58 J)r, A. Hubert Cox— South Stqfordshire Fire-clays. 



It seems clear, thea, that tlie clays exaijiined consist entirely of 

 crystalline material, and that amorphous matter is absent altogether. 

 On the other hand, Dr. Mellor has recently adduced evidence from 

 the heating-curves of some South Staffordshire fire-clays for the 

 existence in them of "clayite", which he regards as being amorphous^f 

 But, as stated above, no amorphous matter could be found by 

 raineralogical examination. 



The mere fact that the clay-base is crystalline and not amorphous 

 does not, however, prevent the fine material form acting in certain 

 respects like a colloid. For example, if suspended in water, it may 

 be readily precipitated by the addition of certain electrolytes, 

 particularly by acids. 



It is unnecessary for the purpose in view in this communication to 

 enter into the vexed question as to the actual mineralogical 

 composition of this clay-base. It certainly seems, however, to act as 

 a single substance with definite optical characters, and therefore 

 presumably definite chemical composition. It is, moreover, significant 

 that the more the fi.ne material of a clay can be purified, the more 

 nearly does it approach kaolin, AI2 Os, 2 Si O2, 2 Ha 0, in chemical 

 composition. It is true that there are certain marked differences of 

 appearance between the substance of the clay-base of these South 

 Staffordshire clays and the kaolin in a china-clay. These differences 

 appear to be intimately connected with the variation in their plasticity, 

 the fire-clay being highly plastic, whereas the china-clay is only 

 slightly plastic. 



2. Arenaceous material was present in all the clays examined, 

 and often in amounts surprisingly large, considering the typical 

 argillaceous appearance of hand-specimens. The arenaceous material 

 consists for the most part of quartz-grains exactly comparable with 

 those in a normal fine-grained sandstone. Grains other than quartz 

 are not abundant, but a certain number of chert-grains were usually 

 to be found. Undoubted felspar was only rarely observed, and the 

 same was true for clastic micas. Isolated examples of heavy minerals, 

 such as tourmaline, zircon, rutile, etc., were usually present, but in 

 amounts so small as to be negligible in affecting the properties of 

 the clays. 



Besides the larger, more or less rounded, and obviously detrital 

 sand-grains, there were also present innumerable minute flakes and 

 chips of quartz. These passed downwards into the finest quartz- 

 dust, almost comparable in grain with the material of the clay-base 

 itself. It is evident that, owing to its very fine grain it would 

 prove almost impossible to separate much of this free silica from the 

 true clay material by any mere process of washing. In view of the 

 occurrence of chert-grains among the sandy material of the clays, 

 it seems very possible that some of this quartz-flour may have 

 been derived from the disintegration of cherty rocks. On the other 

 hand, it may have liad its origin in the small secondary quartzes 

 that would result from the alterations undergone by many of the 

 minerals of the old land surface then undergoing denudation. In 



^ Trans. Eng. Ceramic Soc, vol. xvi, p. 83, 1916-17. 



