Rilandite is a rare chromium-bearing silicate mineral typically found as thin crusts or films on fossil wood in sandstone. It is distinguished by its distinct resinous luster and deep reddish-brown color, most commonly identified in the Morrison Formation of Colorado.
Is this rilandite?
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 rilandite with a known reference. Rilandite 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: grayish brown.
- 3Read the lusterHold the specimen under a strong light. Luster: brilliant on fresh surface, otherwise dull, pitchy.
- 4Match the color rangeCompare against the expected color range: black.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: platy.
Often confused with
Rilandite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Jasper is the harder of the two (Mohs 6.5-7 vs. 2-3); streak differs, so Rilandite leaves grayish brown, Jasper leaves white; luster reads brilliant on fresh surface, otherwise dull, pitchy on Rilandite and waxy on Jasper.
How to tell apart: Streak differs, so Rilandite leaves grayish brown, Cinnabar leaves scarlet; luster reads brilliant on fresh surface, otherwise dull, pitchy on Rilandite and adamantine, inclining to metallic when dark; dull in friable material on Cinnabar.
Often found alongside rilandite
Minerals reported to co-occur with rilandite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Cr₆SiO₁₁·5H₂O
- Mohs hardness
- 2-3
- Density
- 2.9-3.1 g/cm³
- Colors
- Streak
- Grayish brown
- Luster
- Brilliant on fresh surface, otherwise dull, pitchy
- Transparency
- Translucent
- Crystal system
- Monoclinic
- Crystal habit
- Platy
- Cleavage
- None
- Rarity
- Rare
- Uses
- Collector
- Host rock
- On the outer surface and in shallow recesses of a petrified log in sandstone
Where rockhounds find rilandite
Classic worldwide localities
- Riland uranium claim, about 21 km east-northeast of Meeker, Rio Blanco Co., Colorado, USA
Field-hunting tip
Where it forms: On the outer surface and in shallow recesses of a petrified log in sandstone. If you start seeing carnotite, organic matter in float, you are in the right ground. Typical habit: platy. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify rilandite?+
Where is rilandite found?+
Is rilandite safe to handle?+
What rocks look like rilandite?+
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Find rilandite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
