506 NEW YORK STATE MUSEUM 



most commoiilj present and we may also find calcite, gypsum, mica^ 

 siderite or carbonate of iron, pyrite, dolomite, iron oxid, etc. 



Quartz. This mineral is present in sedimentary clays mostly in 

 the form of fine grains, or sometimes in crystals, while in reside 

 ual clays the particles are usually angular. It may be colorless, but 

 the grains may be often superficially colored either red or yellow 

 by iron oxid. It is a very hard mineral and scratches glass easily. 

 Feldspar might be mistaken for it, but feldspar will not scratch 

 glass. 



Flint or amorphous silica is sometimes present in clays. It 

 usually has a muddy color, and a conchoidal fracture. It might 

 be found in either residual or sedimentary clays. 



Quartz and flint are infusible except at very high temperatures; 

 but the presence' of other minerals in the clay acting as fluxes often 

 (pluses them toi soften at a much lower teimperature. In addition 

 quartz serves to diminish the shrinkage of a clay, and, if not natur- 

 ally present in sufiicient quantity, has to be added during the pro- 

 cess of manufacture. The admixture of quartz also tends to de- 

 crease the plasticity, the more so, the coarser the grain. Tho size 

 of the quartz grains aflects the ease with which they can be fluxed ; 

 for, as fusion begins on the outside of a quartz grain, the larger 

 the grain the longer it will take to reach the center. Therefore if 

 the heat is not continued long enough, it may happen that the 

 outside of the grain has been softened and the center is unaflectedi 



Feldspar. Since kaolinite is formed by the decomposition of 

 feldspar, it seems but natural that we should find some undecom- 

 posed grains of the latter in almost every clay. The fragments 

 \vould be scaly or rhombohedral in form. Feldspar is slightly 

 softer than quartz, and while the latter scratches glass, the former 

 'will not. It is commonly pihk, red, yellow or even white. Few 

 fragments fail to show a white coating on the surface of the grains, 

 or lining the cracks and cleavage planes of th.e mineral, indicating 

 the presence of some kaolinite. 



Calcite. This mineral may occur in clays in the form of little 

 rhombohedral grains, soft enough to be scratched with a knifci 



