Chloritoid


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chloritoid

[′klȯr·ə‚toid]
(mineralogy)
FeAl4Si2O10(OH)4 A micaceous mineral related to the brittle mica group; has both monoclinic and triclinic modifications, a gray to green color, and weakly pleochroic crystals.

Chloritoid

 

a mineral of the nesosilicate class, with a chemical composition of (Fe, Mg)2(Al, Fe3+)Al3[SiO4]2O2(OH)4. Chloritoid containing up to 16.6 percent MnO is called ottrelite.

Chloritoid crystallizes in the monoclinic or triclinic system; both modifications are equally common and often form dense concretions. The mineral usually occurs in the form of poorly developed tabular crystals and concretions (chloritoid porphyroblasts in metamorphosed schists) and in the form of plumose aggregates. Generally, its color is dark blue or dark green, sometimes ranging to nearly black. Chloritoid has a hardness of 6.5 on Mohs’ scale and a density of about 3,600 kg/m3. It is brittle and exhibits perfect cleavage parallel to the flattening of the crystals. A rock-forming mineral in certain metamorphosed schists and contact hornfels, chloritoid is also found in deposits of emery, in hydrothermal quartz veins, and in the rocks surrounding veins.

References in periodicals archive ?
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 quartz.
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.
Chloritoid is commonly observed as 1 to 2-mm crystals in the finer-grained beds.