Iddingsit

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Olivine crystal with a brown edge from Iddingsite (thin section, LPL), basalt, Vogelsberg

Iddingsite (also Oroseit ), named in honor of Joseph Paxson Iddings , is a pseudomorphic conversion product of the mineral olivine . Iddingsite is not an independent mineral, but a submicroscopic mineral mixture of clay minerals ( chlorite group , smectite group ), iron oxides ( goethite , hematite ) and ferrihydrides . It arises in the intermediate to highly hydrothermal range (<400 ° C) when basalts are weathered and can definitely be regarded as a phenocrystalline insofar as visible crystals can be recognized that are porphyry-like embedded in a fine-grained matrix. The composition of iddingsite 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, Iddingsit can neither be assigned a definite structure nor a clear chemical formula. An approximate formula is MgO * Fe 2 O 3 * 3 SiO 2 * 4 H 2 O whereby CaO can also substitute for MgO. On earth, the occurrence of iddingsite is limited to volcanic rocks and sub-volcanic rocks (formed by magma injection near the surface); iddingsite does not occur in deep igneous rocks and metamorphic rocks. Iddingsite can be of extraterrestrial origin and is found in meteorites . Iddingsite is of great importance to modern science because it was discovered in Martian meteorites; Radiometric dating can be used to determine the point in time when liquid water was present on the surface of Mars.

introduction

Iddingsite is pseudomorphic after olivine. This means that during the transformation process of olivine crystals, which is also known as iddingsitization , the internal structure or chemical composition changes, while the external shape is retained. But there are also phases in which the atomic arrangement is only distorted and no new final structure is established. The composition of iddingsite, starting from the original olivine crystal, is subject to continuous changes and goes through many stages of structural and chemical change (Gay, Le Maitre 1961). Because of its occurrence in meteorites, iddingsite has recently become an object of research again. The following illustration is an example of rust on Mars and consists of iddingsite, which is itself a mixture of clay minerals and oxides. Liquid water is required to form iddingsite - a fact that has recently enabled scientists to date the presence of liquid water on Mars (Swindle, TD et al, 2000). Radiometric dating using the potassium-argon method showed an age range of 1300 to 650 million years BP for the presence of liquid water on Mars (Swindle, TD et al, 2000).

composition

Iddingsit does not have a final chemical composition, so no precise calculations can be made. For a hypothetical end product made from iddingsite, the following approximate composition was calculated: SiO 2 = 16%, Al 2 O 3 = 8%, Fe 2 O 3 = 62% and H 2 0 = 14%. During the conversion process, based on the ideal composition of the olivine (MgO = 42.06%, FeO = 18.75% and SiO 2 = 39.19%) there is generally a loss of SiO 2 , FeO and MgO, whereas the Al content 2 O 3 and H 2 O increases steadily. Chemically, the oxidation of Fe 2+ and the addition of water lead to an increase in the Fe 2 O 3 content with a simultaneous loss of MgO (Gay, Le Maitre 1961). The chemical formula for iddingsite can be approximated as MgO * Fe 2 O 3 * 3 SiO 2 * 4 H 2 O, with Ca for Mg in a ratio of 1: 4 (Ross, Shannon 1925). As the conversion process progresses, trace amounts of Na 2 O and K 2 O can also be added (Gay, Le Maitre 1961).

Geological occurrence

The geological occurrence of iddingsite is limited to extrusive volcanic rocks or sub-volcanic rocks; it does not exist in deep igneous rocks and metamorphic rocks. It arises during the last cooling phase of lavas as a reaction product of olivine with gases and water (Ross, Shannon 195). The formation of iddingsite does not depend on the original composition of the respective olivine, rather it is influenced by the oxidation state and the water content. Water-rich magmas are a prerequisite for the formation of iddingsite (Edwards 1938). The conversion of olivine to iddingsite takes place under strongly oxidizing conditions at low pressure and medium temperatures (<400 ° C).

structure

Because of the variety of possible conversion phases of olivine, the structure of iddingsite can only be characterized with great difficulty. Iddingsit tends to be visually homogeneous. This fact suggests an underlying structure. It has been found that the structural transformations are controlled by sequences of axially densely packed oxygen layers. These oxygen layers run perpendicular to the X-axis and are thus oriented parallel to the Z-axis of the olivine unit cell. These oxygen ion layers within the olivine undoubtedly exert a strong control over the structure of the conversion products.

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

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

Olivine has orthorhombic symmetry and crystallizes in the space group Pbnm (Brown, 1959). Olivine-like structures arise from the penetration of foreign ions into the olivine structure during the transformation process that begins (Gay, Le Maitre 1961). Its 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 (angstroms). The olivine crystal is set up 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 diffractometric patterns from iddingsite vary from real olivine patterns to extremely diffuse stain patterns. This in turn suggests a deformation of the olivine structure, which was caused by the incorporation of foreign atoms (Gay, Le Maitre 1961).

Goethite-like structures are quite common, as goethite crystallizes in the same space group as olivine (Brown, 1959). Goethite can therefore also grow within the olivine structure and make use of the densely packed oxygen layers in olivine (Gay, Le Maitre 1961). Goethite-like structures have the unit cell dimensions a = 4.6, b = 10.0 and c = 3.0 (Angstrom) (Brown, 1959). X-ray diffractometric patterns of goethite-like structures are diffuse, although the material shows a regulated orientation and the axis directions can even coincide with those of olivine (Brown 1959). An identical Z-axis is preferred here (Gay, Le Maitre 1961).

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 close packing of oxygen atoms and its structural alignment is also comparable to that of olivine (Gay, Le Maitre 1961). If twinning occurs, then hematite-like iddingsite presents itself as follows: the a-axis of the olivine runs parallel to the c-axis of the hematite, the b-axis of the olivine is more or less parallel to the [010] plane of the hematite and the c -Axis of the olivine lies more or less parallel to the [210] -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 density test packing. Spinel structures have a twisted orientation compared to olivine and are determined by densely packed layers (Gay, Le Maitre 1961). The rotation can be described as follows: the a-axis of the olivine runs 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 the Olivine is more or less parallel to the (110) face of the spinel. Transformations with spinel structures occur relatively seldom in iddingsite, but their presence is noticeable with a clear diffraction spot and is therefore easy to identify.

Silicate structures are the most variable of the structures listed. They usually consist of arrays of hexagonal cylinders whose long axes are parallel to the X-axis of the olivine and whose hexagonal sides are parallel to the Z-axis of the olivine. Diffraction effects of these structures can be traced back to the formation of layered silicate structures, the layers of which, however, are severely disturbed in their stacking (Gay, Le Maitre 1961).

Physical Properties

Iddingsite is pseudomorphic after olivine. The olivine crystals are usually surrounded by a thin layer made of yellow-brown or greenish cryptocrystalline material (Brown 1959). However, iddingsite can also penetrate cracks. The color of iddingsite is variable, shades of color range from yellow-brown to orange-brown to deep ruby ​​red and orange-red. Iddingsite is weakly pleochroic. The same color tones can be observed under singly polarized light; the colors only become darker in the later phases of transformation due to the amplification effect caused by pleochroism. In the course of the conversion process, the refractive index n beta , which is 1.9 , usually increases . As the conversion proceeds, the birefringence and the dispersion also increase. Some iddingsite samples show cleavages after the transformations have taken place, but most of the samples are solid and without cleavage surfaces (Gay, Le Maitre 1961). Thin sections from Lismore in Australia show a lamellar habitus with a well-developed cleavage area and two subordinate cleavage areas intersecting at right angles. Their refractive index n alpha is between 1.68 and 1.70, n gamma between 1.71 and 1.72 and the birefringence is 0.04 (Brown, 1959). On average, the density of iddingsite is 2.65 and the hardness 3 (hardness of calcite). Due to the structural change that occurs during the conversion process, all physical data are subject to a certain range of variation.

swell

  1. ↑ NEEP602 Course Notes (Fall 1997) ( English ) University of Wisconsin-Madison. Retrieved May 11, 2019.
  2. ↑ Iddingsite Mineral Data ( English ) http: //webmineral.com.+ Retrieved May 11, 2019.
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  • 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.
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  • Eggeton, Richard. Formation of Iddingsite Rims on Olivine: a Transmission Electron Microscope Study . Clays and Clay Minerals, Col. 32. No. 1, 1-11, 1984.
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  • Ross, Shannon. The Origin, Occurrence, Composition and Physical Properties of the Mineral Iddingsite . Proc. US 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 TD et al. Noble Gases in Iddingsite from the Lafayette meteorite: Evidence for Liquid water on Mars in the last few hundred million years . In: Meteoritics & Planetary Science , 35, 107-115, 2000.