uminium (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
kaemmererite 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 alterati