IDOCRASE 



4.11 



Fig. 73. Crystalline forms of idocrase. 



pi'ism. The prism-taces are sometimes very numerous, and when this is the case the faces 

 are narrow and the crystal almost cylindrical. The basal planes perpendicular to the 

 prism-faces are usually moderately large ; between the basal plane and the prism-faces there 

 are faces of a square pyramid (Fig. 73a), or of a square pyramid in combination with an 

 eight-sided pyramid (Fig. 736). These 

 faces are usually very small in size, but 

 sometimes extremely numerous ; indeed, 

 crystals of idocrase have been met with 

 which are richer in faces than crystals 

 of any other mineral. 



Idocrase shows only a suggestion of 

 cleavage ; the fracture is imperfectly 

 conchoidal to uneven. The mineral is 

 brittle and the hardness (H = 6J) is 



rather less than that of quartz. It can be fused before the blowpipe with moderate ease,. 

 and, after being fused, is decomposed by acids ; before fusion it is unattacked by acids. 

 The specific gravity varies between 3'3 and 3'5 according to the cheniical composition. 

 The brown crystals are rather heavier than the green ; thus the specific gravity of brown 

 crystals from Vesuvius has been determined to be 3'45, and that of green crystals from the 

 Ala valley to vary between 3"39 and 3"43. The mineral, therefore, sinks in pure methylene 

 iodide, but floats in the heaviest liquid (sp. gr. = 3-6). 



In the matter of transparency different specimens of idocrase may differ widely. The 

 majority of crystals are only translucent to semi-transparent, the free end of an attached 

 crystal being frequently more transparent than the other. Only transparent or very 

 translucent crystals are cut as gems : by polishing they acquire a very good lustre, which is- 

 vitreous in character, though on fractured surfaces it inclines to be greasy. The refraction 

 is moderately strong, but the double refraction is feeble. The mineral presents a great 

 range of colour, this being due to the iron and manganese which enter into its composition. 

 Yellow, blue, and red, as well as almost colourless crystals, are met with ; but the colours 

 most commonly seen are various shades of brown and green, and it is crystals of these 

 shades which are used almost exclusively for cutting as gems. The dichroism of idocrase is 

 moderately strong ; in the case of green idocrase from the Ala valley the two images seen 

 in the dichroscope have the maximum colour difference when one is pure green and the 

 other yellowish-green. 



Idocrase occurs at numerous localities as a contact-mineral embedded in limestone, also- 

 in crystalline schists, gneiss, and serpentine. These modes of occuri'ence are well illustrated 

 at the two localities in Italy already mentioned. The occurrence of the mineral under 

 other conditions is rare, and for the present purpose need not be considered. 



The green idocrase of the Ala valley is found on the Testa Ciarva, a bare, precipitous- 

 rock on the Mussa-Alp. in a band of serpentine more than a metre in thickness : the 

 crystals are here associated with chlorite, and are attached to pale-green massive idocrase. 

 It is at this same place that hessonite, the yellowish-brown variety of garnet, occurs ; the 

 two minerals are not, however, found side by side. Fig. 1 of Plate XV. represents a 

 specimen from this locality, having one large and several small crystals attached to the 

 matrix. Fig. 2 of the same plate shows a stone cut from one of these crystals, the colour 

 of which is a fine grass-green tinged with yellow. This has, therefori, a certain similarity 

 in appearance to chrysolite, and is even sometimes known by this name. The yellowish 

 tinge of the latter stone is, however, more pronounced, and its dichroism is scarcely 



