Troctolite is an intrusive igneous rock consisting of plagioclase feldspar and olivine. It is a member of gabbroic rocks family. It is compositionally similar to gabbro. The main difference is that it does not contain pyroxene or contains very little while it is a major mineral in gabbro. It can be described as pyroxene-depleted gabbro.

Troctolite is an olivine-bearing gabbroic rock without pyroxene. Gray mineral is Ca-rich plagioclase, orange is olivine. Olivine has lost its original green color due to weathering. Orange spots are composed of various weathering products of olivine which is collectively known as iddingsite. Flakstadøya, the Lofoten Archipelago, Norway. Width of sample 15 cm.

Classification diagram of olivine-bearing gabbroic rocks. The field of troctolite is annotated. The rock contains variable amount of plagioclase and olivine (10-90%) but very small amounts (less than 10%) of pyroxene¹.

The genesis of troctolite is also strongly tied to gabbro. These two rocks usually occur together in the same magma intrusion because troctolite can not form directly as magma crystallizes. There is no way how magma can crystallize into olivine and plagioclase without pyroxene. Pyroxene crystals must be separated from the melt by a mechanism that leads to the formation of cumulate rocks. In these rocks some minerals occur in much higher (or lower) concentration than expected. Anorthosite is an example of cumulate rock which is also a member of the same gabbro family. Anorthosite is composed of almost pure plagioclase. Troctolite may be similar to anorthosite and these rocks may smoothly grade into each-other as demonstrated below.

Anorthosite on the left and troctolite on the right. Flakstadøya, the Lofoten Archipelago, Norway.

This sample can be described as coarse-grained leuco-troctolite. Flakstadøya, the Lofoten Archipelago, Norway. Width of sample 12 cm.

Here is a sample of olivine gabbro from the same intrusion on Flakstadøya. The difference is that this rock contains lots of black pyroxene augite. Width of sample 12 cm.

An outcrop of mafic intrusion on Flakstadøya. The rock surface cropping out is troctolite. The layer of the rock is several hundred meters wide. It grades to anorthosite (in front) and gabbro (behind the scene).

A closer look of the same outcrop reveals the spotted gray-orange appearance of the rocky surface.

Troctolite as demonstrated above has a speckled appearance which is the reason it carries such a name – troctolite means troutstone in Greek. ‘Troutstone’ is a synonym of troctolite in English and German (forellenstein) also².

References

1. Le Maitre, R. W. (2005). Igneous Rocks: A Classification and Glossary of Terms: Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks, 2nd Edition. Cambridge University Press.
2. Jackson, J. A. (1997). Glossary of Geology, 4th Edition. American Geological Institute.