THE CHEMISTRY AND CRYSTALLOGRAPHY OF ALMANDINE GARNETS FROM EMERALD CREEK, IDAHO

Petrographic and electron microprobe analyses were performed on garnets from the Emerald Creek area of Northern Idaho. The garnets are found within a biotite schist (metamorphosed Wallace formation) and are mined commercially both for their gem quality and for industrial uses. This locality is part of the same syncline that contains the site locality for the Idaho state gemstone, a six star garnet.

The Wallace formation schist and phyllite contains muscovite, biotite, quartz, garnet and magnetite with lesser amounts of chlorite and staurolite. The samples (a combination of pre- and syn-deformational garnets) were selected from a similar stratigraphic unit within one major syncline. The purpose of the study is to characterize geochemical changes along the plunging syncline and the consequent changes in the garnet structure. Preliminary results have established the bulk geochemistry of the garnets, and indicate compositional inhomogeneity across the syncline. Many garnets within the study show deformational features such as fractures and are slightly oxidized on their rims. These were not analysed as part of the study. The garnets belong to the almandine series and petrographically are uniformly isotropic - with rare exceptions they show no visible zoning. Both pre- and syn-deformational garnets are compositionally identical. Electron microprobe analyses show an antipathetic relationship between Mn and Fe. In all the garnets studied the cores were rich in Mn whereas the rims were relatively enriched in Fe and Mg as is characteristic of regional, medium to high-grade metamorphism of pelitic rocks. Ca zoning is also present with higher Ca values towards the rims. Preliminary two-mineral geothermometry suggests temperatures between 500-600ºC with the temperature increasing throughout garnet growth.