15^0 Journal of Applied Microscopy 



It is probable that in the structure formulae the hydroxyl belongs rather with 

 the Al than the Si, because the relation between the hydroxyl group and alumina 

 group is constant, but is not with the silica group. Also because in reactions this 

 constancy mentioned remains, while the silica molecule may be split, and finally 

 because on destruction of the alumosilicate there result aluminates and not silicates. 



The following may be, therefore, taken as the structure formula; of the 

 chlorite nucleus HoAljSiO,;, and the mica nucleus HoAloSigOg. 



OH 



The remaining alumosilicates readily pass into the second type, and differ 

 from it only in additional /(^r/rj- of SiOg groups; these additional pairs differ 

 from the first pair in that they readily separate or recombine. From the two 

 groups represented, however, the SiOj can only be separated by the strongest 

 methods, involving the destruction of the alumosilicate. 



Groups with the Chlorite Nucleus.- — -The minerals belonging to these 

 groups have been little studied. Very few simple salts are known, e. g., marga- 

 rite CaAl2SiOg and some chlorites. They form very complex isomorphic 

 mixtures with each other, and also isomorphic mixtures with metasilicates. In 

 the group may be included all minerals with the formulae RoAl2SiO,j or 

 m R2Al2SiOg . A, and include: 



1. The staurolite and clintonite groups — usually very complicated 



formulae. 



2. The chlorite group. 



3. The mellilite group. 



As yet no simple formulae will express their composition, and there is there- 

 fore no substantial basis for structure formulae. 



Groups with Mica Nucleus. — These groups are much more important, 

 and have been more thoroughly studied. As before stated, characteristics are : 

 I^asy transformation into one another ; production, on earth's surface, of clays 

 by weathering ; constant nucleus in structure formula little affected by most 

 reactions ; of SiOg atoms, two can only be removed by destruction of the com- 

 pound, but the rest can be easily removed or added. 



