Minerals resist weathering to different degrees, therefore the mineral composition of a rock is another considerable factor in determining its durability. Where pyrite is present in abundance it easily weathers out, leaving iron-stained pits and releasing sulphuric acid which decomposes the rock. Abundance of mica, especially where segregated along the stratification planes, permits easy splitting of the rock under weathering. Likewise the mica often weathers more quickly than the surrounding minerals, giving a pitted appearance; in marbles and limestones its irregular occurrence may spoil the appearance. Flint or chert in abundance is deleterious to limestones and marbles, because, being more resistant, it stands out in relief on the weathered surface, interferes with smooth cutting and polishing, and often causes the rock to split along the lines of the flint concretions. Abundance of tremolite may also be disadvantageous to limestones and marbles, because it weathers to a greenish-yellow clay and leaves a pitted surface.
The crushing strength of a rock has an obvious relation to its structural uses. The rock must be strong enough for the specified load. Most hard rocks ordinarily considered for building purposes are strong enough for the loads to which subjected, and this factor is perhaps ordinarily less important than the structural and mineral features already mentioned.
It is often necessary to know the modulus of elasticity and other mechanical constants of a rock, as in cases where it is to be combined with metal or other masonry or to be subjected to exceptional shock.
The beauty and coloring of a rock are its esthetic rather than its utilitarian features. They are particularly important in the construction of buildings and monuments for public or ornamental purposes.
Crushed Stone
The largest use of rock or stone is in the crushed form for road building, railway embankments, and concrete, and the prospect is for largely increased demands for such uses in the future. For the purpose of road building, it is necessary to consider a stone's resistance to abrasion, hardness, toughness, cementing value, absorption, and specific gravity. Limestone cements well, but in other qualities it is not desirable for heavy traffic. Shales are soft and clayey, and grind down to a mass which is dry and powdery, and muddy in wet weather. Basalt and related rocks resist abrasion, and cement well. Granites and other coarse-grained igneous rocks do not cement well and are not resistant to abrasion. Many sandstones are very hard and brittle and resist abrasion, but do not cement.
The application of geology on a large scale to the study of sources and qualities of crushed stone is now being required in connection with the great state and national projects of highway building. This work is by no means confined to a mere testing of the physical qualities of road-building materials found along the proposed route, but includes a careful study of their geologic occurrence, distribution, and probable amounts. In certain of the northern states specialists in glacial geology are preferred for this purpose.
Stone for Metallurgical Purposes
The use of limestone and other rock for metallurgical fluxes is dependent very largely on chemical composition. Comparatively few limestones are sufficiently pure for this purpose. For furnace linings, the quartzite or ganister must be exceptionally pure. The field search for rocks of the necessary composition has required geologic service.