Sector-hourglass zoning--Sector-hourglass zoning typically occurs in chloritoid
, and less commonly in biotite and staurolite (Fig.
The most common brittle micas are dioctahedral margarite, [([Al.sub.2]).sup.VI][([Si.sub.2][Al.sub.2]).sup.IV][O.sub.10][(OH).sub.2]Ca, dominantly trioctahedral clintonite and xanthophyllite, [[(Mg > Fe,Al).sub.2-3].sup.VI] [([Si.sub.1.3][Al.sub.2.7]).sub.IV][O.sub.10] [(OH).sub.2]Ca, and chloritoid
, [[(Al,[Fe.sup.2+]).sub.2-3].sup.VI] [([Si.sub.2][Al.sub.2]).sup.IV][O.sub.10][(OH).sub.2](Mg,[Fe.sup.2+]).
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 = 0.93.
Analysis of the stabilities of chloritoid
and staurolite and some equilibria in the system FeO-Al2O3-SiO2-H2O-O2.
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.
Mica schist have a paragenesis of quartz + muscovite + biotite with minor garnet, chloritoid
and plagioclase, and locally andalusite porphyroblasts cutting the [S.sub.n+1] regional metamorphic foliation (Fig.
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 preponderance of "Nuristan" in the dealers' reports means that I must take it as correct until I meet someone who found the material himself and can testify otherwise.
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.