chemical vignettes ROBERT C. BRASTED University of Minnesota Minneapolis, 55455
Nature's Geological Paint Pots and Pigments Geology, Inorganic and General Chemistry, Soils and Ecology
Most of us, on both sides of the lecture table, have txaveled over or t,hrough (even better, amongst,) our magnificent western and southwest,ern arid regions including the parks systems of t,he Painted Desert, Bryce Canyon, Zion, Yosemite, and the Grand Canyon and a host of ot,hers. We are overwhelmed by the structural geology and especially the "geology of colors." We eit>herrack our memories from courses long past taken for answers to questions posed by these wonders or KC resolve t o do some reading or course reviem at the first opportunity to remedy our deficiencies. After all, how many t,imes can we come up with the same astute answer "I don't know!!!" when t,he kids ask the numberless, "~vhys." Here is a wealth of good descriptive inorganic chemistry (including a lot of pract,ical red-ox processes) that is applicable and useful in our lecture approach to iron chemistry. It is hard to find anything more visiblc or spectacular than these colors of t,he vest. Most of the colors and shades of color that we see in these arcas can he attributed to one or another of the many iron species which are undergoing some stage of t,he weathering process. Keep in mind t,hat iron is the fourth most abundant element, in the eart,hls crust. Iron(I1) silicates are "t,abledn by t,he geologists as pyroxenes, amphiboles, dark micas, or garnets. After t,he iron minerals have been pushed up to the oxidizing atmosphere some of the iron(I1) becomes iron(II1) and the silicates may be converted to oxides or hydrated oxides as in the hematite (Fep08),limonite (a group name for hydrated oxides), goethite and lepidocrocite (FeO(OH)). I n the proper rat,io of oxidation states and hydration a veritable spectrum of colors may be produced: reds, yellows, browns, and even purples. These are, then, some of the pigments for the geological paint pots of nature. I n a brilliant and concentrated form there are mud springs in certain of our parks dhat are called just that "Paint Pot,s." Our understanding of the development of colors is placed on a firmer foundat,ion if we review momentarily the t e h h e d r a l geomehry of the silicates formed deep
'The editor is indebted to Professor W. D. Keller for the ideas used in this Vignette (W. 11. K I : I . L ~"Geochemicd , Weathering of Rocks: Source of Raw hlaterials for Good Living,'' The Geochemical Education Committee, J. Geol. Educ., 14, No. 1 , 17-22 (February, 1966)).
in the earth's crust at temperatures that may exceed 1000°C. The Fe(I1)-0-Si bond is one of the mealtest in the structure so that on exposure to the veathering conditions of t,he earth's surface, water, oxygen, some acids (carbonic for example), the total energy of the mineral assumes a lower energy state when t,he iron(l1) is converted to iron(IlI), event,ually forming some phase of iron(II1) oxide. A skeleton equation may he in the form Fe(I1)-OSi
(silicate)
-
+ 0~+ HzCOs FelOs + H,SiO, (silicic acid) + energy
Ot,her iron containing species are formed (as already mentioned), hydrated oxides and carbonates. Professor I
