There are several varieties of stone practically known as granite, but true granite consists of crystals of quartz and felspar mixed with particles of mica.
The quartz is a very hard glassy substance in grey or colourless amorphous lumps, occasionally in crystals.
The mica is in dark grey, black, brown, flexible, semi-transparent glistening scales, which can easily be flaked off with a knife.
Granite generally contains more felspar than quartz, and more quartz than mica.
The colour of the stone depends upon that of the predominating ingredient, felspar.
"An average granite may be expected to contain from two to three fifth parts of crystals of quartz or crystalline quartz; about the same, more or less, of felspar, also partly crystalline and chiefly in definite crystals; and the remainder (one-tenth part) of mica. But the mica may form two or three tenths, and the quartz three-fifths or more, while the proportion of the felspar, as well as the particular composition of the felspar, both vary extremely."1
If the granite contains a large proportion of quartz, it will be hard to work; but, unless the felspar is of a bad description, it will weather well.
The felspars that occur most commonly in granite are potash felspar (orlhoclase) and a lime and soda felspar (oligoclase).
Sometimes both these varieties are found in the same stone.
Of the two, potash felspar is more liable to decay than the other.2
Mica is easily decomposed, and it is therefore a source of weakness.
1 Ansted's Practical Geology.
If the mica or felspar contain an excess of lime, iron, or soda, the granite is liable to decay.
"The quantity of iron, either as the oxide or in combination with sulphur, must affect the durability of granite, as well as of all other stone.
"The iron can generally be seen with a good glass, and a very short exposure to the air, especially if assisted in dry weather by artificial watering (better still, if 1 per cent of nitric acid be added to the water), ought to expose this.
"The bright yellow pyrites crystallised in a cubical form appear to do little harm. The white radiated pyrites (marcasite), on the contrary, decompose quickly.
"Where the iron stains are large, uneven, and dark coloured, the stone may fairly be rejected, at any rate for outside work.
"When the discoloration is of a uniform light yellow, it is probable that little injury will be done to the stone in a moderate time, and unless appearance is a matter of great importance, such granite would not be rejected.
"In the red granites, the discoloration from iron does not show so easily, but still sufficiently to guide the engineer if bad enough to cause rejection."1
The quality of granite for building purposes depends upon its durability, and upon the size of the grains. The smaller these are, the better can the granite be worked, and the more evenly will it wear.
"In using granite for ornamental purposes, the coarser-grained stones should be placed at a distance from the eye, the finer-grained stones where they can be easily inspected. Without attention to this point, very little better effect .is produced than by a stone of uniform colour."1
Syenite and Syenitic Granite are generally included by the engineer and builder under the general term granite.
True Syenite consists of crystals of quartz, felspar, and hornblende, the latter constituent taking the place of mica in ordinary granite. It derives its name from the granite of Syene, in Upper Egypt, though it has been shown that the latter is really a syenitic granite of the composition mentioned below.
Syenitic Granite consists of quartz, felspar, mica, and hornblende, the last-named constituent being added to those of ordinary granite.
The syenites and syenitic granites are generally of darker colour than ordinary granite, caused by the grains of hornblende.
"The syenitic granites are on the whole tougher and more compact than the ordinary granites, take on a fine polish, and are exceedingly durable.
"They occur less abundantly in nature; but their rarer use most frequently arises from the darker tints imparted to them by the hornblende." 1
Talcose Granite contains, in addition to the ingredients of common granite, talc, a material which scales off in thin flakes, having a whitish colour and unctuous feel.
Such granites are said not to weather well.
Protogene contains talc instead of mica.
Chloritic Granite contains chlorite, an olive-green mineral, generally granular, and of a pearly lustre.
Schorlaceous Granite contains pieces of schorl, "a black, hard, brittle, mineral crystallised in masses or long crystals, sometimes columnar, and radiating from a centre."2
Graphic Granite is composed of long parallel prisms of quartz and felspar, the ends of which when broken across look like the letters of cuneiform inscriptions.
This granite contains very little mica, and is not much used for building purposes.
Porphyritic Granite is the name given to those varieties in which large, distinct, independent crystals of felspar occur at random interspersed through the mass.
These crystals are sometimes called "horse's teeth."