[Rockhounds] giant garnets exposed: a field trip gone awry

[DonH]

Lanny wrote:

> Just for the heck of it, to fill out Don's story, I decided to provide 
> more information on the Moses Butte locality. Go to the following url:
> 
> http://www.lrream.com/mosesgarnet.html


Hi,

A good portion of that rock in the Goat Mtn/Moses Butte area, as well as 
on the high narrow ridge where the Freezeout Lookout Tower once was, is 
a garnet amphibolite.  It is black, with layered white stringers of 
felsic minerals and rust-red garnets.  It has a gneissic texture.  I 
just found this paper from American Mineralogist describing the area:


Vol. 41 JANUARY-FEBRUARY, 1956 Nos. 1 and 2

KYANITE, ANDALUSITE, AND SILLIMANITE IN THE
SCHIST IN BOEHLS BUTTE QUADRANGLE, IDAHO*

Anna Hietanen

Abstract

Kyanite, andalusite, and sillimanite are found together in the 
cordierite-bearing mica
schist of the Prichard formation of the Precambrian Belt series exposed 
in the Boehls
Butte quadrangle in the southern part of the Idaho panhandle. 
Microscopic studies of
this schist suggest that the following inversions took place: (1) 
sillimanite->kyanite,
(2) sillimanite->andalusite, (3) kyanite->andalusite, (4) 
kyanite->sillimanite, (5) andalusite->
sillimanite. These inversions can be in part related to the fluctuation 
of temperature
and stresses during the complex regional and thermal metamorphism to which
the schist was subjected. In some thin sections all three modifications 
occur side by side,
suggesting that they were crystallized close to the physical-chemical 
conditions in which
all three may exist together. The association of epidote and plagioclase 
(An36) in the
calcium-rich beds of the same area suggests that the temperature during 
the crystallization
was close to 400' C.