Dorrite is an extremely rare mineral belonging to the sapphirine group, primarily found in pyrometamorphic environments. It typically occurs as small anhedral grains in association with magnetite and other oxides, requiring advanced petrographic methods for positive identification.
Is this dorrite?
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 dorrite with a known reference. Dorrite sits at Mohs ~5, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: gray.
- 3Read the lusterHold the specimen under a strong light. Luster: submetallic.
- 4Match the color rangeCompare against the expected color range: brownish black, dark brown, reddish brown.
- 5Look at form & habitCrystal system: triclinic. Typical habit: anhedral grains, prismatic.
Often confused with
Dorrite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Sapphirine is the harder of the two (Mohs 7.5 vs. ~5); streak differs, so Dorrite leaves gray, Sapphirine leaves white; luster reads submetallic on Dorrite and vitreous on Sapphirine.
How to tell apart: Streak differs, so Dorrite leaves gray, Aenigmatite leaves reddish brown; luster reads submetallic on Dorrite and vitreous to greasy on Aenigmatite.
Often found alongside dorrite
Minerals reported to co-occur with dorrite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Ca₄(Mg₃Fe₉)O₄[Si₃Al₈FeO₃₆]
- Mohs hardness
- ~5
- Density
- 3.959 g/cm³
- Streak
- Gray
- Luster
- Submetallic
- Transparency
- Opaque
- Crystal system
- Triclinic
- Crystal habit
- Anhedral grains, prismatic
- Cleavage
- Good on {010} and {001}
- Rarity
- Rare
- Uses
- Collector
- Host rock
- A product of oxidizing, high-temperature, low-pressure metamorphism of alkalic rocks, in a pyrometamorphic zone in sediments
Where rockhounds find dorrite
Classic worldwide localities
- Durham ranch, Powder River basin, 13 km northeast of Reno Junction and 25 km south of Gillette, Campbell Co., Wyoming, USA
Field-hunting tip
Where it forms: A product of oxidizing, high-temperature, low-pressure metamorphism of alkalic rocks, in a pyrometamorphic zone in sediments. If you start seeing esseneite, titanian andradite, magnetite-magnesioferrite-spinel in float, you are in the right ground. Typical habit: anhedral grains, prismatic. Train your eye for that shape before scanning the outcrop.
Common questions
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Find dorrite on the map
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