364 



THE POPULAR EDUCATOR. 



Fig. 32. 



MINERALOGY. VI. 



SILICATES (continued) . 



A.NDAITJSITE occurs in the older metamorpMc rocks. It is 

 usually of a grey or flesh-red colour. When crystallised it 

 appears in the trimetric system, exhibiting a distinct lateral 

 cleavage. Its lustre inclines to pearly, but the mineral is never 

 fibrous. It is composed of 2 atoms of silica, and 3 of alumina. 

 Chiastolite is the name given to crystals of andalusite when 

 the form of a cross is exhibited when the crystal is cut across 

 and polished. The peculiarity is owing to impurities mixed with 

 the andalusite, and symmetrically arranged by the powers of 

 crystallisation. Chiastolite is found in bunches of crystals in 

 mica slate, in the neighbourhood of granite. It is remarkably 

 soft ; sometimes it possesses only a hardness = 3. 



Staurotide is dark brown or black ; this is owing to the fact 

 that some of the alumina is replaced by 

 peroxide of iron. It always appears as a twin 

 crystal in the shape of a cross, hence its 

 name (Fig. 32). 



Kyanite, as its name implies, is a light-blue 

 mineral, not unlike sapphire. It possesses the 

 same composition as andalusite, and appears in 

 the same rocks as long, thin-bladed crystals 

 penetrating the gangue, and is often accom- 

 panied by staurotide and garnet. 



Topaz has also a similar composition to andalusite ; but 

 of the oxygen is replaced by fluorine thus the mineral is a 

 fluosilicale of alumina. It is trimetric, occurring in right 

 rhombic prisms, and having a perfect basal cleavage. Its colour 

 is yellow. The transparent varieties are used in jewellery, while 

 the coarse, opaque forms are sometimes substituted for emery 

 in polishing hard substances. 



Oriental Topaz is a yellow variety of sapphire, while False 

 Topaz is quartz tinted yellow ; however, it may readily be dis- 

 tinguished from the gem by its want of cleavage. 



Lapis-lazuli is remarkable for its rich blue colour. It is found 

 in granite and meta/norphic limestone, and is brought from 

 China, Persia, and Siberia. It belongs to the monometric system, 

 crystallising as dodecahedrons. Its composition is somewhat 

 complicated, being a silicate of alumina, combined with sulphates 

 of soda and lime ; chlorine and iron are also present in small 

 quantities. It is much valued in mosaic work, and is used in 

 jewellery. Formerly this stone, reduced to powder, was the 

 ultramarine of the painter a very expensive colour ; but now 

 the pigment can be produced artificially. 



Beryl is the head of a small family, the members of which 

 all contain Glucina. It is found in six-sided prisms in granite 

 and metamorphic rocks ; it has a waxy appearance and a pale- 

 green colour. The beryl of the Mourne mountains, in Ireland, 

 tends towards a cerulean blue. When beryl is transparent, and 

 but slightly tinted with green, it is kno\m as Aquamarine. 

 The largest known specimen is in the possession of Don Pedro, 

 and was found in Brazil; it weighs 18 Ibs. troy, and is without 

 a flaw. 



Emerald probably owes its deeper colour to the presence of 

 a trace of oxide of chromium. The best emeralds come from 

 Grenada, where they are found in dolomite. This gem is much 

 prized in jewellery. All the members of the beryl family are 

 harder than quartz. 



Euclase and Chrysoberyl are closely allied to emerald. 

 Zircon is a silicate of zirconia ; it occurs in dimetric prisms, 

 and is usually found in single crystals scattered in a matrix of 

 basaltic lava or syenite. Its colour is red, shading off into 

 neighbouring tints, or yellow, tending towards grey. When the 

 red specimens are transparent it is called hyacinth. 



Jargon is a smoky variety brought from Ceylon, and is often 

 sold as inferior diamonds. 



Hyacinth, when heated in a crucible with lime, loses its red 

 and assumes a clear yellow tinge. It is frequently substituted 

 for straw-coloured diamonds, but the deception is readily ex- 

 posed by the inferior hardness of the zirconia minerals to the 

 diamond. 



Garnet is a well-known gem ; it belongs to the monometric 

 system, usually appearing in dodecahedra. Besides the trans- 

 parent variety, which is used in jewellery, there are numerous 

 varieties which exhibit all the dark shades of green and red 

 to black. 



The typical composition of the group is 2 atoms of silica, 1 

 of alumina, and 3 of lime. In some garnets protoxide of iron 

 takes the place of the last ingredient. This divides the family 

 into two groups, the Lime Garnets and the Iron Garnets. It 

 must be remembered, however, that many garnets contain both 

 lime and iron, and that other elements, such as chromium 

 manganese, occasionally take part in their composition. 



The lime garnets are 



Grossular or Gooseberry Garnet, which is found in greenish 

 trapezohedrons. 



Cinnamon, Stone or Essonite owes its name to its colour, which 

 approaches that of cinnamon bark. It contains a little iron. 

 The best specimens are from Ceylon and Sweden. 



Topazolite is a variety of the last, when its colour is so light 

 as to bear a resemblance to topaz. 



Succinite and Romanzojite are also lime garnets. 



Of the iron garnets the most noteworthy are 



Precious Garnet or Almandine. The deep-red colour is often 

 so rich, that the gem has to be cut into thin plates to exhibit its 

 beauty. The best specimens are from Ceylon and Greenland. 

 The garnet is the carbuncle of the ancients, and the term 

 almandine is probably taken from Alabanda, the place where, 

 according to Pliny, the carbuncles were cut and polished. 



Melanite, as its name implies, is black as velvet ; it contains 

 manganese. 



Colophonite derives its name from its resinous lustre, 

 kolophonia being the Greek for resin. 



Garnets are found especially in mica slate, and when the rock 

 is exposed to the action of the air the soft mica wears away, 

 lea*, ing the garnet prominent. In Ireland there is a rock wholly 

 composed of garnets, without any visible cemont. The stones 

 are too small for the jeweller's use, but might probably afford 

 a good substitute for emery. 



Idocrase has been previously alluded to as offering a good 

 example of dimorphism ; it possesses the same composition as 

 garnet, and yet belongs to the dimetric system, usually ap- 

 pearing in square prisms. It is sometimes called Vesuvian, 

 having been first found in the lava of that volcano. Its colour 

 is brown. 



Tourmaline is frequently found in prisms penetrating 

 granite, gneiss, mica slate, granular limestone, and other meta- 

 morphic rocks. Its colour passes through nearly all shades. 

 The crystals are very brittle, and exhibit a vitreous lustre. It 

 belongs to the hexagonal system, and its prisms, whio.h usually 

 exhibit numerous secondary faces, are generally terminated by 

 a low pyramid. It becomes electrically excited when heated, 

 and is the mineral always used to exhibit pyro-electricity. It 

 belongs strictly to the plutonic rock, and in Cornwall is a con- 

 stituent part of certain granites. The best known varieties 

 are 



Schorl, the black variety. 

 Rubellite is red tourmaline. 



Indicolite is blue. 

 Achoite is transparent. 



The composition of tourmaline is very complex, but the most 

 remarkable of its ingredients is boracic acid. 



Axinite also contains boracic acid, but in other respects is 

 related to the garnets. It becomes pyro-electric, and fuses into 

 a green glass before the blow-pipe. Its lustre is vitreous, its 

 colour brown, and the edges of the prisms, which are triclinic, 

 are peculiarly sharp. 



Epidote was so named by its discoverer, Hauy, from the 

 Greek e-rriSiStani (ep'-i-di-do'-mi), I increase, because the base of 

 the primary form undergoes an increase in some of the secondary 

 forms. It belongs to the monoclinic system, generally appearing 

 in right rhomboidal prisms, more or less modified. The most 

 usual colour is green. 



Pistacite is a bright-green variety, frequently found in horn- 

 blende rocks. 



Zoisite is grey. 



Maganesian Epidote, which contains 14 per cent, of man. 

 ganese, is dark red. The typical composition is 3 atoms of 

 silica, 2 of alumina, and 3 of lime. 



lolite is named from its blue colour ; sometimes it is called 

 Dichroite, because it exhibits two colours when looked at in two 

 different directions. It occurs in prisms of the trimetric system. 

 Its appearance is exactly that of blue quartz, or blue glass. It 

 is composed of silica in combination with alumina, magnesia, 

 and a little iron. 



Chrysolite, like other members of this group, is used as a 



