194 



HARDWICKE'S SCIENCE-GOSSIP. 



the orthoclastic crystals, when twinned, usually show 

 a median divisional line, on either side of which they 

 depolarise in complementary colours ; 'plagioclastic 

 crystal exhibit numerous bands of different colours. 



Crystalline System, Chemical Composition, and Specific 

 Gravity of the Principal Varieties of Felspar. 



* M = Monoclinic. 

 T = Triclinic. 



The mica found in granite is usually either musco- 

 vite or biotite, but other kinds are sometimes found. 

 All the micas are characterised by their highly perfect 

 cleavage parallel to the basal plane ; and by the thin 

 laminae being flexible and elastic. The percentage 

 chemical composition of the above two forms is given 

 by J. D. Dana as follows : — 



Biotite 



Muscovite 



Silica 40*91 



Alumina . . . . 17*79 



Iron oxides . . . io"oo 



Magnesia. . . . 19*04 



.Potash .... 9-96 



[Silica 46*30 



Alumina . . . .36*8 



(Potash .... g - 2 



jlron sesquioxide . \ 4*5 

 Fluoric acid . . . 0*7 



I Water i*8 



99'3 



It will be seen from these analyses that whereas 

 muscovite contains no magnesia, biotite contains 

 upwards of 19 per cent, of it. The latter is in 

 consequence often called magnesia mica ; muscovite 

 is sometimes called potash mica. Biotite crystallises 

 in the hexagonal system ; muscovite in the rhombic. 

 They have each a specific gravity of 2*7 to 3*1. 

 Biotite is 2*5 to 3 in the scale of hardness ; muscovite 

 2 to 2*5. Lepidolite, a rhombic mica, frequently 

 occurs in granite as a substitute for muscovite, with 

 which it agrees in chemical composition, excepting 

 that the potash is partially replaced by lithium. It 

 usually occurs in fine scales or granules. Lepidome- 

 lane resembles biotite, crystallising in the hexagonal 

 system ; but it contains more iron and less magnesia ; 

 and the folia are brittle. It occurs in some Irish and 

 Cornish granites. Hexagonal micas viewed at right 

 angles to the basal plane between crossed nicols 

 appear dark ; under similar conditions the rhombic 

 micas show tolerably strong chromatic polarisation. 

 The hexagonal species are uniaxial ; the rhombic 

 biaxial. 



Quartz, the remaining essential constituent of 

 granite, is much more simple in its composition than 

 the previously mentioned minerals. It is an oxide of 

 silicon, the only oxide of silicon known. It consists of 

 53*33 of oxygen, and 46*67 of silicon. Its hardness 

 is 7, and specific gravity 2*5 to 2*8. It crystallises 

 in the hexagonal system, and is usually found in 

 the form of six-sided prisms, terminated by six-sided 

 pyramids. But, as previously remarked, it has not 

 assumed a definitely crystallised form in granite, 

 excepting when it occurs in cavities. It is insoluble 

 in all acids, excepting fluoric acid ; but it is acted 

 upon by hot solutions of potash, although but slightly 

 in the case of the purer crystallised varieties. Under 

 the microscope the quartz in granite is seen to contain 

 nnmerous cavities ; some of them are extremely 

 minute, whilst others are large enough to be seen 

 with the naked eye. These cavities usually contain 

 water, or aqueous solutions of chlorides and sulphates 

 of sodium, potassium and calcium. The important 

 bearing of these fluid enclosures will be referred to 

 further on. 



Amongst the other minerals occurring in granite 

 are schorl — usually developed in granite near its 

 contact with other rocks — hornblende, magnetite, 

 apatite j and, less frequently, garnets, pyrites, talc, 

 beryl, iolite, andalusite, topaz, cassiterite, and 

 hcematite. Chlorite, epidote, and pinite, are also 

 found, as alteration products, and kaolin resulting 

 from the decomposition of the felspars. 



Most large masses of granite present differences in 

 texture, and are often traversed by veins, "due in 

 some instances to the segregation of the surrounding 

 minerals in rents of the original pasty magma, and 

 sometimes to the protrusion of a less coarsely crystal- 

 line part of the granitic mass into fissures of the 

 main rock."* Crystalline concretions or fragments 

 are also frequently found in some granites. These 

 also sometimes owe their origin to segregations of 

 the granitic mass, when they are usually ovoid and 

 porphyrite ; or they are entrapped masses of other 

 rocks, and are then either schistose or irregular in 

 form. 



Dr. Haughton gives the following average compo- 

 sition of granite, as the mean of eleven analyses : — ■ 



Silica 72*07 



Alumina 14*81 



Iron peroxide 2*22 



Potash s*ii 



Soda 2*79 



Lime 1*63 



Magnesia *33 



Loss by ignition 1*09 



Total 100*05 

 Mean specific gravity 2*66 



There are many varieties of granite characterised 

 by the presence or absence of certain minerals, or 

 by the state of development of them. In some, for 

 instance, the mica is absent ; in others, the felspar ; 



* Geikie, ' Text-Book of Geology.' 



