Bethlehem Church, Elbert County, Georgia, USAi

Regional Level Types
Bethlehem Church	School
Elbert County	County
Georgia	State
USA	Country

HORNBLENDE GABBRO
(Bethlehem Church, 13 Miles East of Elberton)
Megascopic characters. - This rock outcrops at a number of places along the public road near Bethlehem Church, where it shows a wide variation in the amount of alteration. In a hand specimen, this rock is seen to contain green hornblende dotted regularly with a dull white mineral. It is a massive to very slightly gneissose rock of porphyritic structure. Hornblende phenocrysts as much as 12 millimeters in length occur here and there as equidimensional to irregular shaped grains. In the hand specimen the rock has the general appearance of a porphyritic diorite.

Microscopic characters. - In this section (No. H2) this rock is seen to contain the following minerals named in the order of their importance: hornblende, zoisite, epidote and plagioclase (bytownite, or anorthite), titanite and a small amount of iron oxide. The amphibole, which shows a maximum extinction angle of 22°, occurs in large poikilitic phenocrysts enclosing titanite, epidote and exceptionally masses of partially decomposed feldspar, and also as small individuals arranged in all directions in respect to each other and mutually interfering with one another. It shows a pale-green body color and a moderate amount of pleochroism. It appears to be entirely secondary, and, in part, derived from the feldspar. Only traces of original feldspar crystals remain, their decomposition having given rise to epidote, zoisite and hornblende. The epidote and zoisite form masses of considerable size, which are composed of numerous irregular shaped individuals; they are also contained as inclusions in the feldspar in the form of small particles. Titanite is present in much larger quantities than is suggested in the accompanying analysis; it occurs as fine grains scattered through the hornblende as well as included in the epidote-zoisite masses. It shows at no place its crystal outline and has been recrystallized. and at places segregated into masses of considerable size.

A rock from the same locality, although somewhat more altered from it original nature may be described as follows:

The rock is seen to be composed largely of hornblende, identical with that described in thin section No. H2, but it is in the process of alteration to chlorite and talc. The hornblende contains'inclusions of titanite and magnetite, of which the latter was practically absent in the previous section. Chlorite occurs as small foliae arranged in all directions in the hornblende, from which it was derived, although showing a 'tendency toward arrangement of the long direction of the leaves in parallel position with the cleavage of the hornblende; it is optically positive with a very small optical angle. Optically, the mineral is very close to pennine. Talc is in the process of formation from the hornblende; it occurs either as massses around its edge and more particularly as foliae along the cleavage lines of the hornblende where the cleavages of the two minerals are parallel. While the chlorite shows a sharp boundary separating it from the hornblende, the talc is separated from the latter by a transition zone and is later in origin than the chlorite, having the appearance of being, in part, derived from it. Besides the minerals mentioned above, a small amount of epidote was seen.


HORNBLENDE GABBRO (ALTERED)
Megascopic characters. - This rock is similar in general appearance to other diorites of the region except that it is more altered. It is medium to fine grained and massive to very slightly schistose. Light greenish grains of epidote appear to be second in abundance to the fine grains of almost black hornblende. Pyrite is quite abundant, while feldspar is scarcely recognizable.

Microscopic characters. - In thin section (No. H86) this rock is seen to contain the following minerals arranged in the order of their abundance: hornblende, epidote, plagioclase, quartz, zoisite, chlorite, pyrite, iron oxide and leucoxene.

The plagioclase is too much altered to permit the determination of its variety, but it belongs to the basic end of the series, and is probably labradorite. Its alteration has lead largely to the formation of epidote and zoisite, with the concomitant separation of silica in the form of quartz. Some hornblende was also formed from the plagioclase; it occurs as ragged poikilitic aggregates enclosing quartz from fine particles up to grains of fair size. It also encloses pyrite and iron oxide in large quantities. Its pleochroism is very strong: a =bluish-green, b =yellowish-green to dark green, and c =pale yellow to almost colorless. It gives evidence of being entirely of secondary origin. The quartz occurs in anhedral, irregular-shaped grains enclosed in, and associated with, both the plagioclase and the hornblende. Its presence in such quantities is very exceptional in these rocks so far as studied. Epidote greatly predominates over zoisite, and the two are found evenly scattered with the quartz and hornblende through the rock section. Chlorite is sparingly developed in the form of spherulitic aggregates, as well as in foliae, and is formed from the alteration of hornblende. Pyrite, in the process of alteration to iron oxide, is very abundant.