Isomorphous Groups

A series of compounds which have analogous chemical compositions and closely similar crystal forms are said to make an isomorphous group. The artificial compounds known as the alums form a striking example. They are double salts of sulphuric acid, similar to the following, KA1 (SO4)2.12H2O, which is known as potash alum. They may vary in their composition by the substitution of Na, Li, NH4, etc .,for the potassium and of Fe”’ and Cr for the aluminium . All these compounds have, therefore, different but analogous compositions, and it is found also that they all crystallize in the Isimetric System with an octahedral habit. Further , if a crystal of one alum is suspended in a saturated solution of another member of the series the crystal will continue alto grow. From this it is proved that the molecules of the different alums are physically so closely alike that they can be substituted for each other in any proportion. Therefore this series of compounds is said to be an Isomorphous Group.

Many such groups are to be found in minerals, and attention is called to them in various places in Section IV. Reference might be made to one of the most prominent of these in the case of the Calcite Group (see page 217). This is a series of minerals all of which are carbonates of similar bivalent metals, and therefore they can be said to have analogous chemical compositions. Further, they all crystallize in the same crystal system and class, and have closely agreeing angles between similar crystal faces. Consequently they conform to the second requirement for an Isomorphous Group, namely, that the mineral of it should show similar crystal forms.