The hardest thing to see is what is in front of your eyes.
In my long-ago undergraduate days in western Kansas students commonly worked deciphering the stratigraphy of the Cretaceous Dakota and/or Kiowa formations. If you did not want a project in the limestones or chalk beds, then the Dakota/Kiowa was about the only possibility within close driving distance. In fact, my senior project was tying to map crossbeds in the Dakota and describing some sections. I remember the rock colors of the Dakota were mostly red or orange (or so it seems). We usually described the non-quartz and -calcite visible minerals as iron oxide or limonite and moved on from there.
|These large concretions (~10-12 feet) have eroded from the Dakota (maybe Kiowa) in Ottawa County, Kansas at Rock City.|
As life progressed, I simply thought all red or orange “stuff” in these sandstones was limonite or some such iron oxide—who cared about trivialities?
Only later in life when my first teaching assignment included Sedimentary Geology did I “start to care,” at least a little! BTW, teaching sedimentary geology was a joy since originally, I was assigned Structural Geology, a course that was not one of my strong points ( have you ever tried working with, and understanding, stereonets).
|A stereonet is a powerful method for displaying and manipulating the 3-dimensional geometry of lines and planes (www.sciencedirect.com)--or so they say, not so much for me!|
Along with Sedimentary Geology I labored big time in Ground Water Resources in Western Kansas (I had never taken any sort of a ground water course), Invertebrate Paleontology, and Intro to Geology. Yep, four different course preps for a kid who was trying to finish his Ph.D. dissertation and was soon to be a new father. Spring semester was about the same with Historical Geology, Intro to Geology, Field Methods, and something else. I finished the first good draft of my dissertation on a dark midnight in mid-February and my son was born the next day. I have trouble, even today, remembering much of that first academic year, 1970-71, except the pay was $9000 for the academic year and no commitment for a second year. However, things were going my way when a new tenure-track contract came in mid-May just as the three of us were heading to Dinosaur National Monument where I had a summer position. Life was good and I did graduate that summer (although the Park Service would not let me miss a day to attend graduation in Salt Lake City). After that hectic year I resumed remembering “things.”
The reason my memory was recently jogged about iron oxide minerals is that Mr. Rockhounding the Rockies (rockhoundingkw.blogspot.com), one of the premier collectors of minerals from the Pikes Peak Batholith (age around 1.08 Ga) gifted me an absolutely gorgeous specimen of goethite, an iron oxide-hydroxide [FeO(OH)]. It reminded me, again, that not all iron oxides/hydroxides look like rust, appear as a coating of clay (as in limonite), attract a magnet (magnetite), are a critical ore of iron (hematite), nor do they all come from sedimentary rocks.
In our basic chemistry/physical
geology courses we learned that iron occurs in two different oxidation states:
1) ferrous iron has a plus 2 charge (written as Fe++ or Iron II )
and needs to share two electrons with oxygen to form a neutral ion; 2) ferric
iron has a plus 3 charge (written as Fe+++ or Iron III) and needs to
share three electrons with oxygen to form a neutral ion. Ferric iron is more stable than ferrous iron, the latter then commonly is oxidized (adds more oxygen) and becomes ferric iron.
|Goethite collected from rocks of the Pikes Peak Batholith near Lake George. Width FOV ~5.6 cm. As with many dark, metallic luster, minerals photography is difficult with my equipment. The specimen is much more attractive than depicted in the photo.|
|Photomicrograph of a 1.0 cm. width FOV section of above. Individual prismatic crystals are east to observe.|
Ferrous oxide is rare as a mineral due to its lack of stability and about the only mineral is wustite, a rare oxide usually found in meteorites and man-made slag from smelters. The cation iron has a ++ oxidation state (plus 2) and the anion oxygen has a - - (minus 2) oxidation charge so they balance out: one iron (++) combined with one oxygen (- -) = FeO.
The major ferric iron mineral is hematite, Fe2O3. Here you can see two units of iron (charge of +++) X 2 = 6 combine with three oxygen units (charge of - -) X 3 = 6 or +++ X 2 irons = 6 and - - X 3 = 6 oxygens. So, it balances.
There also is a major iron oxide mineral termed magnetite, Fe3O4 that appears not to balance! However, magnetite is actually composed of both ferric and ferrous iron and should be written as: FeO-Fe2O3, one part of each (one unit of ++iron (2) and two units of +++ iron (6) = 8. One unit of - - oxygen (2) and three units of - - oxygen (6) = 8. Wow, it balances.
Iron minerals become even more complicated when one considers the iron hydroxides where the OH ion with a charge - - (minus 2) combines with iron. As far as I can tell, ferrous (Iron II) can combine with a hydroxide ion, but only as a solution in the lab: Fe++(OH)2. One iron ++ and two hydroxides - . So, one iron ++(2) combines with two hydroxides - -(2) and it balances.
Ferric (+++ or Iron 3) iron may combine with hydroxide to form a really rare and complex mineral called bernalite [Fe(OH)3]: One Fe+++ (3) combines with three hydroxides each with a charge of minus 1– to equal 3, and it balances.
Another major group of iron and oxygen minerals are the Ferric (Iron III) oxide-hydroxides: ferric iron plus the hydroxide ion plus oxygen. The major mineral in this group is goethite, FeO(OH). In goethite there is one unit of ferric iron with an oxidation state of +++ that combines with one oxygen (oxidation state of - -) and one hydroxide (oxidation state of -). So, three of iron equals the two of oxygen plus the one of hydroxide, 3 = 3.
In reality, there are at least three named polymorphs of goethite---exact same chemical formula but crystallizing in different crystal systems: akageneite, lepidocrocite and feroxyhyte.
But, what about limonite, that rusty clay or black streak or “ironstone” or whatever that is common in the orange or red Dakota Sandstone of my youth. Is the mineral ferric or ferrous iron and is it an oxide, or a hydroxide or an oxide-hydroxide? It turns out that limonite is not even a mineral [often written as Fe+3O(OH)-nH2O] but a combination of several “real” minerals---goethite, lepidocrocite, akaganeite, maghemite, hematite, pitticite, and “jarosite group” minerals and the term is used “for unidentified massive hydroxides and oxides of iron, with no visible crystals, and a yellow-brown streak” (MinDat.org). Commonly, limonite is composed of goethite.
I am still not certain that I can identify goethite from limonite in many orange to red sedimentary rocks since both have similar colors (red, reddish brown, yellow brown, brownish black), similar hardness (5.0-5.5 or 4.5-5.0 in limonite[Mohs]), dull to metallic to adamantine luster, and a yellowish brown to orange-yellow streak, and often massive. However, the goethite from the rocks of the Pikes Peak Batholith is different in that it often forms spectacular crystals.
The Pike Peaks goethite is composed of slender, flattened crystals that are elongated along the C-Axis, vertically striated, and exhibit a metallic luster. They form “clumps” of radiating crystals and appear to be black or brownish black in color. However, the streak is brown to brownish yellow to yellow orange. The crystals are secondary in nature and are derived by weathering (an oxidizing environment) of many different iron-bearing minerals. Mr. Rockhounding the Rockies has collected his goethite specimens from the same cavities that produce amazonite and smoky quartz (see his web site for many photos).
To learn more about goethite, and especially Goethe, check out my Blog posting on April 23, 2012: Goethite, Goethe, and Kaninchen.
And finally, words of advice from Johann Wolfgang von Goethe (1749-1832): Every day we should hear at least one little song, read one good poem, see one exquisite picture, and, if possible, speak a few sensible words.
The hardest thing to see is what is in front of your eyes. See top of article. Johann Wolfgang von Goethe