| Description |
The chlorites are common minerals in many metamorphic ricks and in hydrothermal depostits where they are frequently associated with minerals of economic importance. As such, they represent a potential source of information about the minerals with which they are associated. Such use of the chlorites is dependent upon the development of a systematic method of distinguishing the numerous varieties. Chlorite minerals are hydrous magnesium aluminum silicates which may through ionic substitution contain appreciable amounts of ferrous iron, ferric iron chromium, manganese, and perhaps nickel. There are two apparently polymorphic sheet type structures involved. The normal chlorite structure has alternating "brucite" and "mica-type" sheets. A " kaelin-type" structure, consisting of alternating tetrahedral and octahedral layers is somewhat less common. Both of these structures offer essentially the same opportunity for ionic substitution of Si++++, Al+++ and rarely Fe+++ in tetrahedral coordination and Al+++, Fe+++, Cr+++, Mg++, Fe++, Mn++ and Ni++ in octahedral coordination. By consideration of the possible combinations of ionic substitution, it becomes evident that six variables must be considered to define all chlorite varieties. These may be listed as follows: 1. The number of tetrahedral poitions, per 18 anions, not filled by silicon ions. |
