Troctolite Identification Guide
Identify troctolite, the 'trout-stone' gabbro, by its speckled olivine-and-plagioclase texture, coarse grain, mineral hardness, and how it differs from gabbro and norite.
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What Troctolite Looks Like
Troctolite is a coarse-grained, intrusive (plutonic) mafic igneous rock — a variety of gabbro composed essentially of plagioclase feldspar and olivine, with little or no pyroxene. Its name comes from the Greek for "trout," because the dark olivine spots on the pale feldspar background give it a speckled, trout-like appearance.
- Color: Mottled — light gray to white plagioclase with dark green, brown, or black olivine spots; weathered olivine turns rusty/orange.
- Luster: Dull to vitreous on fresh mineral grains.
- Transparency: Opaque.
- Texture: Phaneritic (coarse, visible interlocking crystals); equigranular speckled appearance.
Field-ID Checklist
- Confirm coarse, interlocking crystals visible to the naked eye (plutonic rock).
- Identify two dominant minerals — pale plagioclase plus dark olivine.
- Look for the speckled 'trout' pattern of dark spots on a light matrix.
- Check for absence/scarcity of pyroxene (which would make it gabbro/norite).
- Note rusty weathering of olivine grains.
- Test mineral hardness — both main minerals scratch glass (6.5–7).
Key Diagnostic Tests
- Mohs hardness: Olivine ~6.5–7, plagioclase ~6–6.5; the rock as a whole scratches glass.
- Streak: White to colorless from the silicate minerals.
- Cleavage/fracture: Plagioclase shows two good cleavages with striations (twinning); olivine has poor cleavage and conchoidal fracture and a granular, sugary look.
- Density: Heavy (mafic, olivine-rich), higher than granite.
- Acid test: No reaction (silicate rock).
- Magnetism: May be weakly magnetic if accessory magnetite is present.
Common Look-Alikes
- Gabbro: The parent family; gabbro contains abundant pyroxene with plagioclase, while troctolite is defined by olivine + plagioclase and little pyroxene.
- Norite: A gabbroic rock with orthopyroxene; lacks troctolite's dominant olivine.
- Anorthosite: Almost entirely plagioclase with very few dark minerals; troctolite has clear dark olivine speckles.
- Dunite/peridotite: Olivine-dominated and much darker/greener overall, with little plagioclase.
- Diorite: Lighter, with hornblende/biotite and sodic plagioclase, lacking olivine.
Where It's Found
Troctolite occurs in layered mafic intrusions and gabbroic complexes. Notable localities include the Stillwater Complex (Montana, USA), the Duluth Complex (Minnesota), the Bushveld Complex (South Africa), Scotland (the Isle of Rum), and many ophiolite and lunar settings — troctolite is a well-known rock type among lunar samples returned by Apollo missions.
Frequently asked questions
How can you tell if it's real troctolite?
Troctolite is a coarse-grained plutonic rock made of pale plagioclase feldspar and dark olivine with little pyroxene, producing a speckled trout-like pattern. The grains are large enough to see, scratch glass, and olivine areas often weather rusty.
What does troctolite look like?
It looks like a light gray to whitish rock peppered with dark green to black or rusty spots of olivine, resembling the speckled skin of a trout — hence its name.
What is the difference between troctolite and gabbro?
Both are coarse mafic plutonic rocks, but gabbro contains significant pyroxene alongside plagioclase, while troctolite is defined by plagioclase plus olivine with little or no pyroxene.
Why is troctolite called trout stone?
The name derives from the Greek word for trout, because the scattered dark olivine grains on the pale feldspar background resemble the mottled, spotted skin of a trout.