Contact Metamorphic Minerals

When an igneous rock magma is intruded into the earth’s crust, it causes though the attendant heat and pressure a greater or less alteration in the surrounding rock. This alteration, or metamorphism, of the rocks lying next to an igneous intrusion usually consists partly in the development of new and characteristic mineral species. The minerals that are formed under these conditions are known as contact metamorphic minerals, since they are produced by a metamorphic change and are to be found at or near the contact line between the rock in which they lie and an igneous rock. Any rock into which an igneous mass is intruded will be affected in a greater or less degree, the amount and character of the change depending chiefly upon the size of the intrude mass and upon the chemical and physical character of the surrounding rock. The most striking and important contact metamorphic changes take place when the igneous rocks is intruded into impure limestones. When a pure limestones is affected, it is recrystallized and converted into a marble, but without any development of new species. But, on the other hand, in the case of an impure limestone the heat and pressure caused by the igneous intrusion will serve to develop new and characteristic minerals in the rock. An impure limestone will ordinarily contain, besides the calcium carbonate of the rock, varying amounts of quartz, clay, iron oxide, etc. Under the influence of the heat and pressure these materials will combine with the calcium carbonate to form new minerals. For instance, the calcite and quartz may react together to form wollastonite, CaSiO3. If the limestone contains dolomite, the reaction of this minerals with quartz may produce pyroxene, (Ca,Mg)SiO3. If clay id present, aluminum will enter into the reaction and such minerals as spinel, MgAl2O4, and grossularite, Ca3Al2Si3O12, may result. If any carbonaceous materials are present, the effect of the metamorphism may covert them into graphite. The common contact metamorphic minerals found in limestones are as follows: graphite, spinel, corumdum, wollastonite, tremolite, pyroxene and the lime garnets, grossularite and andradite.

As mentioned in a preceding paragraph, an igneous rock in cooling often gives off large amounts of mineralizing vapors. These consist largely of water vapor, but often include boron and fluorine gases. Under the influence of these vapors, other minerals are often formed in the contact zone of limestones. These particular minerals are commonly spoken of as pneumatolytic minerals, since they are formed, partly at least, through the agency of minerals gases. They consist chiefly of calcium and aluminum silicates which contain hydroxyl, fluorine or boron. The most common of the pneumatolytic contact minerals are chondrodite, vesuvianite, scapolite, phlogopite, tourmaline and fluorite.