Analysis of the stabilities of chloritoid
and staurolite and some equilibria in the system FeO-Al2O3-SiO2-H2O-O2.
The stability of chloritoid
lies between the stabilities of dioctahedral phyllosilicates and most of chlorites.
Much of the andalusite and almost all staurolite in these rocks has been retrograded to pleochroic blue-green needles of chloritoid
with Fe/Fe+Mg = 0.
Mica schist have a paragenesis of quartz + muscovite + biotite with minor garnet, chloritoid
and plagioclase, and locally andalusite porphyroblasts cutting the [S.
Lustrous, sharp, chocolate-brown rhombohedral crystals of siderite to 5 cm make parallel stacks, some with faden and chloritoid
crystals of quartz, others with sharp white dolomite rhombs to 3 cm.
Some specimens are loose, thumbnail-size blades and flaring clusters of blades, but the most dramatic are miniature to cabinet-size matrix pieces, with brookite crystals standing up singly and in forests of sharp blades on groups of quartz crystals (some of the quartz is chloritoid
and greenish); respectable anatase and fluorapatite crystals are also to be espied on a very few of these specimens.
For example, the world's best chloritoid
was at first said by one dealer to have come from "the Northern Areas," and said by a few others to have come from "Nuristan, Afghanistan.
The icy-clear fluorite crystals are perfect dodecahedrons to 4 cm, perching nicely on bristly greenish matrix composed mainly of elongated prisms of chloritoid
The type material (Greece) is from an emery and is associated with: corundum (partly altered to diaspore), chloritoid
, Ti-rich hematite, ilmenite, gahnite, "tourmaline," rutile, and calcite.
It revolved around a remarkable specimen brought to the show by dealer Herb Obodda: a single, hexagonal-tabular, silver-dollar-size crystal of blackish-green chloritoid
, probably the largest such in the world.
Scientifically speaking these inclusions are finely divided chlorite, chloritoid
, montmorillonite and hematite, but they truly look barfy; and yet the specimens are oddly beautiful.
is commonly observed as 1 to 2-mm crystals in the finer-grained beds.