THE FELSPAR GROUP 425 



the obliquely disposed terminal face represented in Figs. 80a and b. On other parts of the 

 crystal the lustre is vitreous, but on this particular face, usually known as the basal plane, 

 the lustre is pearly ; and from the cleavage cracks parallel to this face brilliant iridescent 

 colours are sometimes reflected. The second cleavage, which, though good, is rather less 

 perfect than the first, is parallel to the pair of faces which truncate the acute side edges of 

 the prism (Fig. 806) ; these faces together form what is known in crystallography as the 

 clino-pinacoid. 



The cleavages of monoclinic and of triclinic felspar are essentially the same, the 

 differences between them being unimportant. In monoclinic crystals the two cleavage 

 planes are exactly perpendicular to each other, and for this reason monoclinic felspar has 

 received the name orthoclase, that is to say, cleaving at right angles. In triclinic felspar 

 the two cleavage directions are not quite at right angles, and so this variety is distinguished 

 as plagioclase, that is to say, obliquely cleaving. The two directions are not, however, very 

 oblique ; in one variety, for example, the angle is barely 93°. The cleavage of felspar is a 

 feature which, taken in conjunction with its hardness, readily distinguishes it, even when in 

 massive fragments, from other minerals. The hardness of felspar is 6 ; in fact, this mineral 

 is the one which is chosen to stand sixth on Mohs' scale of hardness. The fracture of 

 felspar in directions other than the cleavage planes is sub-conchoidal to uneven. 



The specific gravity varies with the chemical composition between 2"5 and 2"7. The 

 greater the amoxuit of lime present the heavier is the felspar, pure lime-felspar or 

 anorthite having the highest specific gravity of any. All felspars are fusible before the 

 blowpipe, but only with difficulty. Neither pure potash- nor soda-felspar is attacked by 

 hydrochloric acid, but anorthite and other varieties containing much lime are readily 

 decomposed. The greater the amount of lime present the more easily is the felspar attacked 

 by hydrochloric acid, and the more easily and completely is it decomposed. 



As already mentioned, common felspar is in most cases dull in colour, cloudy, and 

 imperfectly transparent. The colours most commonly seen are very pale shades of yellow, 

 brown, and red, but more intense shades are also met with. The variety of potash-felspar, 

 known as amazon-stone, has a fine green colour and is sometimes cut as an ornamental stone, 

 but the yellow, brown, and red felspars are never used for this purpose. Quite colourless 

 felspars, more or less perfectly transparent, are by no means rare ; the only variety 

 used for ornamental stones is that from the surface of which there is reflected a beautiful 

 milky light. 



The precious stones which are included in the felspar group are not much used nor are 

 they of any great value. Nevertheless they have a certain importance. They will be dealt 

 with below in some detail : first, amazon-stone, the use of which, for ornamental purposes, 

 depends solely upon its colour; and secondly, those felspars which display a reflection of 

 milky light or a play of colour, namely, moon-stone, sun-stone, labradorescent felspar, 

 and labradorite. Other members of the felspar group are never used for this purpose. 



AMAZON-STONE. 



In colour amazon-stone is verdigris-green, sometimes tinged with blue. All shades of 

 this particular colour, from the palest to the darkest, are met with, but only those stones 

 which exhibit perfectly pure dark shades of colour are cut and polished. Stones showing 

 white, yellow, or red patches and streaks are frequently met with, but are useless for 

 ornamental purposes. Amazon-stone is opaque or only slightly translucent, and it acquires 



