96 
inous lustre. It is opaque (1mm.) with a brownish black colour. 
Under the microscope the fine powder is translucent and reddish- 
brown in colour. Most of the fragments are isotropic, but some are 
strongly hirefringent, indicating the association of a small amount 
of cryptocrystalline matter with a colloid. The masses are very 
brittle and are penetrated in every direction by minute cracks, pro- 
bably shrinkage cracks due to dehydration. The density given by 
Dana is 2.5 to 3.0, but a careful determination of the density of the 
Westonia mineral gave for four small (2 to tom.) fragments, 2.26, 
2.26, 2.27, 2.28 : the mean being 2.27. These determinations were 
made by diluting methylene iodide, using quartz (2.65), selenite 
(2.32) and opal (2.10) as indicators. 
The chemical composition sheds light on this low density. Some 
apparently almost clean mineral was taken for a rough analysis. 
This was found to he rapidly decomposed by cold 5E hydrochloric 
acid, the iron going into solution and gelatinous silicic, acid, in the 
form of the original mineral fragments, remaining. Treatment in 
this way revealed embedded granules of siderite amounting approxi- 
mately to five per cent, of the whole. The analytical results obtained 
were — 
Fe 2 0.. . . . . 36.8 per cent. 
Si0 2 . . . . 31.6 ” 
H 2 0 above 100° 6.0 
H’O at 100° . . 21.8 ” 
Allowing for the siderite these are not unlike Dana’s figures,* 
the ratio of Si(\ to F<\,0 being approximately 2 to 1, but the water 
is distinctly higher, Dana's average being 21 per cent. The 
material analysed had been exposed to the air in the author’s 
laboratory for several weeks before analysis, but had not been 
long out of the mine, and with lapse of time might well have lost 
more water before reaching a condition of equilibrium in air. The 
extra, water, which is all lost rapidly at 100°, accounts for the low 
density. 
The chemical composition of hisingerite has not been defi- 
nitely settled. Hintzef does not look upon it as deserving of 
specific rank. He says — 
u Amongst the hydrous amorphous silicates there is scarcely 
one to be found which can be considered as a ‘'mineral,' that is, 
as a chemical compound or isomorphous mixture 
“A second section of hydrous substances consists essentially 
of ferric silicate, partly of lighter, mostly green, colour, such as 
the chloropal of Unghvar in Hungary, partly of brownish black 
to black colour, such as the hisingerite of Riddarhytten in Sweden: 
the black as a rule of greasy lustre, dense with conchoidal frac- 
* System of Mineralogy, 1896, Ed., p. 702. 
t Handbuch der Mineralogie, II., pp. 1827, 1830. 
