Stony Irons

PART IV

Pallasites- These meteorites are mixtures of olivineGroup of silicate minerals, (Mg,Fe)2SiO4, with the compositional endpoints of forsterite (Mg2SiO4) and fayalite (Fe2SiO4). Olivine is commonly found in all chondrites within both the matrix and chondrules, achondrites including most primitive achondrites and some evolved achondrites, in pallasites as large yellow-green crystals (brown when terrestrialized), in the silicate portion and FeNi-metal that probably formed through impact events in the upper mantleMain silicate-rich zone within a planet between the crust and metallic core. The mantle accounts for 82% of Earth's volume and is composed of silicate minerals rich in Mg. The temperature of the mantle can be as high as 3,700 °C. Heat generated in the core causes convection currents in region of small, differentiated asteroids. Later collisions exposed this mixed zone and delivered samples to Earth. To date, three compositional clusters and three ungroupedModifying term used to describe meteorites that are mineralogically and/or chemically unique and defy classification into the group or sub-group they most closely resemble. Some examples include Ungrouped Achondrite (achondrite-ung), Ungrouped Chondrite (chondrite-ung), Ungrouped Iron (iron-ung), and Ungrouped Carbonaceous (C-ung). pallasites have been identified thus far, each likely representing one or more distinct parent bodies:

1. Main-group

olivine with Fa contents between 10.5 and 13%; metalElement that readily forms cations and has metallic bonds; sometimes said to be similar to a cation in a cloud of electrons. The metals are one of the three groups of elements as distinguished by their ionization and bonding properties, along with the metalloids and nonmetals. A diagonal line drawn with 8–12% Ni

2. Eagle Station subgroup

olivine with Fa contents between 19 and 20%; metal richer in Ni, Ir, and Ge and poorer in AuThe astronomical unit for length is described as the "mean" distance (average of aphelion and perihelion distances) between the Earth and the Sun. Though most references state the value for 1 AU to be approximately 150 million kilometers, the currently accepted precise value for the AU is 149,597,870.66 km. The, As, and Ga than main-group

3. PyroxeneA class of silicate (SiO3) minerals that form a solid solution between iron and magnesium and can contain up to 50% calcium. Pyroxenes are important rock forming minerals and critical to understanding igneous processes. For more detailed information, please read the Pyroxene Group article found in the Meteoritics & Classification category. subgroup

pyroxene coexists with olivine, metal ungrouped and different from MG and ES

4. Ungrouped: Choteau, Milton (possibly CV-clan-related), NWA 10019

Utilizing the pressure constraints on the tridymiteSilica group mineral in which the tetrahedra occur in sheets. Tetrahedra alternately point up or down to share oxygen with tetrahedra of other sheets, forming six-sided rings perpendicular the sheets. Tridymite has a fairly open structure and accommodates Na+, K+ and Ca2+; charge balance is achieved by Al3+ ↔ Si4+. inclusions present in the Fukang pallasiteOne of two main classes of stony-iron meteorite, the other being mesosiderites. Pallasites are igneous in nature and characterized by crystals of olivine, sometimes peridot (green gem quality clear olivine crystals), embedded in a matrix of Fe-Ni metal. The type specimen, weighing 680 kg, was found in the mountains near, the maximum size limit for the main-group pallasite parent bodyThe body from which a meteorite or meteoroid was derived prior to its ejection. Some parent bodies were destroyed early in the formation of our Solar System, while others like the asteroid 4-Vesta and Mars are still observable today. would be ~600 km in diameter; a minimum size still large enough to enable differentiationA process by which a generally homogeneous chondritic body containing mostly metal, silicates and sulfides will melt and form distinct (differentiated) layers of different densities.   When the melting process continues for a long enough period of time, the once chondritic body will re-partition into layers of different composition including would be ~40 km (Della-Giustina et al., 2011).

Mesosiderites- The mesosiderites are complex polymict assemblages of FeNi metal and brecciated silicates including orthopyroxeneOrthorhombic, low-Ca pyroxene common in chondrites. Its compositional range runs from all Mg-rich enstatite, MgSiO3 to Fe-rich ferrosilite, FeSiO3. These end-members form an almost complete solid solution where Mg2+ substitutes for Fe2+ up to about 90 mol. % and Ca substitutes no more than ~5 mol. % (higher Ca2+ contents occur, plagioclaseAlso referred to as the plagioclase feldspar series. Plagioclase is a common rock-forming series of feldspar minerals containing a continuous solid solution of calcium and sodium: (Na1-x,Cax)(Alx+1,Si1-x)Si2O8 where x = 0 to 1. The Ca-rich end-member is called anorthite (pure anorthite has formula: CaAl2Si2O8) and the Na-rich end-member is albite, and olivine. Lithic clasts of cumulateIgneous rock composed of crystals that have grown and accumulated (often by gravitational settling) in a cooling magma chamber. and basaltic eucritesMost common type of achondrite meteorite and a member of the HED group. Eucrites are basalts composed primarily of pigeonite and anorthite (An60-98). Eucrites have been placed into three subgroups based on mineralogical and chemical differences. • Non-cumulate eucrites represent the upper crust that solidified on a magma ocean after, diogenitesDiogenites belong to the evolved achondrite HED group that also includes howardites and eucrites. They are named after the Greek philosopher Diogenes of Apollonia, of the 5th century BCE, who was the first to suggest that meteorites come from outer space (a realization forgotten for over 2,000 years). They are, and dunites are present. Mesosiderites possess specific thermal histories ranging from little recrystallized to melted (subgroups 1–4), reflecting different cooling rates based on their burial depth following a rapid heating event, probably an impact. Group 1 cooled the fastest at ~1°C/m.y. with group 3 having the slowest cooling rate of ~0.1°C/m.y. Group 4 reached the highest temperature of ~1350°C before quickly cooling. Recognition of clear differences in bulk compositions among these groups led to further group divisions (see the Bondoc page for details regarding these subdivisions).

There is not yet a consensus for the origin of the mesosiderites, and different theories currently exist to explain their formation. A recent model based on smoothed-particle hydrodynamicsStudy of fluid in motion. calls for the disruption and re-accretion of a 200–400 km differentiated asteroid with a molten coreIn the context of planetary formation, the core is the central region of a large differentiated asteroid, planet or moon and made up of denser materials than the surrounding mantle and crust. For example, the cores of the Earth, the terrestrial planets and differentiated asteroids are rich in metallic iron-nickel.. The impactor is calculated to have been a 50–150 km body with an impact speed of 5 km/s. This event initially caused rapid cooling (~0.1°C/y.) from thermal equilibration, followed by very slow cooling (~0.5°C/m.y.) as the brecciated material was deeply covered by a massive debris blanket. The Ar–Ar ages of mesosiderites of 3.7–4.1 b.y. reflect this very slow cooling. Weakly shocked olivine was sequestered into the core at the time of the catastrophic impact. Subsequently, molten metal was mixed with crustal fragments during re-accretion. The O-isotopic values for the mesosiderites are virtually identical to those of the HED suite meteorites, which implies that a genetic link (i.e., same parent body) exists between these meteoriteWork in progress. A solid natural object reaching a planet’s surface from interplanetary space. Solid portion of a meteoroid that survives its fall to Earth, or some other body. Meteorites are classified as stony meteorites, iron meteorites, and stony-iron meteorites. These groups are further divided according to their mineralogy and classes (Greenwood et al., 2006). Stony-irons represent only 2.8% of the total known meteorites.

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