Iddingsite
Encyclopedia
Iddingsite is an alteration of olivine
that consists of a mixture of clay minerals, iron oxide
s and ferrihydrite
s. It is absent from deep-seated rocks and is found on meteorite
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
.
Iddingsite forms from the weathering of basalt
in the presence of liquid water and can be described as a phenocryst
, i.e. it has megascopically visible crystals in a fine-grained groundmass of a porphyritic
rock. It 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 rock
s that are formed by injection of magma
near the surface.
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 (Swindle 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 et al., 2000).
-like structures, hematite
structures, spinel
structures and silicate
structures. (Gay and 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 and 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 and 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 and 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 they are parallel with their z-axis (Gay and 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 and 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 +/- [010] plane of hematite and the c-axis of olivine is parallel to the +/- [210] 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 and 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 and 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. 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 (Gay and Le Maitre 1961).
material (Brown 1959). 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
. 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 have no cleavage at all (Gay and Le Maitre 1961). Thin sections of Lismore
, Australia
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 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.
Olivine
The mineral olivine is a magnesium iron silicate with the formula 2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface....
that consists of a mixture of clay minerals, iron oxide
Iron oxide
Iron oxides are chemical compounds composed of iron and oxygen. All together, there are sixteen known iron oxides and oxyhydroxides.Iron oxides and oxide-hydroxides are widespread in nature, play an important role in many geological and biological processes, and are widely utilized by humans, e.g.,...
s and ferrihydrite
Ferrihydrite
Ferrihydrite 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 hydrothermal hot springs and scales, soils, and areas affected...
s. It is absent from deep-seated rocks and is found on meteorite
Meteorite
A meteorite is a natural object originating in outer space that survives impact with the Earth's surface. Meteorites can be big or small. Most meteorites derive from small astronomical objects called meteoroids, but they are also sometimes produced by impacts of asteroids...
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
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...
.
Iddingsite forms from the weathering of basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
in the presence of liquid water and can be described as a phenocryst
Phenocryst
thumb|right|300px|[[Granite]]s often have large [[feldspar|feldspatic]] phenocrysts. This granite, from the [[Switzerland|Swiss]] side of the [[Mont Blanc]] massif, has large white [[plagioclase]] phenocrysts, [[triclinic]] [[mineral]]s that give [[trapezium|trapezoid]] shapes when cut through)...
, i.e. it has megascopically visible crystals in a fine-grained groundmass of a porphyritic
Porphyritic
Porphyritic is an adjective used in geology, specifically for igneous rocks, for a rock that has a distinct difference in the size of the crystals, with at least one group of crystals obviously larger than another group...
rock. It is a pseudomorph
Pseudomorph
In mineralogy, a pseudomorph is a mineral or mineral compound that appears in an atypical form , resulting from a substitution process in which the appearance and dimensions remain constant, but the original mineral is replaced by another...
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 rock
Subvolcanic rock
A subvolcanic rock, also known as a hypabyssal rock, is an igneous rock that originates at medium to shallow depths within the crust and contain intermediate grain size and often porphyritic texture. They have textures between volcanic and plutonic rocks. Subvolcanic rocks include diabase and...
s that are formed by injection of magma
Magma
Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...
near the surface.
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 though many stages of structural and chemical change (Gay and Le Maitre 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 (Swindle 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 et al., 2000).
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 H20 = 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 CaO can be substituted by MgO by a ratio of 1:4. (Ross 1925). There are also some trace constituents of Na2O and K2O that enter iddingsite as the alteration process progresses (Gay and Le Maitre 1961).Geologic occurrence
The geologic occurrence of Iddingsite is limited to extrusive or hypabyssal 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 (Ross 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 and 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, goethiteGoethite
Goethite , named after the German polymath Johann Wolfgang von Goethe, is an iron bearing oxide mineral found in soil and other low-temperature environments. Goethite has been well known since prehistoric times for its use as a pigment. Evidence has been found of its use in paint pigment samples...
-like structures, hematite
Hematite
Hematite, also spelled as haematite, is the mineral form of iron oxide , one of several iron oxides. Hematite crystallizes in the rhombohedral system, and it has the same crystal structure as ilmenite and corundum...
structures, spinel
Spinel
Spinel is the magnesium aluminium member of the larger spinel group of minerals. It has the formula MgAl2O4. Balas ruby is an old name for a rose-tinted variety.-Spinel group:...
structures and silicate
Silicate
A silicate is a compound containing a silicon bearing anion. The great majority of silicates are oxides, but hexafluorosilicate and other anions are also included. This article focuses mainly on the Si-O anions. Silicates comprise the majority of the earth's crust, as well as the other...
structures. (Gay and 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 and 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 and 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 and Le Maitre 1961) Goethite-like structures have cell dimensions a=4.6, b= 10.0 and c = 3.0 (angstrom
Ångström
The angstrom or ångström, is a unit of length equal to 1/10,000,000,000 of a meter . Its symbol is the Swedish letter Å....
) (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 they are parallel with their z-axis (Gay and 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 and 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 +/- [010] plane of hematite and the c-axis of olivine is parallel to the +/- [210] 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 and 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 and 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. 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 (Gay and Le Maitre 1961).
Physical properties
Iddingsite is a pseudomorph that usually has crystals rimmed by a thin zone of yellowish brown or greenish cryptocrystallineCryptocrystalline
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 cryptocrystalline. Carbonado, a form of diamond, is also...
material (Brown 1959). 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
Pleochroism is an optical phenomenon in which a substance appears to be different colors when observed at different angles with polarized light.- Background :Anisotropic crystals will have optical properties that vary with the direction of light...
. 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
Birefringence, or double refraction, is the decomposition of a ray of light into two rays when it passes through certain anisotropic materials, such as crystals of calcite or boron nitride. The effect was first described by the Danish scientist Rasmus Bartholin in 1669, who saw it in calcite...
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 (Gay and Le Maitre 1961). Thin sections of Lismore
Lismore
-Places:Australia*Lismore, New South Wales, city*Lismore, Victoria, townCanada*Lismore, Nova Scotia, villageIreland*Lismore, County Waterford, town*Lismore Castle*Lismore, County Down, a townland in County Down,Scotland*Lismore, Scotland, island...
, Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
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 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.
External links
- http://fti.neep.wisc.edu/neep602/FALL97/LEC16/lecture16.html Lecture at University of Wisconsin
- http://webmineral.com/data/Iddingsite.shtml Page on Webmineral