Chlorite

Chlorite, a group of green micaceous minerals which are hydrous silicates of aluminium, magnesium and ferrous iron. The name was given by A.G. Werner in 1798, from χλωρῖτις, “a green stone.” Several species and many rather ill-defined varieties have been described, but they are difficult to recognize. Like the micas, the chlorites (or “hydromicas”) are monoclinic in crystallization and have a perfect cleavage parallel to the flat face of the scales and plates. The cleavage is, however, not quite so prominent as in the micas, and the cleavage flakes though pliable are not elastic. The chlorites usually occur as salt (H=2-3) scaly aggregates of a dark-green colour. They vary in specific gravity between 2.6 and 3.0, according to the amount of iron present. Well-developed crystals are met with only in the species clinochlore and penninite; those of the former are six-sided plates and are optically biaxial, whilst those of the latter have the form of acute rhombohedra and are usually optically uniaxial. The species prochlorite and corundophilite also occur as more or less distinct six-sided plates. These four better crystallized species are grouped together by G. Tschermak as orthochlorites, the finely scaly and indistinctly fibrous forms being grouped by the same author as leptochlorites.

Chemically, the chlorites are distinguished from the micas by the presence of a considerable amount of water (about 13%) and by not containing alkalis; from the soft, scaly, mineral talc they differ in containing aluminium (about 20%) as an essential constituent. The magnesia (up to 36%) is often in part replaced by ferrous oxide (up to 30%), and the alumina to a lesser extent by ferric oxide; alumina may also be partly replaced by chromic oxide, as in the rose-red varieties kämmererite and kotschubeite. The composition of both clinochlore and penninite is approximately expressed by the formula H8(Mg,Fe)5Al2Si3O18, and the formulae of prochlorite and corundophilite are H40(Mg,Fe)23Al14Si13O90 and H20(Mg,Fe)20Al8Si6O45 respectively. The variation in composition of these orthochlorites is explained by G. Tschermak by assuming them to be isomorphous mixtures of H4Mg3Si2O9 (the serpentine molecule) and H4Mg3Al2SiO9 (which is approximately the composition of the chlorite amesite). The leptochlorites are still more complex, and the intermixture of other fundamental molecules has to be assumed; the species recognized by Dana are daphnite, cronstedtite, thuringite, stilpnomelane, strigovite, diabantite, aphrosiderite, delessite and rumpfite.

The chlorites usually occur as alteration products of other minerals, such as pyroxene, amphibole, biotite, garnet, &c., often occurring as pseudomorphs after these, or as earthy material filling cavities in igneous rocks composed of these minerals. Many altered igneous rocks owe their green colour to the presence of secondary chlorite. Chlorite is also an important constituent of many schistose rocks and phyllites, and of chlorite-schist it is the only essential constituent. Well-crystallized specimens of the species clinochlore are found with crystals of garnet in cavities in chlorite-schist at Achmatovsk near Zlatoust, in the Urals, and at the Ala valley near Turin, Piedmont; also as large plates at West Chester in Pennsylvania and at other American localities. Crystals of penninite are found in serpentine at Zermatt in Switzerland and in the green schists of the Zillerthal in Tirol.

Closely allied to the chlorites is another group of micaceous minerals known as the vermiculites, which have resulted by the alteration of the micas, particularly biotite and phlogopite. The name is from the Latin vermiculor, “to breed worms,” because when heated before the blowpipe these minerals exfoliate into long worm-like threads. They have the same chemical constituents as the chlorites, but the composition is variable and indefinite, varying with that of the original mineral and the extent of its alteration. Several indistinct varieties have been named, the most important of which is jeffersonite. 

(L. J. S.)

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