This must be weighed and deducted from the
weight of the silica. In a complete examination of a silicate it should
be treated with the precipitate containing alumina, ferric oxide, &c.
EXAMINATION OF SILICATES.
The student interested in the analysis of rocks and rock-forming
minerals is advised to consult a valuable paper by Dr. W.F. Hillebrand
in the _Bulletin of the United States Geological Survey_, _No._ 148, to
which I am very largely indebted in the revision of the following pages.
~Moisture.~--Five grams of the powdered sample is dried between
watch-glasses in the water-oven for two hours, or till its weight is
constant; and the loss is reported as water lost at 100° C. The rest of
the determinations are made on this dried mineral.
~Combined Water, &c.~--Weigh up 1 gram of the substance, and ignite
over the blowpipe for some time in a platinum-crucible, cool in a
desiccator, and weigh. Record the loss as "loss on ignition," not as
"combined water."
~Silica.~--The ignition should have been performed in an oxidising
atmosphere in a muffle or over a slanting blowpipe flame; this will
ensure the oxidation of any pyrites or other sulphide present, which if
unoxidised would injure the crucible in the next operation. The ignited
residue is mixed with 6 or 7 grams of anhydrous sodium carbonate.
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