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 
place of kaolin, whereby the same amount of silica but only one- 
third of the potash originally present in the orthoclase are lost. 1 As 
the final product of feldspathic 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. 
i H. Rosenbusch, Elemente der Gesteinslehre, 3d edition, p. 79. 
2 H. Niklas: Die Kolloidchemie und ihre Bedeutung fur Bodenkunde, Geologie uud Mineralogie. Inter. 
Mittl. fur Bodenkunde, Vol. Ill, 1913, p. 395. 
a Mohs scale. 
