PHYSICAL PROPERTIES OF ROAD MATERIALS. 7 



PRIMARY MINERALS. 



Quartz (PI. VII, fig. 1) is without doubt the most widely distrib- 

 uted mineral in the earth's crust and is found in greater or less abund- 

 ance in all kinds of road materials excepting the more basic types of 

 unaltered igneous rocks (traps) and certain varieties of basic crystal- 

 line schists (amphibolites, eclogites). It has a specific gravity of 2.66 

 and is extremely hard, standing No. 7 jn the Mohs scale of hardness. 1 

 It is characterized in the rock sample by irregular, rounded shape, 

 without parting planes (cleavage), and breaks readily into splintery 

 glass-like fragments. Quartz sometimes appears as a secondary min- 

 eral in finely crystalline aggregates (chalcedony), often associated 

 with opal or hydrated silica. It will be shown later on that where 

 quartz is present in large quantities, especially when firmly consoli- 

 dated, as in the case of fine-grained igneous and massive metamorphic 

 rocks, the resulting material is extremely hard and offers great resist- 

 ance to wear. 



Orthoclase (microcline) and Plagioclase feldspars (PI. VII, figs. 2 

 and 3) are among the principal ingredients of igneous and metamor- 

 phic rocks and certain types of sandstones. They vary in specific 

 gravity from 2.54 to 2.76, according to composition, and in hardness 

 from 6 to 6.5. (Mohs scale.) These minerals occur as rock constitu- 

 ents frequently in well-defined, tabular-formed or lath-shaped crys- 

 tals characterized by perfect cleavage along planes at, or approaching, 

 right angles to each other. Because of this cleavage, many coarse- 

 grained feldspathic rocks break down readily under impact into frag- 

 ments having very characteristic cubical shapes. When present, 

 however, in fine-grained aggregates closely intergrown with quartz, 

 augite, or other minerals, the effect of cleavage is not apparent 

 and the resulting rocks are extremely hard and tough. 



Augite and Hornblende (PI. VII, figs. 4 and 5) form the chief iron- 

 bearing or dark silicate constituents of basic igneous rocks (traps) 

 and the crystalline schists derived from them. They have a hard- 

 ness of 5 to 6.5, 2 specific gravity of 2.93 to 3.71, and occur usually in 

 more or less elongated, prismatic shapes, with well-defined cleavage 

 parallel to the prism planes. Owing to their peculiar form, these 

 crystals interlock very compactly with adjoining minerals, especially 

 lath-shaped, plagioclase feldspar, thereby developing the most dura- 

 ble types of road luaterial (traps). 



Biotite (black mica) and Muscovite (white mica) are at home chiefly 

 in granitic rocks and in micaceous schists and gneisses. Their spe- 

 cific gravity varies from 2.7 to 3.2, with hardness of from 2 to 3. 2 



1 This scale as described in Merrill's Rocks, Rock-weathering and Soils, p. 12, is based upon the relative 

 hardness of the following minerals arranged in ascending order: (1) Talc, (2) gypsum, (3) calcite, (4) fluorite, 

 (5) apatite, (6) orthoclase, (7) quartz, (8) topaz, (9) corundum, and (10) diamond. 



2 Mohs scale. 



17715°— Bull. 348—16 2 



