CHEMICAL PROPERTIES OF CLAY. 43 



superficial!}- red or yellow by iron oxide. It breaks with, a glassy, 

 shell-like fracture, and is a very hard mineral, being seven in 

 the scale of hardness. 1 It will, therefore, scratch glass, and is 

 much harder than most of the other minerals commonly found in 

 clay, with the exception of feldspar. Quartz at times forms 

 nodules, which have no' crystalline structure, and are termed flint. 

 These are sometimes seen in the Pensauken gravels of the State. 

 Quartz pebbles are not at all uncommon in some of the Cape May 

 and Cohansey clays, and most of the sand grains found in the 

 coarse, gritty surface clays are quartz. This mineral also forms 

 most of the hard pebbles found in the "feldspar" beds of the 

 Woodbridge district. 



Both quartz and flint are highly refractory, being fusible only 

 at cone 35 of the Seger series (see Fusibility, Chap. IV), but 

 the presence of other minerals in the clay may exert a fluxing 

 action and cause the quartz to soften at a much lower tempera- 

 ture. (See Silica, below, and also The Fire Clays and Fire- 

 Brick Industry, Chap. XVI.) 



Feldspar. — Feldspar is a mineral of rather complex composi- 

 tion, being a mixture of silica and alumina, with either potash, 

 or with lime and soda, and occurring usually in red, pink or 

 white grains. When fresh and undecomposed, the grains have 

 a bright lustre, and split off with flat surfaces or cleavages. 

 Feldspar is slightly softer than quartz, and while the latter, as 

 already mentioned, scratches glass, the former will not. Feld- 

 spar rarely occurs in such large grains as quartz, and, further- 

 more, is not as lasting a mineral, being easily attacked by the 

 weather or soil waters, and so decomposed to a whitish clay. 

 This change can be seen in the so-called "feldspar" deposits of 

 the Woodbridge district, which were originally a mixture of 

 quartz and feldspar pebbles; the latter, however, have been 

 mostly changed to kaolinite, and it is possible to> find pieces 

 showing all stages in the change from feldspar to kaolin in the 

 same bank. 



1 The hardness of minerals is expressed in terms of Mohs' scale, which is 

 made up of ten minerals, No. i being the softest and No. 10 the hardest. The 

 series includes the following minerals: i, Talc; 2, Gypsum; 3, Calcite; 

 4, Fluorite ; 5, Apatite ; 6, Orthoclase ; 7, Quartz ; 8, Topaz ; 9, Corundum ; 

 10, Diamond. 



