KYANITE 413 



between 3-29 and 3-30. The lustre on natural crystal-faces is vitreous and frequently very 

 brilliant, and is still further increased by polishing. 



Axinite is found attached to the walls of cavities in ancient silicate rocks of various 

 kinds. The best crystals known occur in the gneiss of Le Bourg d'Oisans in Dauphine 

 (Department Isere). The mineral is found at other places in the Western Alps, less 

 frequently in the eastern (Tyrolese) Alps, at Botallack in Cornwall, and at other localities. 

 The crystals are nearly always small in size, and have very little thickness, so that cut stones 

 have only an insignificant appearance and a correspondingly low value. 



KYANITE. 



Kyanite, as its name implies, is usually of a blue colour, but may be white, pale yellow, 

 grey, or black. Crystals of a deep cornflower-blue are rare, the commonest are a pale sky- 

 blue. When sufficiently transparent, which is not often the case, this mineral forms very 

 pretty gems. 



Kyanite is comparable to a certain extent with sapphire. It has, indeed, been 

 occasionally mistaken and sold for this stone ; and the name sappare, by which it is known 

 to jewellers, recalls the same stone. This latter name arose out of an error made by the 

 Geneva mineralogist, Saussure, junior, who read the label attached to a supposed specimen 

 of sapphire as sappare ; the mistake has long been recognised, but the name remains, 

 having become tirmly established, especially among French jewellers. 



Kyanite, like topaz, is a silicate of aluminium, but its composition differs from that of 

 topaz in the absence of fluorine. Its composition is represented by the formula AlgOg.SiOg ; 

 a small part of the alumina is replaced by ferric oxide, and it is probably to this constituent 

 that the colour of the mineral is due. An analysis of fine blue kyanite from St. Gotthard 

 gave: 



Per cent. 

 Silica (SiOj) ... ... 36-67 



Alumina (AljOg) . . . 63-11 



Ferric oxide (FgOs) • ... 1-19 



100-97 



The mineral frequently occurs in definite crystals, which belong to the triclinic system ; 

 the form most commonly met with is represented in Fig. 75. The crystals are flat, elongated 

 prisms, usually with a flattened six-sided cross-section, and not infrequently they are slightly 

 bent. They occur usually embedded in gneiss, crystalline schists, and other similar rocks.. 

 There is a very perfect cleavage parallel to the broad face, which in the figure is turned 

 towards the front, and there is another less distinct cleavage parallel to one of the two 

 narrower prism-faces next to the broad face. These two narrower prism-faces are striated 

 vertically, that is to say, parallel to the edges, while the broad cleavage-face is striated 

 horizontally. Terminal faces are usually absent, and when present are irregularly developed. 

 At each end of the prism there is usually a plane face oblique to the length of the prism, 

 along which the crystal has broken (Fig. 75) ; this plane of perfect parting (not true 

 cleavage) gives rise to numerous fine horizontal cracks, which can be seen on the faces of the 



