EPIDOTE 



417 



EPIDOTE. 



Epidote is another mineral which is used as a gem only occasionally when of specially 

 fine quality. Almost the only locality at which transparent, finely-coloured crystals of gem- 

 quality are to be found is the Knappenwand in the uppermost part of the Untersulzbachthal, 

 in Pinzgau, Salzbm-g. There are many other localities, but they seldom yield stones which 

 are sufficiently transparent and finely colom-ed for use as gems. 



The chemical composition of epidote is expressed by the formula : 



H20.4Ca0.3(Al,Fe)203.6Si02. 



The mineral is thus a hydrous silicate of calcium and aluminium, in which a variable 

 proportion of the aluminium is replaced by ferric iron. An analysis of epidote from the 



Knappenwand is given below : 



Per cent. 



Silica (SiOa) .... ... 37-83 



Alumina (AljOg) 22-63 



Ferric oxide (FegOs) 14-02 



Ferrous oxide (FeO) 0-93 



Lime (CaO) 23-27 



Water 2-05 



100-73 



Crystals of epidote are of very frequent occurrence. They belong to the monoclinic 

 system and are usually prismatic in habit, the direction of elongation being parallel to the 



Fig. 78. Crystalline form of epidote. 



axis of symmetry, that is to say, perpendicular to the single plane of symmetry. Owing to 

 ■the predominance of one pair of parallel faces of the prism the crystals usually present a 

 flattened appearance. The elongated prism-faces are very often distinctly striated parallel 

 to their length, that is to say, parallel to their mutual intersections, while the small 

 terminal faces of the crystals are usually smooth. It frequently happens that two such 

 prismatic crystals grow together in twin position, and when this is the case some of the 

 terminal faces then form re-entrant angles with each other. As a rule, terminal faces are 

 developed at only one end of the crystal, the other being attached to the matrix. A few 

 of the forms most commonly taken by epidote are represented in Figs. 78a to c ; the 

 first two of these, a and b, are simple crystals, the third, c, is a twin-crystal. 



There is a definite cleavage parallel to one of the prism-faces, and one, rather less 

 definite, parallel to another prism-face. The hardness is 6J, being thus rather less than 

 that of quartz. The specific gravity ranges from 3-25 to 3-5 according to the amount 

 .of iron present. The specific gravity of crystals from the Knappenwand, which are 



2d 



