MINERALOGY OF MARBLE FROM THE J. M. HUBER MINE, GEORGIA MARBLE DISTRICT

Representative samples of Georgia marble from the J. M. Huber mine were collected by members of the UGA Geology Club in the spring of 2011 and studied by the Earth Materials class in the fall of 2011. The purpose of the study is to document the mineral assemblages in the marble and to provide introductory mineralogy students a chance to practice their new skills on a research project. Most of the samples are white, layers of greenish-gray marble occur in some samples and one sample is orange. Commercially prepared polished thin sections were examined with the petrographic microscope and analyzed on the electron microprobe.

A granoblastic texture of carbonate grains (90 to 99+ modal %) dominates the marble. Round grains of quartz (relict quartz sand ?) form distinct layers in the marble. Amphibole, chlorite, phlogopite, colorless white mica, and plagioclase occur in the marble. A mineral identified in hand sample as garnet proved to be humite. Zoisite and epidote are less common accessory minerals. Reflected light examination revealed oxides and sulfide phases in most of the samples.

All samples contain calcite and dolomite. Fine-grained inclusions of one carbonate in another a is reminiscent of exsolution. Compositions of the carbonates are fairly uniform (calcite XMg = 0.02 to 0.06; Xfe = 0.002 - 0.006) dolomite is near end-member, but contains some Fe (dolomite XFe = 0.05 to 0.20). Amphibole compositions cluster around the tremolite-actinolite-magnesiohornblende field boundaries (Si = 6.8 to 7.9 apfu, mg# = 84 -99). Chlorite is Mg-rich and slightly more aluminous than clinochlore (Al = 4.0 to 5.2 apfu). Humite is the Mg-rich variety, norbergite (MgO = 55-56wt % MgO). Hematite (2.0 -2.7 wt % TiO2) , pyrite, pyrrhotite, and rare sphalerite occur in the marble.

Calcite-dolomite geothermometry revealed temperatures in the range of 368 to 646̊C, consistent with the amphibolite facies mineral assemblages (e.g. calcite + dolomite + phlogophite + tremolite) in the marble. Lower temperatures correspond to the greenschist facies and are probably related to the observed exsolution in some carbonate grains. These temperatures are consistent with the occurrence of staurolite, garnet, kyanite prograde assemblages found in pelitic schists in the area.