File:Troctolite (Laramie Anorthosite Complex, Mesoproterozoic; Sybille Pit, Laramie Range, Wyoming, USA) 1.jpg

English: Troctolite from the Precambrian of Wyoming, USA.

This rock is from the Sybille Pit, an old, inactive titanium mine in Wyoming's Laramie Range. A black-colored, iron- and titanium-oxide rock, known as nelsonite, was the mining target. This occurrence is called the Sybille Fe-Ti Oxide Deposit. The mine is in a troctolite body that intruded the host anorthosite of the region. The nelsonite occurs within the troctolite.

The Sybille Pit has: 1) host anorthosite 2) intruded troctolite 3) Fe-Ti oxide concentrations 4) composite dikes 5) granite dikes

Troctolite at the Sybille Pit has plagioclase feldspar and olivine (= brownish-colored on weathered surfaces) and apatite (= pale green to tan/buff-colored & can see some hexagonal crystal cross-sections), a little pyroxene, and a little biotite. It has fewer mafics than normal troctolites, so this occurrence is often called "leucotroctolite", but it's not light-colored. A high plagioclase content makes it dark-colored. The plagioclase is An-45 and the olivine is Fa-60. Xenoliths (some very large) of the host anorthosite occur in the troctolite. The troctolite is a post-Laramie Anorthosite intrusion, but is dated close to the host rocks. The floor of the Sybille Pit is the floor of the troctolite body, which is small and blob-like (a sill?). Plagioclase crystals in the troctolite have nice striations in places. Some cavities are filled with secondary zeolite and prehnite.

Host rocks: Laramie Anorthosite Complex, early Mesoproterozoic, 1.431 to 1.436 Ga

Locality: Sybille Pit, northern side of Sybille Canyon, southern Laramie Range, southeastern Wyoming, USA (41° 46' 42.59" North latitude, 105° 21' 05.77" West longitude)