268 R. H. Rastall — Minerals of Lower Greensand. 



of sphene, together with, a greater abundance of colourless pyroxene 

 (diopside), which shows the characteristic jagged appearance of this 

 mineral, due to the imperfection of the cleavage. Its maximum 

 extinction angle is about 30°, and some care is needed to discriminate 

 it with certainty from kyanite, which has about the same extinction 

 angle. The well-marked transverse cleavage or parting of kyanite, 

 parallel to c 001, will generally serve to distinguish them. 



Fig. 6. — Sphene. Aspley Guise, x 300. 



The most abundant heavy mineral is zircon ; the crystals are often 

 bo sharp and well developed that the crystallographic characters 

 can be determined and some of the angles measured approximately. 

 The most common forms are 100, 110, 111, 001, 311 in various 

 combinations, the usual ones being 110, 111, 311, and 100, 110, 111. 

 The basal plane 001 is often well developed, apparently a somewhat 

 unusual circumstance. The simple combination of a prism and 

 pyramid 110 and 111 was also observed, though not frequently. 

 According to Krushtchov the bipyramid 31 1 is specially characteristic 

 of zircons in gneissose rocks, but rare or absent in those found in 

 true granites. Both types are here almost equally plentiful. One 

 small zircon was observed enclosing a minute but perfect crystal of 

 sphene. Quite abundant also are well-rounded pink crystals with 

 the optical properties of zircon and showing a well-developed zonary 

 structure as figured by Krushtchov. It is still an open question 

 whether these are zircon or xenotime, a mineral which is probably 

 isomorphous with zircon and may even form mixed crystals with it. 

 It is noticeable that in the pink crystals the tabular and rounded 

 inclusions are less conspicuous than in the undoubted zircons and of 

 a different type. The birefringence also seems to be slightly weaker; 

 this is said to be characteristic of xenotime. There is also a pink 

 mineral with slight pleochroism and birefringence higher than that 

 of zircon. This commonly occurs as rounded grains with a curiously 

 pitted surface. Those crystals which show prismatic forms are 

 optically positive. Inclusions of any kind are rare. This is probably 

 merely a variety of zircon. 



