[graham] SAPONITE 201 



formula, assuming AI2O3 equivalent to 3MgO, i.e., that the substance is 

 a mixture of the compound 6MgO-6Si02-9H20 with a small amount of 

 the corresponding compound 2Al203-6Si02-9H20. 



If Clarke's formula really expresses the true composition of 

 saponite, it would seem better to write it in the form (MgOH)2Si205+ 

 2H2O, or H2Mg2Si207 + 2H20, seeing that two-thirds of the water, 

 representing two molecules, are given off at a temperature below 

 100°C, and moreover are again absorbed by the material when it is 

 exposed to a moist atmosphere at the ordinary temperature. 



According to this view, saponite would be comparable with 

 picrosmine, H2Mg2Si207, differing from it only in containing two 

 additional molecules of water, which are held very loosely. 



Little is known concerning the actual constitution of saponite. 

 Clarke favours the view that the mineral is best represented by the 

 orthodisilicate formula 



/ H2 

 (MgOH)o = Si207 



^ H2 

 To indicate that two molecules of water are very loosely held, 

 the formula might still be written as an orthodisilicate, H2Mg2Si207 

 •2H2O; or it might be represented as 



,Si03. 



Mg( )Mg + 2H20 



II 

 H2 



which is the constitution ascribed by Clarke to picrosmine. Other 



constitutional formulae, as for example the metadisilicate (MgOH)2 



Si205 + 2H20, might of course be suggested for saponite, but in the 



present state of our knowledge concerning the genesis of the mineral, 



and especially in view of the existing uncertainty even of its correct 



empirical formula, it is not possible to confidently assign any particular 



one of these to the mineral. 



The saponite described above was collected by Dr. J. A. Bancroft 

 near the western end of the Canadian Northern Railway tunnel, 

 immediately beneath the Corporation Quarry. This tunnel, which 

 has been recently completed, passes from east to west under Mount 

 Royal, and below the quarry it traverses the intrusive contact between 

 the nepheline syenite and Trenton limestone. It is here that the 

 saponite occurs, as a deposit on corroded crystals of calcite which coat 

 fissures and cavities in the rock. 

 Geological Department, 



McGill University, 

 Montreal. 



