QUANTITATIVE MINERALOGY OF TOURMALINES FROM THE SPRUCE PINE PEGMATITE IN MITCHELL COUNTY, NORTH CAROLINA

A suite of tourmaline samples were collected from Spruce Pine, North Carolina and were analyzed using a microprobe in order to quantify the chemical makeup of the mineral. These tourmalines are from the border zone of a pegmatite composed of quartz, feldspar, mica, and garnet. The crystals themselves are very small but numerous and are intermixed with biotite. Previous literature on these minerals has classified them as both Fe end member schorl and Mg end member dravite depending on the source. Most of the schorl classification was related to the deep black color of the hand samples and the few references to dravite were a result of more detailed analyses. It is our contention that, in some ways, both are right. A detailed analysis under a microprobe showed that most of the properly oriented crystals exhibit complex zoning and the chemical variations among these zones is considerable. In fact, some of the zones on the outer portions of the crystal seem to have been altered and filled in with a new zone with distinct chemical variance. From the core of the crystals and through each zone the elemental variations we were looking for the most were between Fe and Mg. Throughout the entire crystal the amount of Fe ranged from around 5.5 weight percent to about 11 weight percent. Conversely, the Mg ranged from around 2 weight percent to about 8 weight percent. Essentially the Fe can deviate by about a 2:1 ratio where the deviation in Mg can be as much as 4:1 in the same crystal. Through the data collected and stoichiometric calculations we will define a generic formula for the core and some of the zones of the tourmaline crystals of the Spruce Pine pegmatite. It is also believed that if more tourmaline crystals in the pegmatite were examined, the variance among each crystal would be appreciable and the chemical analyses would reflect that these minerals would be somewhere in between the two end members of the solid solution series.