Iddingsite is an alteration of
olivinethat consists of a mixture of clay minerals, iron oxides and ferrihydrides. Iddingsite forms from the weathering of basaltin the presence of liquid water and can be described as a phenocrystmeaning it has megascopically visible crystals in a fine-grained groundmass of a porphyriticrock. Iddingsite is a pseudomorph that has a composition that is constantly transforming from the original olivine that pass though many stages of structural and chemical change to create a fully altered iddingsite. Because iddingsite is constantly transforming it does not have a definite structure or a definite chemical composition. The chemical formula for iddingsite has been approximated as MgO * Fe2O3 * 4 H2O where CaO can be substituted by MgO. The geologic occurrence of iddingsite is limited to extrusive or subvolcanic igneous rocks that are formed by injection of magma near the surface. Iddingsite is also absent from deep-seated rocks and is found on meteorites. Iddingsite is a very important mineral to modern science as it has been found on Martian meteorites and its ages have been calculated to obtain absolute ages when liquid water was at or near the surface of Mars.
Iddingsite is a pseudomorph meaning that during the alteration process the olivine crystals had there internal structure or chemical composition changed, but the external form has been preserved. This is not true for all phases of the alteration of olivine because the atomic arrangement becomes distorted and causes a non-definite structure to form. Iddingsite has a composition that is constantly transforming from the original olivine passing though many stages of structural and chemical change (Gay, Le Maitre 1961). Iddingsite has been a subject researched in recent years because of its presence in the Martian meteorites.Figure 1 is an example of Martian rust which is a sample of iddingsite made up of a clay and oxide mixture (http://fti.neep.wisc.edu/neep602/FALL97/LEC16/lecture16.html). The formation of iddingsite requires liquid water, giving scientists an estimate as to when there has been liquid water on Mars (Swindle T.D. et al, 2000). Potassium-argon dating of the meteorite samples showed that Mars had water on its surface anywhere from 1300 Ma to 650 Ma ago (Swindle T.D. et al, 2000).
Iddingsite is a mineral that lacks a definite chemical composition, so exact compositions cannot be calculated. An approximated composition for a hypothetical end product of iddingsite has been calculated as being SiO2 = 16%, Al2O3 = 8%, Fe2O3 = 62% and H20 = 14%. Figure 2 shows the composition and refractive index of various stages of olivine alteration. Column 1 show the ideal composition of olivine with no alteration taken place. Column 2 shows what the composition of a sample of iddingsite taken from nepheline bearing olivine basalt from Vogelsberg Germany. Column 3 shows the composition of a sample found in trachy basalt from Gough Island. Column 4 shows the composition of a sample taken from pyroxene trachyte, Mt. Moroto, Uganda. This table shows that through out the alteration process of olivine, there is a decrease in SiO2, FeO and MgO and an increase in Al2O3 and H2O. The chemical process associated with the alteration consists of the addition of Fe2O3 and the removal of MgO (Gay, Le Maitre 1961). The chemical formula for iddingsite is approximated as MgO * Fe2O3 * 4 H2O where CaO can be substituted by MgO by a ratio of 1:4. (Ross, Shannon 1925). There are also some trace constituents of Na2O and K2O that enter iddingsite as the alteration process progresses (Gay, Le Maitre 1961).
The geologic occurrence of Iddingsite is limited to extrusive or hypabyssal rocks and is absent from deep-seated rocks. Iddingsite is an epimagmatic mineral derived during the final cooing of lava in which it occurs from a reaction between gasses, water and olivine (Ross, Shannon 1925). The formation of iddingsite is not dependent on the original composition of the olivine. It is however dependent on oxidation conditions, hydration and the magma from which iddingsite forms must be rich in water vapor (Edwards 1938). The alteration of olivine to iddingsite occurs in a highly oxidizing environment under low pressure and at intermediate temperatures. Temperature needed for the alteration process has to be above temperatures that could cause the olivine to solidify, but below temperatures that would cause structural reorganization (Gay, Le Maitre 1961). Structure:The structure of iddingsite is difficult to characterize because of the complexity of the possible alterations that can occur from olivine. Iddingsite has the tendency to be optically homogeneous which indicates that there is some structural control. Structural rearrangements are controlled by hexagonal sequences of approximately close-packed oxygen sheets. These oxygen layers are perpendicular to the x-axis of an olivine cell. One of the close-packed directions is parallel to the z-axis of an olivine cell. These ion arrangements within olivine control the structural orientation of the alteration products. X-ray diffraction patterns found that there are five structural types of iddingsite that can occur during different stages of alteration. They are: olivine-like structures, goethite-like structures, hematite structures, spinel structures and silicate structures. (Gay, Le Maitre 1961)Olivine has an orthorhombic structure with a space group of Pbnm. (Brown, 1959) Olivine-like structures represent the stage that breaks down olivine with chemical changes introduced by alterations. (Gay, Le Maitre 1961) These structures have the cell dimensions a = 4.8, b = 10.3 and c = 6.0 (angstrom), a space group Pbnm and a d-spacing of 2.779(angstrom). Olivine axes are oriented in the following way: is parallel to X-axis, b is parallel to Y-axis and c is parallel to Z-axis. (Brown, 1059) X-ray diffraction patterns taken from iddingsite vary from true olivine pattern to patterns that are very diffuse spots. This is an indication of a distorted structure caused by atomic replacement creating a distorted atomic arrangement. (Gay, Le Maitre 1961)Goethite-like structures are common because goethite is in the same space group as olivine (Brown, 1959). This allows for goethite to grow within the olivine making the close packed planes common for both structures. (Gay, Le Maitre 1961) Goethite-like structures have cell dimensions a=4.6, b= 10.0 and c = 3.0 (angstrom) (Brown, 1959). Diffraction spots caused by goethite are diffuse even though the material is well oriented. These structures are aligned parallel to the original olivine with a-axis (goethite) parallel to a-axis (olivine), b-axis (goethite) parallel to b-axis (olivine) and c-axis (goethite) parallel to c-axis (olivine) (Brown 1959). The preferred orientation of olivine and goethite are when there are parallel with there z-axis (Gay, Le Maitre 1961).Hematite-like structures occur in a similar fashion as goethite. Hematite has a triagonal crystal system and experiences twinning by having an approximately hexagonal close-packed oxygen framework and has a structural orientation similar to olivine (Gay, Le Maitre 1961). When twinning occurs, the orientation of hematite-like iddingsite is as follows: a-axis of olivine is parallel to the c-axis of hematite, the b-axis of olivine is parallel to the +/-  plane of hematite and the c-axis of olivine is parallel to the +/-  plane of hematite (Brown, 1959). This hematite structure is very well oriented and occurs because of the high stability of the anion framework and because the cations can be made to migrate throughout the structure (Gay, Le Maitre 1961). Spinel Structures consist of multiple oxide structures that are cubic and have cubic close packing. The spinel structures have a twined orientation and are controlled by close packed sheets (Gay, Le Maitre 1961). This twined orientation is can be described as: the a-axis of olivine is parallel to the (111) spinel face. The b-axis of olivine is parallel to +/- (112) and the c-axis of olivine is parallel to +/- (110) spinel face. These alterations tend to be rare in iddingsite but when they are present they show a sharp diffraction spot making them easily identified. Silicate structures are the most variable among all of the structures discussed. Figure 3 is an illustration of a common silicate structure consisting of a hexagonal array of cylinders whose length is parallel to the x-axis of the olivine and the side of the hexagonal cell is parallel to the z-axis of olivine. Diffraction effects caused by this structure can be attributed to the formation of sheet silicate structures that have a very disordered stacking of layers (Gay, Le Maitre 1961).
Iddingsite is a pseudomorph that usually has crystals rimmed by a thin zone of yellowish brown or greenish cryptocrystalline material (Brown 1959). The color of iddingsite varies from red-brown to orange-brown to deep ruby re to orange-red. The color of iddingsite in plane polarized light is the same until the later alteration stages when it turns into a darker color due to the strengthening effect of pleochroism. An increase in beta refractive index, which typically is 1.9 can be seen in most types of iddingsite, as the alteration process proceeds. Iddingsite also exhibits an increase in birefringence and dispersion as the alteration process proceeds. Some samples that have completed their alterations have miscellaneous cleavage thereby making it not a very good diagnostic tool. Most samples don’t have any cleavage at all (Gay, Le Maitre 1961). Thin sections of Lismore, Australia had shown to have a lamellar habit with one well developed cleavage and two subsidiary cleavages at right angles to each other. It has an alpha of 1.7 to 1.68 and a gamma of 1.71 to 1.72 and a birefringence of .04 (Brown, 1959). On average iddingsite has a density of about 2.65 g/cc and a harness of 3 (calcite) (http://webmineral.com/data/Iddingsite.shtml). Variability in these values are expected due to the differences in crystal structure that can occur from different stages in the alteration process.
*Brown George. "A structural Study of Iddingsite from New South Wales, Australia". American Mineralogist. 44; 3-4, Pages 251-260, 1959.
*Borg Lars, Drake Michaels. "A review of meteorite evidence for the timing of magmatism and of surface or near-surface liquid water on Mars". Journal of Geophysical Research. Vol. 110, E12S03 pages 1-10, 2005.
*Edwards Andrew. "The Formation of Iddingsite". Am Mineral. Pages 277-281, 1938.
*Eggeton, Richard. "Formation of Iddingsite Rims on Olivine: a Transmission Electron Microscope Study". Clays and Clay Minerals, Col. 32. No. 1, 1-11, 1984.
*Gay Peter; Le Maitre R W. "Some Observations on Iddingsite". American Mineralogist. 46; 1-2, pages 92-111. 1961.
*Ross, Shannon. "The Origin, Occurrence, Composition and Physical Properties of the Mineral Iddingsite". Proc. U.S. Nat., Mus., 67 1925.
*Smith, Katherine et al. "Weathering of Basalt: Formation of Iddingsite". Clays and Clay Minerals, Col. 35. No. 6, 418-428, 1987.
*Sun Ming Shan. "The Nature of Iddingsite in Some Basaltic Rocks of New Mexico". American *Mineralogist. 42; 7-8, 1957.
*Swindle T.D. et al. "Noble Gases in Iddingsite from the Lafayette meteorite: Evidence for Liquid water on Mars in the last few hundred million years". Meteoritics and Planetary Science 35; 107-115, 2000.
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