PHYSICAL PROPERTIES OF ROAD MATERIALS. 9 



feldspar (PI. VII, fig. 2) through a process of hydration and dissolu- 

 tion by which all of the potash and two-thirds of the silica present 

 in the feldspar may be removed. Under somewhat different condi- 

 tions of weathering secondary white mica (sericite) will result in 

 placo of kaolin, whereby the same amount of silica but Only one- 

 third of the potash originally present in the orthoclase are lost. 1 As 

 tho final product of foldspathic rock decay, kaolin is found, usually 

 in large quantities, in clays and similar residual deposits. 2 Kaolin 

 occurs either in well-defined, small crystal plates (kaolinite) resem- 

 ling muscovite (PI. VII, fig. 9), in physical properties (hardness 

 2-2. 5, 3 specific gravity 2.6-2.63), or in the form of minute, amorphous 

 grains of very indefinite composition, often associated with iron 

 oxide and opaline silica. In this condition kaolin acts as a colloid 

 and influences very greatly the plasticity and cementing value of 

 rock powders by increasing in volume, and becoming glue-like 

 when wet and binding the mineral particles firmly together when dry. 



Chlorite and Epidote (PI. VII, figs. 7 and 10) are frequently 

 associated as alteration products of augite, hornblende, dark mica 

 (biotite), and plagioclase, and are consequently most abundant 

 in trap rocks or in darker varieties of crystalline schists (Table 1, 

 Nos. 12, 14, and 26). The process of alteration is similar to that 

 already stated regarding kaolin and the products themselves are in 

 many cases not unlike this mineral in physical properties. Chlorite 

 is a soft green mineral found either in the form of well-developed, 

 thin plates and folia resembling mica, or as very fine scales and fibers 

 of indefinite composition (viridite) frequently accompanied by 

 amorphous silica, calcite, and iron oxide. In the latter development, 

 chlorite bears a close resemblance to kaolin in its effects on the 

 cementing properties of road materials. 



Epidote in contrast to chlorite, occurs as granular or prismatic 

 crystal aggregates, derived chiefly from plagioclase feldspar when 

 accompanied by iron-bearing compounds. It is a hard (6-7 3 ), tough 

 mineral with high specific gravity (3.25-3.5), yellowish green color 

 and, when present in appreciable quantities, apparently increases 

 the wearing qualities of rocks. 



Other secondary minerals. — Among other products of rock decay 

 may be mentioned limonite, serpentine, talc, opal, and zeolitic com- 

 pounds. These minerals are, with the exception of the harder 

 crystalline zeolites (chabazite and natrolite), very indefinite in 

 crystal form or entirely amorphous (opal and limonite), and are not 

 readily separated by means of the microscope. 



1 H. Rosenbusch, Elemente der Gesteinslehre, 3d edition, p. 79. 



2 H. Niklas: Die Kolloidchemie und ihre Bedeutung fur Bodenkunde, Geologie und Mineralogie. Inter. 

 Mittl. fur Bodenkunde, Vol. Ill, 1913, p. 395. 



3 Mohs scale. 



