Troctolite

Troctolite is a mafic intrusive rock type. It consists essentially of major but variable amounts of olivine and calcic plagioclase along with minor pyroxene. It is an olivine-rich anorthosite, or a pyroxene-depleted relative of gabbro. However, unlike gabbro, no troctolite corresponds in composition to a partial melt of peridotite. Thus, troctolite is necessarily a cumulate of crystals that have fractionated from melt.

Troctolite is found in some layered intrusions such as in the Archean Windimurra intrusion of Western Australia, the Voisey's Bay nickel-copper-cobalt magmatic sulfide deposit of northern Labrador, the Stillwater igneous complex of Montana and the Tertiary Rhum layered intrusion of the island of Rùm, Scotland. Troctolite is also found in the Merensky Reef of the Bushveld Igneous Complex, South Africa and in the Lizard complex in Cornwall.

References

Troctolite 76535

Troctolite 76535 is a lunar sample discovered and collected on the Apollo 17 mission in 1972 in the Taurus–Littrow valley. It has a mass of about 156 grams and is about 5 centimeters across at its widest point. It was collected as part of a "rake sample" of lunar soil at Geology Station 6, near the base of the North Massif.

Description

Troctolite 76535 is a coarse-grained plutonic rock that is believed to have had a slow cooling history. The rock originates from early in the Moon's history. Geologists have described it as a coarse-grained olivine-plagioclase cumulate with a granular polygonal texture.

Olivine and plagioclase are of about equal quantities within 76535, while the remaining approximate 4% is made up of primarily orthopyroxene. Studies have shown that the rock is plutonic in origin and originates from about the middle to lower crust of the Moon.

Investigations have shown that the rock may have formed as a cumulate at depth, thus possibly making the sample an important link in the understanding of the geologic timeline of the Moon.