
- Streak
- White
- Luster
- Pearly to greasy when massive
- Transparency
- Translucent
- Crystal system
- Monoclinic
- Crystal habit
- Earthy
- Cleavage
- Perfect on {001}
- Rarity
- Uncommon
- Uses
- Collector, Industrial
- Host rock
- In some low-temperature hydrothermal argillic alteration zones, along veins and pervasively replacing potassic feldspar
Where rockhounds find rectorite
Classic worldwide localities
- Blue Mountain district, Garland Co., and in the Jeffrey quarry, North Little Rock, Pulaski Co., Arkansas, USA
- Buckeye Gulch, eight km from Leadville, Lake Co., Colorado, USA
- Fort Sandeman district, Baluchistan Province, Pakistan
Field-hunting tip
Where it forms: In some low-temperature hydrothermal argillic alteration zones, along veins and pervasively replacing potassic feldspar. If you start seeing quartz, cookeite, ankerite in float, you are in the right ground. Typical habit: earthy. Train your eye for that shape before scanning the outcrop.
Common questions
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Find rectorite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
