Identifying Silicon Minerals

Silicon exists as the acid elements in the large group of minerals known as the silicates. Some of these are readily soluble in acids, but the greater part are quite insoluble. The test employed differ somewhat in the two cases.

1. Test for a Soluble Silicate. If the silicate is soluble, it should be powdered and dissolved in boiling hydrochloric acid. When this solution is evaporated a jellylike material will separate out just before dryness is reached. This silica jelly, as tit is called, mineral. On continued evaporation it will be dehydrated and converted into a sandy and insoluble substance having the composition of silicon dioxide, SiO₂.
2. Test for an Insoluble Silicate. In the case of an insoluble silicate, the mineral must be decomposed by fusion with sodium carbonate before treating it with an acid. Make a mixture of one part of the powdered mineral to three parts of sodium carbonate and fuse thoroughly before the blowpipe on a loop of platinum wire. It is best to make two or three such beads. The fusion serves to decompose the silicate and to render the resulting mass wholly soluble in acids. The beads are powdered and dissolved in boiling dilute nitric acid. The evaporation is conducted as explained in experiment 1 and a similar silica jelly is obtained
   Frequently it is desirable to make test for the bases which are present in the silicate. In this case, after the formation of the jelly, continue the evaporation to complete dryness. This converts the silicon into the insoluble oxide both leaves the bases in the form of various soluble salts. Treat the residue in the test tube with a little water and hydrochloric acid, warn and filter from the insoluble silica. Add an excess of ammonium hydroxide to the filtrate to precipitate any aluminum or ferric iron as their respective hydroxides. Filter if necessarily, and to the filtrate add a little ammonium oxalate to precipitate any calcium as calcium oxalate. Filter again, and to the filtrate add moiré ammonium hydroxide if necessary and then a little hydrogen sodium phosphate, which will precipitate any magnesium present as ammonium magnesium phosphate.
3. Decomposition of Silicates by Acids. Certain silicates, when their powder is treated with boiling hydrochloric acid, are decomposed, the bases going into solution and the silicon separating as the dioxide, SiO2. in this case there would be no jelly formed when the solution never becomes perfectly clear owing to the silica, which remains in suspension in the solution. It gives the solution a translucent appearance. The surest proof that the mineral has been decomposed is to filter the solution and test for various bases in the filtrate in a similar manner to that described under test 2.
4. Test with the Salt of Phosphorus Bead. When the powder of a silicate is heated in a salt of phosphorus bead, the bases are dissolved, leaving the silica present as an insoluble translucent skeleton.

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