Iddingsite

Iddingsite is a microcrystalline rock that is derived from alteration of olivine. It is usually studied as a mineral, and consists of a mixture of remnant olivine, clay minerals, iron oxides and ferrihydrites. Debates over iddingsite's non-definite crystal structure 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 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, passing through 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 * 3Si2O2 * 4 H2O [2] 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 near the surface. It is absent from deep-seated rocks and is found on meteorites. 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.

It was named after Joseph P. Iddings, an American petrologist. [2]

Iddingsite
Igneous rock
Iddingsite
Olivine weathering to iddingsite
Composition
Olivine, clays, ferrihydrites
Iddingsite
Idyngsyt BA
Photomicrograph of iddingsite
General
CategorySilicate mineral (not a proper mineral)
Formula
(repeating unit)
MgFe2Si3O10•4(H2O)
Crystal systemOrthorhombic
Identification
ColorBrown
Crystal habitFoliated
CleavagePerfect
Mohs scale hardness3
LusterVitreous
StreakNone
DiaphaneityTransparent to translucent
Specific gravity2.5 - 2.8
Optical propertiesBiaxial (-)
References[1]

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.[3]

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.[4] Potassium-argon dating of the meteorite samples showed that Mars had water on its surface anywhere from 1300 Ma to 650 Ma ago.[4]

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.[5] There are also some trace constituents of Na2O and K2O that enter iddingsite as the alteration process progresses.[3]

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.[5] 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.[6] 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.[3]

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.[3]

Olivine has an orthorhombic structure with a space group of Pbnm.[7] Olivine-like structures represent the stage that breaks down olivine with chemical changes introduced by alterations.[3] 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.[7] 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.[3]

Goethite-like structures are common because goethite is in the same space group as olivine.[7] This allows for goethite to grow within the olivine making the close packed planes common for both structures.[3] Goethite-like structures have cell dimensions a=4.6, b= 10.0 and c = 3.0 Å.[7] 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).[7] The preferred orientation of olivine and goethite are when they are parallel with their z-axis.[3]

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.[3] 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.[7] 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.[3]

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.[3] 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.[3]

Physical properties

Iddingsite is a pseudomorph that usually has crystals rimmed by a thin zone of yellowish brown or greenish cryptocrystalline material.[7] 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.[3] Thin sections of Lismore, New South Wales, 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 0.04.[7] On average iddingsite has a density of about 2.65 g/cm3 and a hardness of 3 (calcite).[8] Variability in these values are expected due to the differences in crystal structure that can occur from different stages in the alteration process.

References

  1. ^ Olivine
  2. ^ a b "Iddingsite". mindat.org. Retrieved March 25, 2019.
  3. ^ a b c d e f g h i j k l m Gay Peter; Le Maitre, R. W. "Some Observations on Iddingsite". American Mineralogist. 46; 1–2, pp. 92–111. 1961.
  4. ^ a b 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.
  5. ^ a b Ross, Shannon. "The Origin, Occurrence, Composition and Physical Properties of the Mineral Iddingsite". Proc. U.S. Nat., Mus., 67 1925.
  6. ^ Edwards, Andrew. "The Formation of Iddingsite". American Mineralogist, pp. 277–281, 1938.
  7. ^ a b c d e f g h Brown George. "A structural Study of Iddingsite from New South Wales, Australia". American Mineralogist. 44; 3–4, pp. 251–260, 1959.
  8. ^ David Bartholmy (31 December 2009), "Iddingsite mineral data", Mineralogy database, retrieved 19 July 2012

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

Boron on Mars

Researchers in December 2016 announced the discovery by the Curiosity rover of the element boron in mineral veins on the planet Mars. No other mission to Mars has found boron. However, boron was found in Martian meteorites that included MIZ 09030 in 2013, MIL 09030, Nakhla, Lafayette, and Chassigny. For boron to be present in the veins there must have been a temperature between 0-60 degrees Celsius and a neutral-to-alkaline pH. The temperature, pH, and dissolved minerals of the groundwater support a habitable environment.Moreover, boron has been suggested to be necessary for life to form. Its presence stabilizes the sugar ribose which is an ingredient in RNA. Ribose would rapidly decompose in water without boron being present. On Earth, boron compounds may have been needed to link the organic compounds that were produced without life into RNA-like molecules that were used for the very first life forms.

Bowen's reaction series

Within the field of geology, Bowen's reaction series is the work of the petrologist, Norman L. Bowen who summarized, based on experiments and observations of natural rocks, the crystallization sequence of typical basaltic magma undergoing fractional crystallization (i.e., crystallization wherein early-formed crystals are removed from the magma by crystal settling, say, leaving behind a liquid of slightly different composition). Bowen's reaction series is able to explain why certain types of minerals tend to be found together while others are almost never associated with one another. He experimented in the early 1900s with powdered rock material that was heated until it melted and then allowed to cool to a target temperature whereupon he observed the types of minerals that formed in the rocks produced. He repeated this process with progressively cooler temperatures and the results he obtained led him to formulate his reaction series which is still accepted today as the idealized progression of minerals produced by cooling basaltic magma that undergoes fractional crystallization. Based upon Bowen's work, one can infer from the minerals present in a rock the relative conditions under which the material had formed.

Goldich dissolution series

The Goldich dissolution series is a way of predicting the relative stability or weathering rate of various minerals on the Earth's surface. S. S. Goldich came up with the series in 1938 after studying soil profiles. He found that minerals that form at higher temperatures and pressures are less stable on the surface than minerals that form at lower temperatures and pressures. This pattern follows the same pattern of the Bowen's reaction series, with the minerals that are first to crystallize also the first the undergo chemical weathering.

Haruj

Haruj (Arabic: هروج‎, also known as Haroudj) is a large volcanic field spread across 45,000 km2 (17,000 sq mi) in central Libya. It is one of several volcanic fields in Libya along with Tibesti, and its origin has been attributed to the effects of geologic lineaments in the crust.

It contains about 150 volcanoes, including numerous basaltic scoria cones and about 30 small shield volcanoes, along with craters and lava flows. Most of the field is covered by lava flows that originated in fissure vents; the rest of the flows originated within small shield volcanoes, stratovolcanoes and scoria cones. Some of these vents have large craters. Volcanism in Haruj blocked ancient rivers and led to the formation of Lake Megafezzan.

Volcanic activity in Haruj commenced about 6 million years ago and continued into the late Pleistocene. There are a number of individual lava flow generations that were emplaced in the Haruj volcanic field, the most recent ones in the Holocene 2,310 ± 810 years ago. There are reports of solfataric activity.

Horizon Guyot

Horizon Guyot is a presumably Cretaceous guyot (tablemount) in the Mid-Pacific Mountains, Pacific Ocean. It is an elongated ridge, over 300 kilometres (190 mi) long and 4.3 kilometres (2.7 mi) high, that stretches in a northeast-southwest direction and has two flat tops; it rises to a minimum depth of 1,443 metres (4,730 ft). The Mid-Pacific Mountains lie west of Hawaii and northeast of the Line Islands.

It was probably formed by a hotspot, but the evidence is conflicting. Volcanic activity occurred during the Turonian-Cenomanian eras 100.5–89.8 million years ago and another stage has been dated to have occurred 88–82 million years ago. Between these volcanic episodes, carbonate deposition from lagoonal and reefal environments set in and formed limestone. Volcanic islands developed on Horizon Guyot as well and were colonised by plants.

Horizon Guyot became a seamount during the Coniacian-Campanian period. Since then, pelagic ooze has accumulated on the seamount, forming a thick layer that is further modified by ocean currents and by various organisms that live on the seamount; sediments also underwent landsliding. Ferromanganese crusts were deposited on exposed rocks.

Joseph P. Iddings

Joseph Paxson Iddings (January 21, 1857 – September 8, 1920) was an American geologist and a petrologist.

The National Academies Press called Iddings "an outstanding leader of petrology".

The New York Times called him "a distinguished petrologist".

Iddings was a member of the National Academy of Sciences, a member of the Geological Society of London, the American Philosophical Society, a fellow of the Geological Society of America, a member of the Scientific Society of Christiania, an honorary member of the Société française de Mineralogie, an honorary curator of petrology in the U.S. National Museum.Yale University established Iddings Scholarship for Graduate Studies.The mineral iddingsite was named after him.

Son of a wholesaler in Philadelphia. He received a master's degree from Yale College in 1877. Then he studied analytical chemistry at the University. Later, he transferred to Columbia University where he studied Geology under Professor John S. Newberry. He spent 1879-1880 at the University of Heidelberg, where he conducted petrographic research under the direction of Karl Rosenbush.

From July 1880 he worked in the Geological survey of the United States.

Since 1892 he has lectured at the University of Chicago, where a Department of Petrology, the first of its kind in the world, was created especially for him. In 1908, he left the University and retired to his country house in Maryland, conducting his own research. He died unmarried and childless in 1920 from chronic nephritis.

Limalok

Limalok (formerly known as Harrie or Harriet) is a Cretaceous-Paleocene guyot/tablemount in the southeastern Marshall Islands, one of a number of seamounts (a type of underwater volcanic mountain) in the Pacific Ocean. It was probably formed by a volcanic hotspot in present-day French Polynesia. Limalok lies southeast of Mili Atoll and Knox Atoll, which rise above sea level, and is joined to each of them through a volcanic ridge. It is located at a depth of 1,255 metres (4,117 ft) and has a summit platform with an area of 636 square kilometres (246 sq mi).

Limalok is formed by basaltic rocks and was probably a shield volcano at first; the Macdonald, Rarotonga, Rurutu and Society hotspots may have been involved in its formation. After volcanic activity ceased, the volcano was eroded and thereby flattened, and a carbonate platform formed on it during the Paleocene and Eocene. These carbonates were chiefly produced by red algae, forming an atoll or atoll-like structure with reefs.

The platform sank below sea level 48 ± 2 million years ago during the Eocene, perhaps because it moved through the equatorial area, which was too hot or nutrient-rich to support the growth of a coral reef. Thermal subsidence lowered the drowned seamount to its present depth. After a hiatus lasting into the Miocene, sedimentation commenced on the seamount leading to the deposition of manganese crusts and pelagic sediments; phosphate accumulated in some sediments over time.

List of gemstones by species

This is a list of gemstones, organized by species and type.

Lithology

The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition. Lithology may refer to either a detailed description of these characteristics, or a summary of the gross physical character of a rock. Lithology is the basis of subdividing rock sequences into individual lithostratigraphic units for the purposes of mapping and correlation between areas. In certain applications, such as site investigations, lithology is described using a standard terminology such as in the European geotechnical standard Eurocode 7.

Olallie Butte

Olallie Butte is a steep-sided shield volcano in the Cascade Range of the northern part of the U.S. state of Oregon. It is the largest volcano and highest point in the 50-mile (80 km) distance between Mount Hood and Mount Jefferson. Located just outside the Olallie Scenic Area, it is surrounded by more than 200 lakes and ponds fed by runoff, precipitation, and underground seepage, which are popular spots for fishing, boating, and swimming. The butte forms a prominent feature in the Mount Jefferson region and is usually covered with snow during the winter and spring seasons.

Part of a stretch of shield volcanoes in Oregon with an unusually low elevation, meaning they have undergone less erosion over time than surrounding volcanic centers, Olallie has been excavated by glacial erosion on its northeastern flank. Its central volcanic plug has also been exposed. Comparisons of its morphology with Mount Jefferson suggest an age for the butte between 70,000 and 100,000 years; there is no evidence that it has erupted within the past 25,000 years. Olallie Butte has a steep, conical shape that serves as a transitional morphology between steep, mafic (rich in magnesium and iron) volcanoes like Mount McLoughlin and Mount Thielsen and flatter, mafic shields. It is made of basaltic andesite.

A Forest Service fire lookout tower was built on the summit in 1915 but abandoned in 1967; the summit also had a cupola cabin from 1920 until its roof collapsed in 1982. Olallie gets its name from the Chinook Jargon word klallali, which means berries. Today, the butte lies within the Warm Springs Indian Reservation. The Pacific Crest Trail passes over the western side of the butte, and there are other trails that reach the mountain's summit. Although the main trail to the summit is not well maintained, it still remains open to hikers.

Olivine

The mineral olivine ( ) is a magnesium iron silicate with the formula (Mg2+, Fe2+)2SiO4. Thus 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 quickly on the surface.

The ratio of magnesium to iron varies between the two endmembers of the solid solution series: forsterite (Mg-endmember: Mg2SiO4) and fayalite (Fe-endmember: Fe2SiO4). Compositions of olivine are commonly expressed as molar percentages of forsterite (Fo) and fayalite (Fa) (e.g., Fo70Fa30). Forsterite's melting temperature is unusually high at atmospheric pressure, almost 1,900 °C (3,450 °F), while fayalite's is much lower (about 1,200 °C [2,190 °F]). Melting temperature varies smoothly between the two endmembers, as do other properties. Olivine incorporates only minor amounts of elements other than oxygen, silicon, magnesium and iron. Manganese and nickel commonly are the additional elements present in highest concentrations.

Olivine gives its name to the group of minerals with a related structure (the olivine group)—which includes tephroite (Mn2SiO4), monticellite (CaMgSiO4) and kirschsteinite (CaFeSiO4).

Olivine's crystal structure incorporates aspects of the orthorhombic P Bravais lattice, which arise from each silica (SiO4) unit being joined by metal divalent cations with each oxygen in SiO4 bound to 3 metal ions. It has a spinel-like structure similar to magnetite but uses one quadrivalent and two divalent cations M22+ M4+O4 instead of two trivalent and one divalent cations.Olivine gemstones are called peridot and chrysolite.

Olivine rock is usually harder than surrounding rock and stands out as distinct ridges in the terrain. These ridges are often dry with little soil. Drought resistant scots pine is one of few trees that thrive on olivine rock. Olivine pine forest is unique to Norway. It is rare and found on dry olivine ridges in the fjord districts of Sunnmøre and Nordfjord. Olivine rock is hard and base-rich.

Peridotite

Peridotite is a dense, coarse-grained igneous rock consisting mostly of the minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium (Mg2+), reflecting the high proportions of magnesium-rich olivine, with appreciable iron. Peridotite is derived from the Earth's mantle, either as solid blocks and fragments, or as crystals accumulated from magmas that formed in the mantle. The compositions of peridotites from these layered igneous complexes vary widely, reflecting the relative proportions of pyroxenes, chromite, plagioclase, and amphibole.

Peridotite is the dominant rock of the upper part of the Earth's mantle. The compositions of peridotite nodules found in certain basalts and diamond pipes (kimberlites) are of special interest, because they provide samples of the Earth's mantle brought up from depths ranging from about 30 km to 200 km or more. Some of the nodules preserve isotope ratios of osmium and other elements that record processes that occurred when the earth was formed, and so they are of special interest to paleogeologists because they provide clues to the early composition of the Earth's mantle and the complexities of the processes that occurred.

The word peridotite comes from the gemstone peridot, which consists of pale green olivine. Classic peridotite is bright green with some specks of black, although most hand samples tend to be darker green. Peridotitic outcrops typically range from earthy bright yellow to dark green in color; this is because olivine is easily weathered to iddingsite. While green and yellow are the most common colors, peridotitic rocks may exhibit a wide range of colors such as blue, brown, and red.

Pisgah Crater

Pisgah Crater, or Pisgah Volcano, is a young volcanic cinder cone rising above a lava plain in the Mojave Desert, between Barstow and Needles, California in San Bernardino County, California. The volcanic peak is around 2.5 miles (4.0 km) south of historic U.S. Route 66-National Old Trails Highway and of Interstate 40, and west of the town of Ludlow. The volcano had a historic elevation of 2,638 feet (804 m), but has been reduced to 2,545 feet (776 m) due to mining.

Researcher Ridge

Researcher Ridge is an underwater ridge in the Northern Atlantic Ocean. It appears to be a chain of seamounts named Gollum Seamount, Vayda Seamount, Bilbo Seamount, Gandalf Seamount, The Shire Seamount, Pippin Seamount, Merry Seamount, Molodezhnaya Seamount, Frodo Seamount, Sam Seamount and Mount Doom Seamount that were likely formed by a hotspot.

South Arch volcanic field

South Arch volcanic field is an underwater volcanic field south of Hawaiʻi Island. It was active during the last 10,000 years, and covers an area of 35 by 50 kilometres (22 mi × 31 mi) at a depth of 4,950 metres (16,240 ft).

Although the field is related to the Hawaiian hotspot, it does not appear to be a precursory volcano, but seems to have formed when the weight of the growing Hawaiian volcanoes caused the oceanic crust to buckle, opening up pathways for magma to ascend in front of the hotspot.

Ultramafic rock

Ultramafic rocks (also referred to as ultrabasic rocks, although the terms are not wholly equivalent) are igneous and meta-igneous rocks with a very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium, and are composed of usually greater than 90% mafic minerals (dark colored, high magnesium and iron content). The Earth's mantle is composed of ultramafic rocks. Ultrabasic is a more inclusive term that includes igneous rocks with low silica content that may not be extremely enriched in Fe and Mg, such as carbonatites and ultrapotassic igneous rocks.

Weathering

Weathering is the breaking down of rocks, soil, and minerals as well as wood and artificial materials through contact with the Earth's atmosphere, water, and biological organisms. Weathering occurs in situ (on site), that is, in the same place, with little or no movement, and thus should not be confused with erosion, which involves the movement of rocks and minerals by agents such as water, ice, snow, wind, waves and gravity and then being transported and deposited in other locations.

Two important classifications of weathering processes exist – physical and chemical weathering; each sometimes involves a biological component. Mechanical or physical weathering involves the breakdown of rocks and soils through direct contact with atmospheric conditions, such as heat, water, ice and pressure. The second classification, chemical weathering, involves the direct effect of atmospheric chemicals or biologically produced chemicals also known as biological weathering in the breakdown of rocks, soils and minerals. While physical weathering is accentuated in very cold or very dry environments, chemical reactions are most intense where the climate is wet and hot. However, both types of weathering occur together, and each tends to accelerate the other. For example, physical abrasion (rubbing together) decreases the size of particles and therefore increases their surface area, making them more susceptible to chemical reactions. The various agents act in concert to convert primary minerals (feldspars and micas) to secondary minerals (clays and carbonates) and release plant nutrient elements in soluble forms.

The materials left over after the rock breaks down combined with organic material creates soil. The mineral content of the soil is determined by the parent material; thus, a soil derived from a single rock type can often be deficient in one or more minerals needed for good fertility, while a soil weathered from a mix of rock types (as in glacial, aeolian or alluvial sediments) often makes more fertile soil. In addition, many of Earth's landforms and landscapes are the result of weathering processes combined with erosion and re-deposition.

Xenolith

A xenolith ("foreign rock") is a rock fragment that becomes enveloped in a larger rock during the latter's development and solidification. In geology, the term xenolith is almost exclusively used to describe inclusions in igneous rock during magma emplacement and eruption. Xenoliths may be engulfed along the margins of a magma chamber, torn loose from the walls of an erupting lava conduit or explosive diatreme or picked up along the base of a flowing body of lava on the Earth's surface. A xenocryst is an individual foreign crystal included within an igneous body. Examples of xenocrysts are quartz crystals in a silica-deficient lava and diamonds within kimberlite diatremes. Xenoliths can be non-uniform within individual locations, even in areas which are spatially limited, e.g. rhyolite-dominated lava of Niijima volcano (Japan) contains two types of gabbroic xenoliths which are of different origin - they were formed in different temperature and pressure conditions.Although the term xenolith is most commonly associated with igneous inclusions, a broad definition could include rock fragments which have become encased in sedimentary rock. Xenoliths have been found in some meteorites.To be considered a true xenolith, the included rock must be identifiably different from the rock in which it is enveloped; an included rock of similar type is called an autolith or a cognate inclusion.

Xenoliths and xenocrysts provide important information about the composition of the otherwise inaccessible mantle. Basalts, kimberlites, lamproites and lamprophyres, which have their source in the upper mantle, often contain fragments and crystals assumed to be a part of the originating mantle mineralogy. Xenoliths of dunite, peridotite and spinel lherzolite in basaltic lava flows are one example. Kimberlites contain, in addition to diamond xenocrysts, fragments of lherzolites of varying composition. The aluminium-bearing minerals of these fragments provide clues to the depth of origin. Calcic plagioclase is stable to a depth of 25 km (16 mi). Between 25 km (16 mi) and about 60 km (37 mi), spinel is the stable aluminium phase. At depths greater than about 60 km, dense garnet becomes the aluminium-bearing mineral. Some kimberlites contain xenoliths of eclogite, which is considered to be the high-pressure metamorphic product of basaltic oceanic crust, as it descends into the mantle along subduction zones.The large-scale inclusion of foreign rock strata at the margins of an igneous intrusion is called a roof pendant.

Yamato 000593

Yamato 000593 (or Y000593) is the second largest meteorite from Mars found on Earth. Studies suggest the Martian meteorite was formed about 1.3 billion years ago from a lava flow on Mars. An impact occurred on Mars about 11 million years ago and ejected the meteorite from the Martian surface into space. The meteorite landed on Earth in Antarctica about 50,000 years ago. The mass of the meteorite is 13.7 kg (30 lb) and has been found to contain evidence of past water alteration.At a microscopic level, spheres are found in the meteorite that are rich in carbon compared to surrounding areas that lack such spheres. The carbon-rich spheres and the observed micro-tunnels may have been formed by biotic activity, according to NASA scientists.

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