File:The Quarterly journal of the Geological Society of London (12734356324).jpg

410 N. STORY-MASKELYNE AND AV. FLIGHT ON THE CHARACTER
small an angle as to give a stauroscopic figure almost identical with
that of a uniaxial crystal.
In parallel light, however, the cleavage-plane exhibits principal
sections parallel and perpendicular to the edge c, a; and a slight
opening of the cross perpendicularly to that edge can be detected, so
that the optic axes would lie in the plane of symmetry. The crystal
is negative. It hardly exhibits any dichroism for light traversing
the principal cleavage-plane perpendicularly.
When heated on platinum-foil a little piece expanded to about six
times its original thickness

after it has been reduced to powder it
does not expand at all. By long boiling in strong hydrochloric acid
it gradually expands, and loses constitutional water at about 110° C.
Heated for many hours at 100° C, it apparently underwent no con-
stitutional change.
The powder dried for an hour and a half at 100° C. lost no weight.
In one portion the loss of weight at higher temperatures was ascer-
tained

in another the total amount of water present was directly
determined. The results are as follow

—
o
o
per cent
iring half an hour at 125-
-130 C. .
.. 1-309
„ half an hour at 125-
-130
.. 0-205
„ one hour at 130
.. 0-025
„ half an hour at 150
.. 0-231
„ half an hour at 150
.. 0-179
1-949
Water evolved at a low red heat .
.. 9-717
All water which passes off at temperatures about 100° C. has been
regarded as constitutional water.
The mineral has the following composition

—
Oxygen Ratios.
Silicic acid 40-833 21-777 .... 10
Alumina 9-801 4-562 1 R fi1 ft „
Iron peroxide .... 6-844 2-054 J O'Olb . . . . 6
Chromium oxide . . trace.
Magnesia 31-338 12-535 .... 6
Soda 0-670
Water 9-717 8-636 .... 4
Carbonic acid .... trace.
99-203
The above ratios correspond with the formula
R 2 3 , Si0 2 +2 (3MgO, 2Si0 2 , 2H 2 0).
On comparing the atomic ratios obtained from the analysis of this
mineral with those given in Prof. Cooke's recently published memoir
on the Yermiculites*, as those of Jefferisite andHallitef respectively,

J. P. Cooke, 'Proc. Amer. Ac. Arts and Sciences,' December, 1873. 'Phil.
Mag.,' April, 1874.
t The choice of the name of this mineral is unfortunate, as it is already in
use as a synonym for the aluminite (Websterite) of