A variety of materials may be processed into railroad ballast. The following general classifications and
accompanying definitions list the most common materials. Detailed examination of individual materials
should be made to determine the specific mineralogical composition.
b. Granite is a plutonic rock having an even texture and consisting chiefly of feldspar and quartz.
Definitions: A plutonic rock is rock formed at considerable depth by chemical alteration. It is
characteristically medium to coarse grained, or granitoid texture.
c. Traprock is any dark-colored fine grained non-granitic hypabyssal or extrusive rock.
Definitions: Hypabyssal – Pertaining to igneous intrusion or to the rock of that intrusion whose depth is
intermediate between that of plutonic and the surface.
d. Quartzite is a granoblastic metamorphic rock consisting mainly of quartz and formed by
recrystallization of sandstone or chert by either regional or thermal metamorphism. Quartzite may also
be a very hard but unmetamorphosed sandstone, consisting chiefly of quartz grains with secondary silica
that the rock breaks across or through the grains rather than around them.
Definitions: Granoblastic – type of texture is a nonschistose metamorphic rock upon which
recrystallization formed essentially equidimensional crystals with normally well sutured boundaries.
Chert-A hard, dense cryptocrystalline sedimentary rock consisting dominantly of interlocking crystals of
quartz.
e. Carbonate rocks are sedimentary rocks consisting primarily of carbonate materials such as limestone
and dolomite.