Abstract
The garnet group minerals are among some of the most important minerals, occuring under a wide range of metamorphic conditions. Because garnet in high-P/T type metamorphism shows compositional zoning controlled by the metamorphic history, much work has been carried out to deduce metamorphic conditions and P-T path from the analysis of such zoned garnet.
This study concerns other information from garnet that is critical to evaluate the formation conditions of metamorphic rocks. The relative growth rate of crystallographic faces depends on not only the atomic structure but also the interfacial energy with surrounding environment(chemical potential gradient and dihedral angle). Thus, morphological research can become a powerful tool not only to analyze mineral growth processes, but also to determine growth conditions, as the growth habit reflects the growth environment. If grain shape can be expressed quantitatively, it may provide useful insight into the petrogenesis of rocks.
A garnet grain of the epidote-amphibolite rock was picked up to analyze the three-dimensional morphology, following thermal extraction method(Kretz,1973). The grain is about 10 mm in size and expresses an elongated shape maintained by the {110} growth surface. The aspect ratio is 1.83, which could not be produced by an isotropic growth. Applying the relation between the maximum aspect ratio and the ratio between the growth rates, it is indicated that the elongated garnet may be formed as a result of different growth rates with the ratio of 1.29 in symmetry both-sides model.
We also calculated cross-sections assuming isotropic growth zoning. Examples are shown in Fig. 8-a, which reveals that "pseudo-anisotropic growth" caused by the variation in the apparent growth breadth. By dividing the cross section into regions so that each region is configured by parallel growth surfaces, the "hkl division map" is drawn. On the other hand, in the single region, the zoning width is constant and their sides are parallel. Thus the chemical profile should be measured in the single hkl region to obtain the accurate growth increment.
