226 Alfred Brammall — The Genesis of Chiastolite. 



Chiastolite is usually described (somewhat loosely) as a variety of 

 andalusite ; but though the two have the same empirical formula 

 (Al 2 3 . Si0 3 ), the same crystal form, and the same (negative) sign of 

 birefringence, the specific gravity of andalusite is constantly, and the 

 degree of hardness usually, somewhat higher than for chiastolite — 

 differences which might conceivably be an effect of higher pressure 

 exerted on the parent rock at the time of its metamorphosis. But the 

 inclusions of carbonaceous matter characterizing chiastolite are 

 significant ; such inclusions are essentially absent from andalusite, 

 which, moreover, frequently occurs in gneissose and acid igneous 

 rocks in such a manner as to leave no doubt as to its pyrogenic origin 

 (whether from the fusion of aluminous xenoliths, or as a normal 

 product of crystallization from a highly alumino-silicic magma, or 

 otherwise, is immaterial to the present discussion). The assertion that 

 chiastolite is merely a variety of andalusite may be true in this sense : — 



(1) That the temperature range within which the orthorhombic 



silicate of alumina of empirical formula Alg O3 . Si O2 can be 

 produced is very wide ; and 



(2) That if the silicate is initiated at a temperature nearer the 



lower than the upper limit — a temperature too low 

 to dissipate (by oxidation) the entangled carbonaceous 

 matter — the latter is constrained by crystallization to take 

 up the diagonal orientation characteristic of chiastolite ; and 



(3) That if the silicate is produced at a temperature sufficiently 



high to dissipate the carbon and under pressure, the product 



is andalusite. 

 But if the upper limit of the ' chiastolite-andalusite ' temperature 

 range be passed at ordinary pressure by heating andalusite to 1320° C. 

 this mineral paramorphs to sillimanite 1 (also orthorhombic but more 

 acicular in habit and of positive birefringence) ; thus 1320° is the 

 ' transition point ' (at ordinary pressure) from andalusite to sillimanite, 

 which may therefore be regarded as distinct 'phases', i.e. definite 

 mineral species ; and by analogy a temperature (much below 1320° C.) 

 at which carbonaceous inclusions in chiastolite can be dissipated may 

 be regarded as the ' transition point ' from chiastolite to andalusite ; 

 and the question arises as to whether chiastolite is merely a variety 

 of andalusite any more than andalusite is a variety of sillimanite. 



Again, chiastolite is clearly a ' low temperature ' product ; and it is 

 highly probable that the temperature range for its production is- 

 consistent with the presence of water, as such, at the time of, and 

 possibly as essential to, the nativity of the mineral. Assuming only 

 the mere presence of water, however, the production of the mineral in 

 rocks of appropriate composition 



(1) Depends upon the maintaining of a comparatively low 



temperature — within a restricted range ; and 



(2) Is effected in the presence of water (meteoric or juvenile) 



which may be essential either (a) as the sole mineralizer or 

 (b) as the carrier of mineralizers more potent than itself — 

 emanations or leachings from igneous magmas. 



1 W. Vernadsky, Bull. Soc. Min., vol. xiii, p. 256, 1890; Compt. Eend., 

 vol. ex, p. 1377, 1890. 



