Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/1057

representing the "Saussurite." The external portions of this latter contain much carbonate; but the central mass, or core, consists of a bluish-white compact silicate, or mixture of silicates which, omitting small quantities, has the following composition:—

 

I. " Saussurite" of the newer Gabbro.

Dried at 100° C.

1. Ignition (water with traces of carbonic acid) ................................................................................................................................................................................................................................................................................................................................................................................................

   4⋅80

1. Silica ................................................................................................................................................................................................................................................................................................................................................................................................

   45⋅70

1. Alumina ................................................................................................................................................................................................................................................................................................................................................................................................

   23⋅00

1. Oxides of iron ................................................................................................................................................................................................................................................................................................................................................................................................

   ⋅50

1. Lime ................................................................................................................................................................................................................................................................................................................................................................................................

   19⋅30

1. Magnesia ................................................................................................................................................................................................................................................................................................................................................................................................

   4⋅75

1. Alkali and loss ................................................................................................................................................................................................................................................................................................................................................................................................

   1⋅95

————
100⋅00

The compound has more affinity, as regards its percentage of silica, with anorthite than with labradorite; but the presence of so much magnesia and its poverty in alkali must lead us to regard it as a mixture rather than as a mineral species worthy to be classed with the felspars^[1].

 

II.

The diallage associated with the above is a well-defined mineral, and but slightly affected by ignition or acids. Subjoined is a rough analysis:—

Dried at 100° C.

1. Ignition (water) ................................................................................................................................................................................................................................................................................................................................................................................................

   1⋅30

1. Silica ................................................................................................................................................................................................................................................................................................................................................................................................

   50⋅10

1. Alumina ................................................................................................................................................................................................................................................................................................................................................................................................

   6⋅77

1. Oxides of iron ................................................................................................................................................................................................................................................................................................................................................................................................

   6⋅90

1. Lime ................................................................................................................................................................................................................................................................................................................................................................................................

   17⋅46

1. Magnesia ................................................................................................................................................................................................................................................................................................................................................................................................

   17⋅47

————
100⋅00

No alkali was found in the specimen examined. The amounts of lime and magnesia, being so exactly equal, indicate a mineral which, in its chemical aspect, is almost equally allied to the augites and the hornblendes^[2].

1. See p. 895 for the history of this mineral. Perhaps the magnesia may be accounted for by the presence of pale-coloured hornblende, which, though almost microscopic, is often present in considerable quantities.—T. G. B.
2. This may explain the extreme facility with which this diallage is replaced by hornblende, see p. 895.