Troctolite

Troctolite, from Greek "troctos" and "lithos" = trout rock due to the look "spotted", due to the presence of olivine crystals, consists essentially of major but variable amounts of olivine and calcic plagioclase along with variable minor pyroxene. It is an olivine-rich, pyroxene-depleted relative of gabbro. However, unlike gabbro, no troctolite corresponds in composition to a partial melt of peridotite. Thus, troctolite is ecessarily a cumulate of crystals that have fractionated from melt. The olivine may be wholly altered to serpentine, which gives the rock a stippled appearance of red, green, brown, yellow, and black spots on a pale background, reminiscent of the skin of a trout. Serpentine alteration is common in plutonic or metamorphic rocks, it being along fracture, as the result of hydrothermal or deuteric processes and may completely convert olivine rocks in a serpentine rock, with a structure characteristic of the olivine fracture pattern ("mesh structure").

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Altered olivine (yellow-orange) in a troctolite from Lofoten Archipelago, Norway. From (Sandatlas). Lødingen, Hinnøya, Norvegia.

Altered olivine (yellow-orange) in a troctolite from Lofoten Archipelago, Norway. From (Sandatlas). Lødingen, Hinnøya, Norvegia.

Bibliography

• Cox et al. (1979): The Interpretation of Igneous Rocks, George Allen and Unwin, London.
• Howie, R. A., Zussman, J., & Deer, W. (1992). An introduction to the rock-forming minerals (p. 696). Longman.
• Le Maitre, R. W., Streckeisen, A., Zanettin, B., Le Bas, M. J., Bonin, B., Bateman, P., & Lameyre, J. (2002). Igneous rocks. A classification and glossary of terms, 2. Cambridge University Press.
• Middlemost, E. A. (1986). Magmas and magmatic rocks: an introduction to igneous petrology.
• Shelley, D. (1993). Igneous and metamorphic rocks under the microscope: classification, textures, microstructures and mineral preferred-orientations.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press.

XPL image, 2x (Field of view = 7mm)

Photo
Olivine partially altered in serpentine (note the radial fracture due to increases of volume for serpentinization). PPL image , 2x (Field of view = 7mm)	Olivine partially altered in serpentine (note the radial fracture due to increases of volume for serpentinization). XPL image , 2x (Field of view = 7mm)	Olivine partially altered in serpentine (note the radial fracture due to increases of volume for serpentinization). PPL image , 2x (Field of view = 7mm)
Olivine partially altered in serpentine (note the radial fracture due to increases of volume for serpentinization). XPL image , 2x (Field of view = 7mm)	Olivine partially altered in serpentine (note the radial fracture due to increases of volume for serpentinization). PPL image , 2x (Field of view = 7mm)	Olivine partially altered in serpentine (note the radial fracture due to increases of volume for serpentinization). XPL image , 2x (Field of view = 7mm)
Olivine partially altered in serpentine (note the radial fracture due to increases of volume for serpentinization). XPL image , 2x (Field of view = 7mm)	Olivine partially altered in serpentine (note the radial fracture due to increases of volume for serpentinization). PPL image , 2x (Field of view = 7mm)	Olivine partially altered in serpentine (note the radial fracture due to increases of volume for serpentinization). PPL image , 10x (Field of view = 2mm)
Olivine partially altered in serpentine (note the radial fracture due to increases of volume for serpentinization). PPL image , 2x (Field of view = 7mm)	Olivine partially altered in serpentine (note the radial fracture due to increases of volume for serpentinization). XPL image , 2x (Field of view = 7mm)	Olivine partially altered in serpentine (note the radial fracture due to increases of volume for serpentinization). XPL image , 2x (Field of view = 7mm)
Olivine partially altered in serpentine (note the radial fracture due to increases of volume for serpentinization). PPL image , 2x (Field of view = 7mm)	Olivine partially altered in serpentine (note the radial fracture due to increases of volume for serpentinization). XPL image , 2x (Field of view = 7mm)	Olivine partially altered in serpentine (note the radial fracture due to increases of volume for serpentinization). PPL image , 2x (Field of view = 7mm)
Olivine partially altered in serpentine (note the radial fracture due to increases of volume for serpentinization). XPL image , 2x (Field of view = 7mm)	Olivine partially altered in serpentine (note the radial fracture due to increases of volume for serpentinization). PPL image , 2x (Field of view = 7mm)	Olivine partially altered in serpentine (note the radial fracture due to increases of volume for serpentinization). XPL image , 2x (Field of view = 7mm)
Olivine partially altered in serpentine (note the radial fracture due to increases of volume for serpentinization). XPL image , 2x (Field of view = 7mm)	Olivine partially altered in serpentine (note the radial fracture due to increases of volume for serpentinization). XPL image , 2x (Field of view = 7mm)	Olivine partially altered in serpentine (note the radial fracture due to increases of volume for serpentinization). XPL image , 2x (Field of view = 7mm)