THE PYROXENE AND AMrHIBOLE GROUPS (NEPHRITE) 459 



The composition of nephrite is exactly the same as that of another member of the 

 amphibole group, namely, actinolite. This mineral is found not infrequently in the form ot 

 long, thin rhombic prisms, belonging to the monoclinic system, which occur embedded 

 singly in talc-schist, for example, in the Zillerthal in the Tyrol ; more often it occurs as 

 radial aggregates of acicular crystals at many localities. All the essential characters of 

 nephrite, namely, the specific gravity, hardness, and cleavage, agree completely with those of 

 actinolite, and in both minerals the colour is of a more or less intense green. Nephrite is 

 thus nothing more than a very finely fibrous to compact actinolite, the prismatic crystals of 

 which are reduced to microscopically fine fibres. Observation of thin sections of nephrite 

 under the microscope shows that the fibres have the same characters as the larger prismatic 

 ■crystals of actinolite, but that these fibres are woven and matted together in an altogether 

 irregular fashion. The disposition of the fibres with respect to each other is to a certain 

 extent characteristic of each occurrence, specimens from different localities differing some- 

 what in their microscopic structure. 



From what has been said regarding the structure of nephrite, it will be readily 

 ■understood that the mineral never shows any external crystalline form. It occurs always 

 ■as irregular masses of larger or smaller si/e or in water-worn blocks. There is never a 

 •cleavage through the whole mass of a specimen, but there is sometimes a distinct separation 

 in one direction, which is due to the material having become schistose in character. In 

 this direction the mass can be broken up with comparative ease, but in others, owing to the 

 toughness of the material, this can be accomplished only with great difficulty. It is almost 

 impossible to break up large blocks of nephrite with a hannner, especially when they are in 

 the form of rounded boulders. The method adopted in such cases is to develop cracks in 

 the mass by subjecting it to sudden changes of temperature, for example, by heating it 

 ■strongly and then plunging it into cold water. The breaking up of the mass into its 

 ■characteristic splintery fragments can then be completed by the help of a hannner. 



Compared with the extraordinary toughness of nephrite its hardness is rather low, 

 being not quite 6 on the scale (H = 5| — 6). The mineral is therefore harder than glass 

 but softer than quartz. It is brittle, but with suitable tools can be worked on the lathe, 

 the process requiring, however, special precautions and care. 



The specific gravity is very nearly 3, but is slightly variable, probably because of the 

 variable amount of iron present. The usual limits are 2-91 and 3'01, but values up to 3'1 

 and even 3'2 have been given. These high values are probably the results either of the 

 presence of foreign matter (magnetite, &c.) or of inaccurate determination. Hence most 

 .specimens of nephrite float in liquid No. 3 (sp. gr. = 3'0), but some, specimens slowly sink. 

 The specific gravity is of importance, since it affords a means whereby nephrite may be 

 distinguished from the very similar but nnich heavier jadeite (sp. gr. = 3'3), which always 

 .sinks rapidly in liquid No. 3, and scarcely floats even in pure methylene iodide. 



Nephrite is not attacked by acids. Heated before the blowpipe it becomes white and 

 cloudy and fuses with difficulty to a grey slag. In contrast to this behaviour jadeite fuses 

 ■easily, even in an ordinary flame, and, moreover, it colours the flame bright yellow, which is 

 not the case with nephrite. 



This mineral is never transparent, and is strongly translucent only in the thinnest of 

 sections or along the sharp edges of splintered fragments. In mass it is either perfectly 

 ■ opaque or feebly translucent. P'ractured surfaces are dull, but smooth polished surfaces 

 Jiave a good lustre of a somewhat greasy character. 



The colour of nephrite depends upon the amount of iron present. Like actinolite it is 

 (Usually of some shade of green, brighter or paler according as there is much or little iron 



