Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/1055

 

Group A. The Serpentines.

These rocks, as usual, present an infinite variety as regards external appearance; but there is considerable resemblance in their chemical composition. A specimen has been selected for careful analysis.

Black Serpentine from "near Cadgwith"—Black to greenish black, opaque; fracture splintery; charged with numerous crystals of a highly cleavable mineral of a pale brass-colour and metallic lustre. Sp. gr. of the mass 2⋅587. Freed as far as possible from the crystals, the black matrix yields a greenish-grey powder, which adheres in considerable quantity to the magnet. Composition:—

Dried at 100° C.

1. Silicates &c. undecomposed by HCl ................................................................................................................................................................................................................................................................................................................................................................................................

   1⋅37

1. Iron bisulphide ................................................................................................................................................................................................................................................................................................................................................................................................

   0⋅41

1. Water (including traces of CO2 and C?) ................................................................................................................................................................................................................................................................................................................................................................................................

   12⋅35

1. Silica (pulverulent) traces of Ti O2 ................................................................................................................................................................................................................................................................................................................................................................................................

   38⋅50

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

   1⋅02

1. Ferric oxide ................................................................................................................................................................................................................................................................................................................................................................................................

   4⋅66

1. Ferrous oxide ................................................................................................................................................................................................................................................................................................................................................................................................

   3⋅31

1. Oxide of nickel ................................................................................................................................................................................................................................................................................................................................................................................................

   0⋅59

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

   1⋅97

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

   36⋅40

1. Sulphuric acid ................................................................................................................................................................................................................................................................................................................................................................................................

   traces

————

100⋅58

Two or three kinds of crystalline fragments are noticeable in the undecomposed residue; one is probably enstatite.

As the amount undecomposed is so small, the quantity of unaltered augite, diallage or enstatite, must be very trifling. Any olivine still existing would of course be dissolved; but the quantity of water found, and the fact that no gelatinous silica is noticeable, preclude the possibility of there remaining any considerable amount of this mineral^[1].

The quantity of ferric oxide is probably wholly in combination as normal magnetite; this would absorb 2⋅09 out of the 3⋅31 of ferrous oxide, leaving 1⋅22 ferrous oxide for the hydrous silicate which forms the bulk of the mass. There exists, therefore, in all probability, 6⋅75 per cent, of normal magnetite disseminated as a fine powder throughout the mass, causing the blackness and opacity for which this serpentine is remarkable.

The undoubted presence of nickel is interesting in connexion with these magnesian rocks; it may exist partly as nickeliferous pyrrhotite, and partly as a base of the hydrous silicate. It has been

1. The amount shown by the microscope is extremely small (see p. 921).—T. G. B.