Brasstown Creek, Young Harris, Towns County, Georgia, USAi

Regional Level Types
Brasstown Creek	Creek
Young Harris	Village
Towns County	County
Georgia	State
USA	Country

TROCTOLITE
(Brasstown Creek, 1 3/4 miles North of Young Harris)

Megascopic characters. - This rock has a dark, yellowish-green color and a massive structure. With unaided eye it can be seen to be composed of yellowish masses of olivine grains surrounded by a fine-grained, greenish ground mass, the individual minerals of which can not be recognized.

Microscopic characters. - Olivine appears to be the only original mineral in this section. It occurs as large grains which have been much crushed, and, in part, recrystallized. The fractures are filled with a central stringer of magnetite and separated from the olivine by greenish-yellow serpentine. At places the magnetite is collected into masses of considerable size. The olivine grains are commonly surrounded by a band of short, stocky, crystals of actinolite arranged with the crystallographic c axis sometimes parallel, but usually perpendicular to the surface of the enclosed olivine. A small amount of enstatite is present in the place of actinolite. In the spaces between the olivine grains, and enclosed by a layer of actinolite, are fine-grained, granular minerals, the exact nature of which could not be determined. These areas were formerly occupied by feldspar, only traces of which remain, and are believed to be formed of actinolite, enstatite, etc. Some large leaves of what appears to be chlorite are found associated with these fine-grained minerals. Both pleonaste and picotite occur, often forming the same individual.

A somewhat fresher sample of this rock has been described as follows:

"The section is over half composed of olivine, which is slightly altered to yellowish serpentine, especially along the cleavage cracks. The serpentine contains numerous magnetite granules, particularly along the central portion, making the position of the original crack. Plagioclase feldspar (probably anorthite) occurs in only a few large remnants. Large areas of it have been entirely replaced by the corrosion products. Occasionally grains of translucent picotite occur.

"The corrosion mantles are only two-ply in this rock, and consist of (1) enstatite in broad plates next to the olivine, usually nearly colorless; (2) pale-green monoclinic amphiboles, in slender needles and narrow laths, usually slightly pleochroic in greenish and yellowish tones and always associated with much pleonaste in parallel arrangement and irregularly intermingled. As olivine preponderates over the feldspar in this rock, the latter is often entirely replaced by the corrosion minerals. In such cases, the 'nests' of minerals replacing the feldspar consist, in some cases, of enstatite alone; in others, enstatite forms the border next to the olivine, with amphibole in the center. Picotite grains surrounded by the corrosion minerals show a gradual transition from dark-brown color in the central portion to the light-green of the pleonaste about the borders; among the olivine, picotite does not show this character."