MINERALOGY OF CLAYS. 43 



it gives rise to soluble sulphates which cause efflores- 

 cence. 



MICA. 



This mineral can be frequently detected by the nak- 

 ed eye, owing to its high lustre, even when it is present 

 in the form of very minute scales. It is seldom absent 

 in clays and is usually found to an appreciable extent 

 in even the best kaolins, for on account of its scaly 

 nature and lightness, it remains suspended in water 

 for a long while and is consequently very hard to re- 

 move by washing; small amounts of white mica are 

 rarely injurious. 



'Mica is usually ound in those clays which have been 

 derived from the breaking down of igneous or meta- 

 morphic rocks, such as granites, gneisses or schists, 

 and two species are recognized in clay, i. e. biotite and 

 muscovite. The former is a complex silicate of iron, 

 magnesia, and alumina, and occurs as six sided plates 

 or irregular scales usually of a dark color. As it easi- 

 ly decomposes with the formation of iron oxide, it is 

 not so apt to be found in clays as the muscovite, which 

 is more resistant to weathering. The muscovite is 

 sometimes called potash mica, although it also con- 

 tains a small amount of iron and magnesia; it is of 

 silvery white or light brown color. 



Mica decreases the plasticty of clay, and tends to 

 make it flaky when wet, if in a finely divided condition. 



White mica tends to increase the refractoriness of a 

 clay, and to exert very little fluxing action, even at 

 moderately high temperatures. 



IRON OXIDE. 



This, next to quartz, is perhaps the commonest min- 

 eral impurity of clay. It occurs as earthy grains, as 



