Troctolite
Mafic intrusive igneous rock (plagioclase + olivine)
A mafic plutonic rock of plagioclase and olivine whose mottled appearance earned it the nickname 'troutstone'.
- Mohs hardness
- 6-7
- Color
- Mottled grey and dark green to black
- Type
- igneous
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Overview
Troctolite is a coarse-grained mafic intrusive rock composed essentially of calcium-rich plagioclase feldspar and olivine, with little or no pyroxene. The contrast between pale feldspar and dark, often greenish olivine gives it a speckled, blotchy appearance.
The name comes from the Greek for "trout stone" (troutstone), because the mottled grey-and-dark pattern resembles a trout's skin. Troctolite is essentially a member of the gabbro family in which olivine, rather than pyroxene, accompanies the plagioclase.
It is relatively uncommon and is most often found within layered mafic intrusions.
Formation & geology
Troctolite forms by the slow crystallization and accumulation of plagioclase and olivine crystals in deep, cooling mafic magma chambers. It typically occurs as cumulate layers within large layered intrusions such as the Stillwater Complex (Montana), the Bushveld Complex (South Africa), and the Duluth Complex (Minnesota).
It is associated with gabbro, norite, anorthosite, and dunite, marking specific stages of magmatic differentiation when olivine and calcic plagioclase crystallized together before significant pyroxene appeared. Some lunar samples are also troctolitic.
How to identify it
Look for a coarse-grained, heavy rock with a distinctive mottled or spotted texture of pale grey plagioclase and rounded dark green-to-brown olivine grains, and a general scarcity of pyroxene. Olivine often weathers to a dull brown or reddish rind.
Hardness is roughly 6-7. Distinguish troctolite from gabbro and norite by the dominance of olivine over pyroxene, from dunite by its abundant feldspar, and from anorthosite by its significant dark olivine content. Fresh olivine's glassy green grains are a strong clue.
Uses & significance
Troctolite has limited commercial use but is quarried locally as a decorative and dimension stone for its attractive spotted pattern. Some troctolite-bearing layered intrusions, such as the Duluth Complex, are explored for copper, nickel, and platinum-group elements.
It has no significant gemstone or metaphysical tradition, but it is scientifically valued for what it reveals about magma crystallization and is also recognized among lunar highland rocks.
Frequently asked questions
Why is troctolite called troutstone?
Its mottled pattern of pale feldspar and dark olivine resembles a trout's speckled skin, and the name derives from the Greek for trout.
What is the difference between troctolite and gabbro?
Troctolite contains plagioclase plus olivine with little pyroxene, whereas gabbro is dominated by plagioclase plus pyroxene.
Where is troctolite found?
Mainly in layered mafic intrusions such as the Stillwater, Bushveld, and Duluth complexes, and also among lunar rocks.
Is troctolite economically important?
Some troctolitic intrusions host copper-nickel and platinum-group metal deposits, though the rock itself has modest direct use.
Troctolite guides
In-depth guides for identifying, valuing, and understanding Troctolite.
