By Ranger Naturalist H. H. Waesche
THE QUESTION is often asked, "What minerals are found in the Grand Canyon?" This cannot be answered in a few words as is usually expected by those who ask the question and who are probably not familiar with the true meaning or inclusiveness of the term "mineral". Most people associate minerals with the metals or with the ores from which various commercial materials are derived. True, these are minerals and many of them are found in the walls of Grand Canyon. Ores from which copper, iron, vanadium, asbestos, plaster of paris, lead and zinc are obtained, are represented there. They have been mined for their values although limited quantity and difficulties peculiar to the region have prevented their exploitation and further development. These, however, are not the dominant minerals of the region.
In the Grand Canyon there are vast quantities of the non-metallic minerals which are quite as interesting as the ores in spite of the fact that they are very seldom used commercially. This leads back to the question of, "What is a mineral?" E. S. Dana, the accepted authority, gives the following definition. "A mineral is a body produced by the processes of inorganic nature, having a definite chemical composition and, if found under favorable conditions, a certain characteristic molecular structure which is exhibited in its crystalline form". If this is the case, every inorganic material in the earth must be either a mineral or composed of minerals with possibly the exception of certain rock glasses, shells formed by organic agencies, and petroleum. Thus it may be seen that the walls of the Grand Canyon are composed of minerals. Probably the only exceptions are some volcanic rocks of a basaltic nature and these are rare. Even the fossils may be classed as minerals as they, for the most part, have had mineral matter substituted for the original material.
It would seem from this discussion that in an area as large as Grand Canyon National Park, there would be a limitless variety of minerals. This is not true, however, and with comparatively few exceptions it is not true of any part of the earth's crust. The past and the present geology of the Grand Canyon limits its mineral possibilities. The major part of the building of the canyon walls has been through processes of sedimentation. Primarily this restricts all of the material above the inner gorge to three principal minerals.
The most important of the three great minerals of Grand Canyon is quartz which is the chief constituent of most sandstones. Sand grains which form them are derived usually from previously crystallized quartz of igneous origin. Also, primary crystalline quartz, deposited from solution, is quite common in cavities in the various Grand Canyon formations. Quartz is, as almost everyone knows, silicon dioxide (SiO2). It is the moat plentiful mineral in the earth's crust.
Second most abundant mineral is calcium carbonate which is deposited from water solutions. This is another of the three most important minerals of Grand Canyon. If crystalline it is known as calcite, (rocks composed of crystalline calcite are called marbles), but if not crystalline, it forms limestones which are chemically similar. Various forms of calcite are plentifully distributed in most of the lime bearing formations of the canyon. Addition of magnesium to the calcite gives a very similar mineral known as dolomite. It can be distinguished ordinarily only by chemical tests.
The third major sedimentary mineral is kaolin which makes up a high percentage of the material composing the shales or mud formations of Grand Canyon as elsewhere. Shales are derived from rocks which on weathering have altered to kaolin. Kaolin is a hydrous aluminum silicate (H4 Al2 Si2 O9).
Minor minerals found in the sedimentary rocks of the region are: iron oxides, manganese oxides, barite, coppers, leads, etc. The common forms of iron are hemetite and limonite (Fe2 O3 and 2 Fe2 O3 - 3H2)). It is these minerals which give the familiar red and yellow colors to so many of the formations. Hemetite or limonite, along with calcite, are often the cementing materials which hold together the sand grains of a sandstone. Manganese oxides are often found in dendritic form in several formations. Barite, a barium sulfate, (Ba SO4) is found along the Kaibab Trail in the Redwall formation. It is probably of secondary origin, replacing the carbonate of the original limestone. In other places in the Redwall, copper has replaced the calcium, leaving green and blue copper carbonates known as malachite and azurite as replacements in the limestone. These minerals were mined for copper near Grand View Point and at other places in the Canyon a number of years ago. The chemical composition of malachite is Cu CO3 • Cu (OH)2; that of azurite is 2 Cu CO3 • Cu (OH)2. The Redwall is also the home of the lead and zinc ore minerals, galena (Pb S) and aphalerite (Zn S), and associated with them one finds some pyrite (Fe S2) and other sulfides of similar occurrence. In the much metamorphosed Bass limestone of Algonkian age is found a fibrous mineral known as asbestos. It is a secondary mineral formed in magnesian limestone as a product of metamorphism.*
In the Inner Gorge of Grand Canyon among the old Archean rocks exists a mineral world entirely different from that of the other formations of the Canyon. Here the minerals are primary ones of igneous origin associated with granite dikes, or secondary ones formed as a result of dynamic metamorphism in the schist. Partial alteration causes the formation of still other minerals and lastly there is a series of minerals resulting from solutions of mineralized waters in the fractures or brecciated zones.
The minerals in the granite are the usual types one would expect to find in that rock with quartz and orthoclase (KAlSi3 O8), a feldspar, predominating. The principal fero magnesium minerals are black mica (biotite, which is a hydrous iron, magnesium aluminum, silicate) and hornblende which is a silicate of calcium and magnesium, with iron and aluminum. Another mica is also common. It is muscovite - a hydrous sodium, potassium aluminum silicate. The microscope shows, in addition, small amounts of another feldspar (microcline (KAlSi3 O8)) and also apatite and soda-lime feldspars.
The schist of the Canyon bottom is made up mostly of mica biotite and quartz, with minor amounts of hornblende. Where the biotite and the hornblende have altered, chlorite has been formed. Chlorite is a hydrous magnesium aluminum silicate colored green by ferrous iron.
Sulfides of iron and copper are commonly found in places where water has been able to circulate in the Archean formations.
The preceding brief description covers most of the minerals that have been found in the Grand Canyon. Of course, other less common ones will be found from time to time, but those listed are the ones a visitor would expect to see if he cared to look for them. Some very interesting varieties of these minerals are distributed through the various formations and descriptions of some of them will be given in future issues of Nature Notes.
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