The origin of asterism in almandine-pyrope garnets from Idaho

Abstract

We have shown that the asterism in garnets emanating from Emerald Creek, Latah County, Idaho, is due to the precipitation of acicular crystals in the garnet crystal host. These inclusions have been identified, using electron microscopy, as tetragonal rutile, the stable phase of TiO₂. One set of rutile (r) needles was found to have the following crystallographic relationship with its garnet (g) host: $[011]_g /\!/[0\bar 10]_r,(02\bar 2)_g /\!/(100)_r,(\bar 400)_g /\!/(001)_r$. From meticulous examinations of the asteriated garnets and the angular relationships between the rutile inclusions and the garnet matrix it was possible to determine the orientations of the other sets of needles for both four and six rayed star garnets. A six rayed star garnet exhibits needles aligned along the $\langle 111\rangle$ and $\langle 011\rangle$ directions of the garnet while a four rayed star garnet exhibits needles aligned along the $\langle 111\rangle$ directions only. These results are believed to be applicable to asteriated garnets emanating from all sources.