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.
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.
Troctolite 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.




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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.