Iddingsite is a

microcrystalline A microcrystalline material is a crystallized substance or rock that contains small crystals visible only through microscopic examination. There is little agreement on the range of crystal sizes that should be regarded as microcrystalline, but the ...

rock that is derived from alteration of

olivine The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers qui ...

. It is usually studied as a

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...

, and consists of a mixture of remnant olivine,

clay mineral Clay minerals are hydrous aluminium phyllosilicates (e.g. kaolin, Al2 Si2 O5( OH)4), sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations found on or near some planetary surfaces. Clay minera ...

iron oxide Iron oxides are chemical compounds composed of iron and oxygen. Several iron oxides are recognized. All are black magnetic solids. Often they are non-stoichiometric. Oxyhydroxides are a related class of compounds, perhaps the best known of wh ...

s, and

ferrihydrite Ferrihydrite (Fh) is a widespread hydrous ferric oxyhydroxide mineral at the Earth's surface, and a likely constituent in extraterrestrial materials. It forms in several types of environments, from freshwater to marine systems, aquifers to hy ...

s. Debates over iddingsite's non-definite

crystal structure In crystallography, crystal structure is a description of the ordered arrangement of atoms, ions or molecules in a crystalline material. Ordered structures occur from the intrinsic nature of the constituent particles to form symmetric pattern ...

caused it to be de-listed as an official mineral by the IMA; thus, it is properly referred to as a rock. Iddingsite forms from the

weathering Weathering is the deterioration of rocks, soils and minerals as well as wood and artificial materials through contact with water, atmospheric gases, and biological organisms. Weathering occurs '' in situ'' (on site, with little or no movement ...

basalt Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90 ...

in the presence of liquid water and can be described as a

phenocryst 300px, feldspathic phenocrysts. This granite, from the Switzerland">Swiss side of the Mont Blanc massif, has large white plagioclase phenocrysts, triclinic minerals that give trapezoid shapes when cut through). 1 euro coins, 1 euro coin (diameter ...

, i.e. it has macroscopically visible

crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macro ...

s in a

fine-grained Granularity (also called graininess), the condition of existing in granules or grains, refers to the extent to which a material or system is composed of distinguishable pieces. It can either refer to the extent to which a larger entity is sub ...

groundmass The matrix or groundmass of a rock is the finer-grained mass of material in which larger grains, crystals, or clasts are embedded. The matrix of an igneous rock consists of finer-grained, often microscopic, crystals in which larger crystals, ...

of a

porphyritic Porphyritic is an adjective used in geology to describe igneous rocks with a distinct difference in the size of mineral crystals, with the larger crystals known as phenocrysts. Both extrusive and intrusive rocks can be porphyritic, meaning a ...

rock. It is a

pseudomorph In mineralogy, a pseudomorph is a mineral or mineral compound that appears in an atypical form ( crystal system), resulting from a substitution process in which the appearance and dimensions remain constant, but the original mineral is replaced ...

that has a composition that is constantly transforming from the original olivine, passing through many stages of

structural A structure is an arrangement and organization of interrelated elements in a material object or system, or the object or system so organized. Material structures include man-made objects such as buildings and machines and natural objects such ...

and

chemical change Chemical changes occur when a substance combines with another to form a new substance, called chemical synthesis or, alternatively, chemical decomposition into two or more different substances. These processes are called chemical reactions and, ...

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 * 3SiO₂ * 4 H₂O where MgO can be substituted by CaO. The geologic occurrence of iddingsite is limited to extrusive or subvolcanic rocks that are formed by injection of

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natura ...

near the surface. It is absent from deep-seated rocks and is found on

meteorite A meteorite is a solid piece of debris from an object, such as a comet, asteroid, or meteoroid, that originates in outer space and survives its passage through the atmosphere to reach the surface of a planet or moon. When the original object ...

s. As it has been found on Martian meteorites, its ages have been calculated to obtain absolute ages when liquid water was at or near the surface of

Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury. In the English language, Mars is named for the Roman god of war. Mars is a terrestrial planet with a thin at ...

. It was named after Joseph P. Iddings, an American petrologist.

Introduction

Iddingsite is a pseudomorph, and during the alteration process the olivine crystals had their internal structure or chemical composition changed, although 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 through many stages of structural and chemical change.Gay Peter; Le Maitre, R. W. "Some Observations on Iddingsite". ''American Mineralogist''. 46; 1–2, pp. 92–111. 1961. Iddingsite has been a subject researched in recent years because of its presence in the Martian meteorites. The formation of iddingsite requires liquid water, giving scientists an estimate as to when there has been liquid

water on Mars Almost all water on Mars today exists as ice, though it also exists in small quantities as vapor in the atmosphere. What was thought to be low-volume liquid brines in shallow Martian soil, also called recurrent slope lineae, may be grains of ...

.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, pp. 107–115, 2000. Potassium-argon dating of the meteorite samples showed that Mars had water on its surface anywhere from 1300 Ma to 650 Ma ago.

Composition

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 H₂O = 14%. Throughout 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 and Le Maitre 1961). The chemical formula for iddingsite is approximated as MgO * Fe₂O₃ * 4 H₂O where MgO can be substituted by CaO by a ratio of 1:4.Ross, Shannon. "The Origin, Occurrence, Composition and Physical Properties of the Mineral Iddingsite". ''Proc. U.S. Nat., Mus.'', 67 1925. There are also some trace constituents of Na₂O and K₂O that enter iddingsite as the alteration process progresses.

Geologic occurrence

The geologic occurrence of Iddingsite is limited to extrusive or

hypabyssal A subvolcanic rock, also known as a hypabyssal rock, is an intrusive igneous rock that is emplaced at depths less than within the crust, and has intermediate grain size and often porphyritic texture between that of volcanic rocks and plutonic ...

rocks, and it is absent from deep-seated rocks. Iddingsite is an epimagmatic mineral derived during the final cooling of lava in which it occurs from a reaction between gases, water and olivine. 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. 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.

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 X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...

patterns found that there are five structural types of iddingsite that can occur during different stages of alteration. They are: olivine-like structures,

goethite Goethite (, ) is a mineral of the diaspore group, consisting of iron(III) oxide-hydroxide, specifically the "α" polymorph. It is found in soil and other low-temperature environments such as sediment. Goethite has been well known since ancient t ...

-like structures,

hematite Hematite (), also spelled as haematite, is a common iron oxide compound with the formula, Fe2O3 and is widely found in rocks and soils. Hematite crystals belong to the rhombohedral lattice system which is designated the alpha polymorph of . ...

structures,

spinel Spinel () is the magnesium/aluminium member of the larger spinel group of minerals. It has the formula in the cubic crystal system. Its name comes from the Latin word , which means ''spine'' in reference to its pointed crystals. Properties S ...

structures and

silicate In chemistry, a silicate is any member of a family of polyatomic anions consisting of silicon and oxygen, usually with the general formula , where . The family includes orthosilicate (), metasilicate (), and pyrosilicate (, ). The name is a ...

structures. Olivine has an orthorhombic structure with a space group of Pbnm.Brown George. "A structural Study of Iddingsite from New South Wales, Australia". ''American Mineralogist''. 44; 3–4, pp. 251–260, 1959. Olivine-like structures represent the stage that breaks down olivine with chemical changes introduced by alterations. These structures have the cell dimensions a = 4.8, b = 10.3 and c = 6.0 Å, a space group Pbnm and a d-spacing of 2.779 Å. Olivine axes are oriented in the following way: a is parallel to X-axis, b is parallel to Y-axis and c is parallel to Z-axis. 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. Goethite-like structures are common because goethite is in the same space group as olivine. This allows for goethite to grow within the olivine making the close packed planes common for both structures. Goethite-like structures have cell dimensions a=4.6, b= 10.0 and c = 3.0 Å. 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). The preferred orientation of olivine and goethite are when they are parallel with their z-axis. 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. 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 +/− 10plane of hematite and the c-axis of olivine is parallel to the +/− 10plane of hematite. 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. 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. 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. A common silicate structure consists 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.

Physical properties

Iddingsite is a pseudomorph that usually has crystals rimmed by a thin zone of yellowish brown or greenish

cryptocrystalline Cryptocrystalline is a rock texture made up of such minute crystals that its crystalline nature is only vaguely revealed even microscopically in thin section by transmitted polarized light. Among the sedimentary rocks, chert and flint are crypt ...

material. The color of iddingsite varies from red-brown to orange-brown to deep ruby red 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 Pleochroism (from Greek πλέων, ''pléōn'', "more" and χρῶμα, ''khrôma'', "color") is an optical phenomenon in which a substance has different colors when observed at different angles, especially with polarized light. Backgrou ...

. 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 Birefringence is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. These optically anisotropic materials are said to be birefringent (or birefractive). The birefri ...

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 have no cleavage at all. Thin sections of Lismore,

New South Wales ) , nickname = , image_map = New South Wales in Australia.svg , map_caption = Location of New South Wales in AustraliaCoordinates: , subdivision_type = Country , subdivision_name = Australia , established_title = Before federation , es ...

Australia Australia, officially the Commonwealth of Australia, is a sovereign country comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands. With an area of , Australia is the largest country by ...

, 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 0.04. On average iddingsite has a density of about 2.65 g/cm³ and a hardness of 3 (calcite). Variability in these values are expected due to the differences in crystal structure that can occur from different stages in the alteration process.

References

Additional sources

*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, pp. 1–10, 2005. *Eggeton, Richard. "Formation of Iddingsite Rims on Olivine: a Transmission Electron Microscope Study". ''Clays and Clay Minerals'', Col. 32. No. 1, 1–11, 1984. *Smith, Katherine et al. "Weathering of Basalt: Formation of Iddingsite". ''Clays and Clay Minerals'', Col. 35. No. 6, pp. 418–428, 1987. *Sun Ming Shan. "The Nature of Iddingsite in Some Basaltic Rocks of New Mexico". ''American *Mineralogist''. 42; pp. 7–8, 1957.

External links

Lecture at University of Wisconsin
{{Webarchive, url=https://web.archive.org/web/20070712194055/http://fti.neep.wisc.edu/neep602/FALL97/LEC16/lecture16.html , date=2007-07-12 Minerals