Black amorphous Si obtained by reduction of sand (SiO2) with carbon; ultrapure semiconductor grade crystals are blue-gray metallic. Bulk Si unreactive towards oxygen, water, acids (except HF), but dissolves in hot alkali. Used in semiconductors, alloys, polymers.
Diagnostic tests:
The surest method of detecting a silicate is to get the mineral in
solution and obtain gelatinous silica by evaporation.
If a silicate is dissolved in boiling acid, HCl or HNO3,
the silica gel separates upon evaporation. At this point, the other
elements in the mineral are in solution and can be tested for additional
unknowns.
If a silicate is decomposed in boiling acid, HCl or
HNO3, the silica separates as an insoluble residue without the formation
of a jelly. This residue, if due to silica, is soluble in boiling KOH.
Fusion in Na2CO3 is necessary if
the mineral is unaffected by acid digestion. Mix the powdered mineral with
equal parts of sodium carbonate into a paste and fuse on charcoal with the
blowpipe. Dissolve the residue in HCl, the silicate will separate as
gelatinous mass upon evaporation. Note, most silicates are soluble in a
borax flux and will give the same gelatinous reaction as the sodium
carbonate.
Silicates are partially soluble in a
Na2(NH4)(PO4) bead. When a powdered
silicate is fused in the bead, the insoluble silicate skeleton or
translucent mass is detectable with a hand lens.
References
Emsley, J., 1991; THE ELEMENTS : Sec. Ed.,
Clarendon Press, Oxford, 251 p.
(* - Mineral Name Is Not IMA Approved)
(! - New Dana classification added or changed from Danas New Mineralogy)
(? - IMA Discredited Mineral Species Name)
There are 1437 minerals with Si in the Mineralogy Database.