244 DE. H. H, THOiTAS OX XEXOLITHIC [vol. Ixxviii, 



To such an assemblage the name sillimanite-buehite is applied 

 (p. 241). ^ 



The mineral, even in the small needles, has a characteristic 

 pleochroism of pale pink to colourless, similar to, but less intense 

 than, that of the larger crystals described below. Inclusions are 

 seldom present, and then consist solely of glass. 



In origin, the sillimanite is partly a product of direct vitrifica- 

 tion and partly a primary phase separated, on fall of temperature, 

 from the original aluminous melt. It furnishes the main source 

 of the alumina present in the other aluminous minerals, such as 

 anorthite, spinel, and cordierite, which are of later appearance and 

 are dependent for their formation on the transfusion of bases from 

 the io;neous magma. ^ 



Large crystals of sillimanite of a deep rose-pink colour occur in 

 a dark glass, in association with cordierite and spinel (S. 18001a), 

 PL VII, fig. 4. There is evidence that these crystals are due to 

 the reheating of an original sillimanite-buehite, and that their size 

 has thereby been increased. 



Their most striking feature is an intense pleochroism which is 

 bright rose-pink parallel to r, the long axis of the crystals, and 

 colourless parallel to a and li (S. 18001). So far as I am aware, a 

 strong pink pleochroism, recalling that of the more highly-coloured 

 varieties of andalusite, has seldom been met with in this mineral. 

 The only record that I can ti'ace of a pink sillimanite is that given 

 by Des Cloizeaux and quoted by Hintze.- 



Crystallo graphically the mineral forms approximately rectangular 

 prisms bounded by the face (110) ; these in cross-section provide 

 diagonal directions of extinction, and show traces of the usual 

 perfect cleavage parallel to (010). 



Felspar. — The chief and earliest-foiTtied felspar of the acci- 

 dental xenoliths occurs in the complex crystalline zone (anorthite- 

 corundum-spinel assemblage) which separates the sillimanite- 

 buehite from the igneous rock, and resulted from the crystalliza- 

 tion of a hybrid melt of special composition due to the mutual 

 influence of the tholeiite-magma and fused aluminous xenolithic 

 material. The felspars occur as large crystals, often reaching 

 several inches in length. Xear the external margin of the crystal- 

 line zone they are mutually interfering, but towards the buchite 

 they become more distinctly separated by glassy matter and more 

 noticeably idiomorphic. 



Their formation has, to a limited extent, been attended by a 

 resorption of the sillimanite of the buchite ; but the felspars are 



^ G. V. Wilson, ' Notes on the Formation of certain Eock -forming INIinerals 

 in & about Glass Furnaces ' Trans. Soc. Glass Technology, vol. ii (1918) 

 p. 177. 



- A. Des Cloizeaux, ' Manuel de Mineralogie ' vol. i (1862) p 179 ; see also 

 C. Hintze, ' Handbuch der Mineralogie' vol. ii (1897) p. 142: ' Zuweilen ist 

 ein Pleochroismus wahrzunehmen : Des Cloizeaux beobachtete an Spaltungs- 

 blattchen mit dem Dichroskop ein farbloses und ein schwach rosenrothes 

 Bild.' 



