Troctolite is a coarse-grained intrusive igneous rock. It has mainly variable amounts of olivine, calcic plagioclase, and minor pyroxene.
Its composition lies between anorthosite, where feldspar is dominant, and dunite rock, where olivine dominates. It is like a gabbro depleted of pyroxene or anorthosite rich in olivine.
The word troctolite comes from the Greek words τρώκτης, which means trout, and λίθος, stone, i.e., trout stone.
This name describes brownish, yellowish, black, green, or reddish spots of whole or altered olivine in a pale plagioclase background. It resembles the skin of a trout.
In Germany, Forellenstein is the name for a spotted troctolite variety.
_1.jpg){kind=link}
Description and appearance
Troctolite is a coarse-grained, dark-colored rock. It is usually dark gray with a mottled appearance of light-colored calcic plagioclase and greenish-orangish olivine specs. Also, it may have a bit smaller, darker specs of pyroxene.
Usually, it is much lighter than gabbro and has a more greenish hue due to the high olivine. Serpentinization of olivine creates orangish specs.
Composition and mineralogy
Troctolite is a mafic (iron and magnesium-rich) rock. It has 45-52 wt.% silica (SiO2).
Mineralogically, troctolite has mainly variable amounts of olivine and calcium-rich plagioclase. Also, it has none to a small amount of pyroxene. Olivine may be partly or fully altered.
Other minerals this rock may have include hornblende and accessory minerals like chromian spinel and ilmenite.
The calcic plagioclase is usually high in anorthite, i.e., labradorite or bytownite, while olivine is iron-rich.
This rock is essentially gabbroid. In gabbroids, plagioclase is more than 65% of the total feldspars. Also, these rocks have less than 10% foids or less than 20% quartz of the QAPF content. Quartz and foids don’t coexist.
Using a (Px), clinopyroxene (Clx), Orthopyroxene (Oxp), and olivine (Ol) ternary classification by volume, it has mostly olivine and plagioclase with clinopyroxene less than 5%.
Increasing pyroxene content and reducing olivine will grade this rock into gabbro.
How does it form?
Troctolite is an intrusive igneous rock. It most likely forms from cumulate crystals from magma fractional crystallization. Partial melting of peridotites will not produce magma of such composition.
Where does troctolite occur?
It occurs mainly as part of layered intrusions. It is associated with gabbros, anorthosites, and ultramafic rocks like dunite and other peridotites.
However, it can also occur in lopoliths. Also, it occurs in mid-ocean ridges and the moon.
1. Layered intrusions
Troctolites occur mostly in layered intrusions together with other mafic and ultramafic rocks. These intrusions include the Stillwater igneous complex in Montana and Duluth in Minnesota, USA. In Canada, it occurs at Voisey’s Bay in North Labrador among the magmatic sulfides.
Also, these rocks occur in the Windimurra Igneous Complex in Australia, Rùm’s Island Tertiary Rhum layered intrusion in Scotland, Merensky Reef at Bushveld Igneous Complex in South Africa, and Lizard Complex in Cornwall, UK.
2. Mid-ocean ridges
A few samples have been collected at mid-ocean ridges, notes a study. These are ocean-spreading centers or divergent ridges that create new oceanic floors.
These ridges include the Mid-Cayman Rise, East Pacific Rise, Hess Deep Rift, Southwest Indian Ridge, and Mid-Atlantic Ridge.
Samples from Hess Deep Rift at the East Pacific Rise were fine and coarse-grained.
Also, xenoliths of troctolite occur in the Mid Ocean Ridge basalts (MORB). Xenoliths are pieces of rocks trapped in different rocks.
3. Occurs on the moon
Troctolite occurs on the moon’s lunar highlands. Samples collected from the moon by Apollo missions confirm it.
These rocks are coarse-grained and probably crystallized beneath the moon’s surface. However, later, meteorite impacts excavated them.
Uses
Troctolite is a durable rock with hardness comparable to granite or gabbro. Uses include making aggregate for roading, construction or concrete, dimensional stones, and landscaping or garden decoration.
As a dimensional stone, you can use it in paving, building, ashlars (cladding), countertops, etc.
Frequently Asked Questions (FAQs)
Troctolite is slightly lighter in color and has less to no pyroxene than gabbro. Also, it has greenish to orangish speckles, while gabbro is mostly grayish or blackish but can also be dark green.