871 



FiK. 5. 



Li'.liiumbisilicate; cleavagedireclioii.s 

 (enlargement 500 X)- 



mixtures of llie same chemical composition, in (he form of llie great, Hal 

 crystals, which are characterised by their tabular sha|)e and their 

 peculiar aspect. Often they are recognisable (Fig. 5) by a system of 



cleavage-direct ions, crossing each 

 other almost normally in three 

 directions of S|)ace ; the plates, 

 as if co\ercd with a line nel- 

 woik, are llius often bordered 

 by right angles. The syniinelry 

 seems lo be rhombic, or pio- 

 biibly moiiocliiiic ; the ci'ystals 

 are biaxial, with posili\e charac- 

 ter of their birefringejice, which 

 is about 0,020, — beijig slightly 

 less than for the metasilicate. 

 The ai)parent axial angle is 

 |)robably rather great ; more or 

 less complicated twins are pre- 

 sent. The figures 6 and 7 may 

 give some representation of the habitus of this compound, as it 

 looks between crossed nicols. The refractive indices are about : 

 'J. 545 and 1.525; it is very difficult to determine them accurately, 

 because the crystals are intergrown with fine, felty needles of tridy- 

 mite, or in some preparation with those of the metasilicate. The 

 peculiar cleavage-directions are in every ease most typical for this 

 compound. 



The specific gravity of the bisilicale was pycnometrically deter- 

 mined on : c/,o=2,454 at 25°.l C. 



It was a difficult thing to fix the borders of the stability-lield 

 of this compound, in contact with a binary liquid. We succeeded, 

 by heating preparations of different com|)osition. inclosed in thin 

 platinum-folium, at a constant temperature during a time ranging 

 from 20 to 60 minutes, in a (juenching-furnace, and chilling (he 

 preparations then suddenly in cold mercury; in this way the momen- 

 tary state of the mixture is fixed, and can be studied by accurate 

 microscopical investigation. 



The residts of those experiments were the following: 



Preparation N». 3. Analysis: Weight Proc oi'SiO.^ (= 74.2 Mol. Proc). 

 Temperaiwe 



in M. V. : 



8890 Wholly crystalline; much bisilicate, very hltle tridyuiite. 



56 



- Proceedings Royal Acad. Amsterdam. Vol. XVI. 



