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THE POPULAR EDUCATOR. 



2 atoms of lime. It is nearly allied to albite. By referring to 

 this table, prepared from the analyses of Dr. Haughton, the 

 student will easily perceive the differences in the composition of 

 these five prominent felspars. 



Leucite and Nepheline are the felspars of volcanic lavas, and 

 bear the same relation to each other as orthoclase and albite. 

 The five felspars above occur in lavas which are too ancient to 

 be reckoned the produce of volcanoes. Leucite crystallises in 

 the deltoidal octahedron, a figure after which the mineral has 

 been named the leucitohedron. The crystals are of a dull, glassy 

 colour. Nepheline appears in hexagonal prisms. When digested 

 in nitric acid they become cloudy; hence its name, vefyeXt], a 

 cloud. 



THE HORNBLENDE FAMILY. When cooling from a molten 

 condition, the constituents of the igneous rocks seem to have 

 divided themselves into two main groups : all the alkalies, the 

 lime, potash and soda, coalesced with the silicates to form the 

 felspars we have just considered, while the iron and magnesia 

 combined in varied quantities with the ailica to produce the 

 members of the hornblende family. 



Hornblende itself occurs in oblique rhombic prisms, more or 

 less modified, of the monoclinic system. It cleaves perfectly, 

 in places parallel with the faces of the prisms. It is also found 

 fibrous, sometimes coarse; at other times fine, with a silky 

 lustre. It varies greatly in colour, passing from white through 

 green to black, according to the quantity of iron it contains. 

 Typical hornblende may be regarded as a compound of 3 atoms 

 of silica with 4 atoms of magnesia, lime, or protoxide of iron, 

 alterations in these latter constituents producing the varieties. 

 The lighter-coloured members of the family are Tremolite, 

 usually found as long, white crystals, penetrating metamorphic 

 rocks, especially the dolomite of Val Tremola, St. Gothard. 

 Actinolite appears in the same way, but is light green. When 

 a quantity agglomerates, a radiated structure results, and the 

 colour darkens. It contains from 6 to 8 per cent, of the peroxide 

 of iron. It is to this radiated structure that it owes its name, 

 which is derived from OKTIV, a ray. 



Asbestos is hornblende in slender fibres, which are easily 

 separable. Usually it is white, occasionally green. It is noted 

 for its resistance of fire, and is used in gas stoves for the flame 

 to play against. The asbestos, becoming red-hot, assumes the 

 appearance of live coal. It may be woven into fabric which is 

 sometimes used as a covering where fire is to be resisted. The 

 ancients are said to have used a fabric made of asbestos for 

 napkins, because they need only be thrown into the fire to be 

 cleansed. Also the wicks of the Temple lamps which burnt 

 continually were of asbestos. Mountain Leather and Mountain 

 Cork are but varieties of the same material, remarkable for 

 being the lightest of mineral products. 



The dark-coloured varieties are Hornblende itself, which we 

 have described, and Amphibole, which is but another name for 

 hornblende. Hornblende is an essential constituent of certain 

 rocks, as syenite, trap, hornblende, slate. Tremolite is usually 

 found in metamorphic limestone and dolomite ; actinolite, in 

 talc, steatite, serpentine, and other magnesian rocks, which also 

 afford asbestos. 



Augite consists of 2 atoms of silica, combined with 3 of 

 magnesia, lime, or iron protoxide. 



Pyroxene is augite proper, nearly allied to hornblende. It 

 differs from it in its crystalline system, ths monoclinic. It 

 offers an example of isomorphism, its basis of lime, protoxide 

 of iron, and protoxide of manganese, replacing each other with- 

 out altering the crystalline form. It would seem that the main 

 difference between augite and hornblende results from the 

 manner of the cooling of the rock; the various conditions of 

 rapid and slow cooling, or being under pressure, or in contact 

 with water, are considered sufficient to account for the differ- 

 ence of structure. The tremolite of the hornblende is repre- 

 sented in the augites by Diopside, while actinolite corresponds 



to Sahlitc. Speaking generally, hornblende is a constituent of 

 the older igneous rocks, and augite of the more modern. 



The MICA FAMILY have a characteristic which at once dis- 

 tinguishes them from all other minerals ; they have a remark- 

 able power of foliation, or of splitting up into leaves. They are 

 found in igneous rocks and rocks immediately derived from 

 them ; and, by the peculiar appearance of mica, it is a promi- 

 nent constituent of any rock in which it is found. There are 

 two kinds of mica, white and black ; the former contains 

 alumina and alkalies, while the colour of the latter is owing to 

 the presence of iron, which is associated with magnesia. 



Muscovite, or white mica, occurs in plates, which may bo 

 regarded as short rhombic prisms of the trimetric system. It 

 is found in Siberia, where it is frequently used as a substitute 

 for glass. It can be divided into thin, elastic laminae of great 

 tenuity. Its composition is 5 atoms of silica, 4 atoms of 

 alumina, and 1 of potash. 



Margarodite is a white mica which contains an atom of water. 

 It is found in the Irish granite ; but the Muscovite does not 

 appear in the British Islands. 



Lepidolite, or lithia mica, is of a pink colour. It is generally 

 found in the neighbourhood of metallic lodes. 



The above micas are binaxial ; that is, they have two optic 

 axes. An explanation of this property was given in our intro- 

 duction to Mineralogy, where the physical properties, etc., of 

 minerals were discussed. 



Black mica is uniaxal, and is generally considered as belonging 

 to the hexagonal system. In Biotite magnesia is a prominent 

 constituent, and, according to Dana, it is not actually uniaxal, 

 but the angle between the optic axes is but 1, whereas in 

 Muscovite it varies from 56 to 75. 



Lepidomelane mica is the only other variety of black mica. 

 Its jet-black colour is duo to the presence of manganese, of 

 which substance it contains about 1-J- per cent. This is the 

 black mica in our granites. 



The TALC FAMILY is constituted of three members the 

 Talcs, the Serpentines, and the Chlorites. 



The Talcs. Talc, when crystallised, which is rare, appears 

 in rhombic or hexagonal prisms. Generally, however, it ia 

 massive, composed of minute, pearly scales. Like mica, it is 

 eminently foliaceous, the laminae being flexible, but not, like 

 mica, elastic. The colour is occasionally white, but usually has 

 a greenish tint. It is very soft, being easily scratched with 

 the nail, and possesses an unctuous feel. Its typical composi- 

 tion is 5 atoms of silica, 6 of magnesia, and 1 of water. 



Meerschaum is a talc, containing a little oxide of iron, and 

 alumina. When first taken from the earth it is soft, and will, 

 with water, make a lather. The Tartars use it for washing their 

 linen. When heated, it gives off water with a foetid odour, 

 becoming a white, hard body. Its chief use is to make the 

 bowls of pipes. 



Soapstone, or Steatite, is a talc which owes its name to its 

 soapy feel. It is generally white ; sometimes a little yellow or 

 greenish. 



Saponite is not actually steatite, but an English variety con- 

 taining more water. It is found at the Lizard, in Cornwall ; 

 and, according to Professor Haughton, its existence is accounted 

 for by the intrusion of granite veins into serpentine rock, the 

 former supplying the alumina, and the latter the requisite 

 magnesia. 



The Serpentines seem to be produced by a metamorphic 

 action on limestones which contain magnesia. They are com- 

 posed typically of 4 atoms of silica, 9 of magnesia, and 6 of 

 water. Serpentine admits of being polished, and is much used 

 in ornamenting buildings. Its most ordinary colour is a 

 mottled green. Chromic iron disseminated through it adds 

 red tints. Some specimens are so bright-coloured, and take so 

 fine a polish, that they are used for brooches and bracelets. 

 When veins of crystalline limestone shoot through groen serpen- 

 tine, the stone is called verd-antique. Many parts of Con- 

 nemara and Donegal in Ireland, and Devonshire and Cornwall 

 in England, yield serpentines. 



Chlorite, the third member of the talc family, is always 

 green, and of a pearly lustre. Usually, chlorite appears in 

 dark, olive-green masses ; but when crystallised, it is found in 

 hexagonal crystals, foliated, like mica, with which it may easily 

 be confounded ; but the laminae are not elastic. Its composition 

 is 2 atoms of silica, 1 ox alumina, 5 of magnesia, and 3 of water 



