Iddingsite

Iddingsit (also Oroseit ), named in honor of Joseph Paxson Iddings, is a pseudomorphic transformation product of the mineral olivine. Iddingsit is not an independent mineral, but a submicroscopic mineral mixture of clay minerals ( chlorite, smectite ), iron oxides ( goethite, hematite) and Ferrihydriden. It arises in the intermediate to high hydrothermal range (< 400 ° C) in the weathering of basalts and may well be regarded as Phänokristall so far can be seen visible crystals, which are porphyritic embedded in a fine-grained matrix. The composition of Iddingsit is subject to constant change; starting from the original olivine, it goes through several stages of structural and chemical transformations. Because of this continuous transformation process can Iddingsit neither a definite structure or a unique chemical formula can be assigned. An approximate formula is Fe2O3 MgO * SiO2 * 3 * 4 H2O which can also occur CaO for MgO. On Earth, the presence of Iddingsit on volcanics and Subvulkanite is limited (formed by near-surface Magmeninjektion ), in deep igneous and metamorphic rocks does not occur Iddingsit. Iddingsit may be extraterrestrial in origin and is found in meteorites. For modern science Iddingsit has great importance, since it was discovered in Martian meteorites; can radiometric dating thus the time to be determined, was present at the liquid water on the surface of Mars.

Introduction

Iddingsit is pseudomorphic after olivine. This means that changes during the conversion process of olivine crystals, which is also referred to as Iddingsitisierung, the internal structure or the chemical composition, the outer shape is preserved, however. But there are also phases in which the atomic arrangement is only distorted and does not set a new final structure. The composition of Iddingsit subject, starting from the original olivine crystal, ongoing modifications, and goes through many stages of structural and chemical changes ( Gay, Le Maitre, 1961). Because of its occurrence in meteorites Iddingsit has recently again become a subject of research. The following figure is an example of rust on Mars and consists of Iddingsit, itself a mixture of clay minerals and oxides ( http://fti.neep.wisc.edu/neep602/FALL97/LEC16/lecture16.html ). To form Iddingsit liquid water is needed - a fact which has recently displaced scientists in the position to make a date for the presence of liquid water on Mars ( Swindle, TD et al, 2000). So were radiometric dating by the potassium - argon method of age range of 1300 to 650 million years BP for the presence of liquid water on Mars ( Swindle, TD et al, 2000).

Composition

Iddingsit has no definite chemical composition, it may, therefore, no precise calculations are performed. For a hypothetical end product from Iddingsit following approximate composition was calculated: SiO2 = 16 %, Al2O3 = 8%, Fe2O3 = 62 % and H20 = 14 %. During the conversion process takes place starting from the ideal composition of olivine (MgO = 42.06 %, FeO = 18.75% and SiO2 = 39.19 %) is generally a loss of SiO2, FeO and MgO instead, whereas the content of Al2O3 and H2O steadily increases. Chemically carried out by oxidation of Fe2 and supply of water, an increase in Fe2O3 content with concomitant loss of MgO ( Gay, Le Maitre, 1961). The chemical formula for Iddingsit can be approximated with MgO * Fe2O3 SiO2 * 3 * 4 H2O reproduced, Ca can occur for Mg in a ratio of 1:4 ( Ross, Shannon 1925). With increasing conversion process also trace amounts of Na2O and K2O may be added join ( Gay, Le Maitre, 1961).

Geological occurrence

The geological occurrence of Iddingsit limited to extrusive volcanic rocks or Subvulkanite, in deep igneous and metamorphic rocks, it does not exist. It is produced during the last cooling phase of lavas as a reaction product of olivine with gases and water ( Ross, Shannon 195). Forming Iddingsit is not depending on the original composition of the particular olivine, but it is influenced by the oxidation state and the water content. Prerequisite to the development of water-rich magmas are Iddingsit ( Edwards 1938). The conversion of olivine to Iddingsit effected under strongly oxidising conditions at low pressure and moderate temperatures (< 400 ° C).

Structure

Because of the variety of possible conversion phases of olivine, the structure of Iddingsit can only be characterized very difficult. Iddingsit tends to behave optically homogeneous. This fact suggests an underlying structure. It has been found that the structural transformations of sequences hrxagonal close-packed oxygen layers are controlled. These oxygen layers are perpendicular to the X axis, and are thus oriented in parallel to the Z- axis of the unit cell olivine. Undoubtedly, these oxygen ion layers practicing within the olivine from a strong control over the structural design of the transformation products.

X-ray diffraction studies on Iddingsit revealed five structural types that can occur during the conversion process ( Gay, Le Maitre, 1961):

• Olivine -like structures
• Goethite -like structures
• Hämatitstrukturen
• Spinel and
• Silicate structures

Olivine has orthorhombic symmetry and crystallizes in the space group Pbnm (Brown, 1959). Olivine -like structures caused by penetration of foreign ions in the olivine structure during the onset of the conversion process ( Gay, Le Maitre, 1961). Your unit cells have the dimensions a = 4.8, b = 10.3 and c = 6.0 ( in Angstroms ), also belong to the space group Pbnm and have a d- value of 2.779 (angstrom ). The olivine crystal is prepared as follows: a is parallel to the crystallographic X - axis b is parallel to the Y axis and c is parallel to the Z axis (Brown, 1959). X-ray diffraction pattern of Iddingsit vary from real Olivinmustern to extremely diffuse speckle patterns. This in turn close to a deformation of olivine, which was caused by the incorporation of foreign atoms ( Gay, Le Maitre, 1961).

Goethite -like structures are quite common, as goethite in the same space group as olivine crystallizes (Brown, 1959). Goethite can therefore grow within the olivine structure and the close-packed oxygen layers in olivine advantage make ( Gay, Le Maitre, 1961). Goethite -like structures have unit cell dimensions a = 4.6, b = 10.0 and c = 3.0 (angstrom ) (Brown, 1959). X-ray diffraction pattern of goethite similar structures are diffuse, while the material exhibits a controlled orientation and axial directions may even match those of the olivine (Brown 1959). Preferred in this case an identical Z- axis ( Gay, Le Maitre, 1961) is.

Hematite -like structures are roughly comparable to the goethite -like structures. Hematite crystallizes in the trigonal crystal system, its crystal lattice consists of a nearly hexagonal closest packing of oxygen atoms and also its structural alignment with the olivine comparable ( Gay, Le Maitre, 1961). If it comes to the twinning is presented hematite similar Iddingsit as follows: the a- axis of the olivine is parallel to the c- axis of the haematite, the b- axis of the olivine is more or less parallel to the plane of the hematite and the c- axis of the olivine is more or less parallel to the plane of the hematite (Brown, 1959). This hematite -like structure is very well aligned and owes its existence to the high stability of the anion lattice, through which cations can migrate relatively freely ( Gay, Le Maitre, 1961).

Spinel structures consist of cubic oxides with cubic yourself kit. Spinel structures have a twisted orientation relative to olivine and be of layers with yourself test kit determines ( Gay, Le Maitre, 1961). The rotation can be described as follows: the a-axis of olivine is parallel to the (111) surface of the spinel, the b- axis of the olivine is more or less parallel to the ( 112 ) surface of the spinel, and the c-axis of olivine is more or less parallel to the ( 110) surface of the spinel. Transformations with spinel structures are relatively rare in Iddingsit ago, their presence makes itself but with a significant Diffraktionsfleck noticeable and are therefore easy to identify.

Silicate structures are among the most variable structures listed. They usually consist of arrays of hexagonal cylinder, whose longitudinal axes are parallel with the X-axis of olivine and the hexagonal sides are oriented parallel to the Z axis of the olivine. Diffraction effects of these structures can be attributed to the formation of layered silicate structures, the respective documents are seriously disturbed in their stacking of ( Gay, Le Maitre, 1961).

Physical Properties

Iddingsit is pseudomorphic after olivine. The olivine crystals are thereby usually surrounded by a thin layer consisting of yellow-brown or greenish cryptocrystalline material (Brown 1959). Iddingsit but can also penetrate into cracks gap. The color of Iddingsit is variable shades of color ranging from yellow-brown to orange brown to deep ruby red and orange-red after. Iddingsit is weakly pleochroic. Under simple polarized light the same color tones are observed only in the later stages of conversion, the colors are darker due to the damage caused by pleochroism enhancement effect. During the conversion process usually increases the refractive index nbeta, which is 1.9. With progressing conversion is also reinforced the birefringence and the dispersion. Some Iddingsitproben own after the transformations Spaltbarkeiten, but most samples are coarse solid and without cleavage planes ( Gay, Le Maitre, 1961). Thin sections from Lismore in Australia show lamellar habit with a good gap formed family of surfaces and two intersecting at right angles subordinate gap area droves. Your refractive index Nalpha is 1.68 to 1.70, 1.71 to 1.72 ngamma and birefringence of 0.04 (Brown, 1959). Average is the density of Iddingsit at 2.65 and the hardness at 3 (hardness of calcite ) ( http://webmineral.com/data/Iddingsite.shtml ). Due to the self-adjusting during the conversion process, structural change, however, all physical data subject to a degree of variation.

Swell

• George Brown. A structural Study of Iddingsite from New South Wales, Australia. American Mineralogist. 44; 3-4, Pages 251-260, 1959.
• Lars Borg, Michael Drake. 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, pages 1-10 E12S03, 2005.
• Andrew Edwards. The Formation of Iddingsite. On mineral. Pages 277-281, 1938.
• Eggeton, Richard. Formation of Iddingsite Rims on Olivine: a Transmission Electron Microscope Study. Clays and Clay Minerals, Col. No. 32. 1, 1-11, 1984.
• 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. No. 35. 6, 418-428, 1987.
• Sun Ming Shan. The Nature of Some Basaltic Rocks of Iddingsite in New Mexico. American Mineralogist *. 42; 7-8, 1957.
• Swindle T.D. et al. Noble gas in Iddingsite from the Lafayette meteorite: Evidence for liquid water on Mars in the load few hundred million years. In: Meteoritics & Planetary Science, 35, 107-115, 2000.
• Http://webmineral.com/data/Iddingsite.shtml

• Mineralogy