12.109 Lecture Notes September 15, 2005 Rock Forming Minerals III

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12.109 Lecture Notes September 15, 2005 Rock Forming Minerals III Structure and composition of: PYROXENES Pyroxenes = chain silicates, with tetrahedral and octahedral chains Pyroxene structure combines tetrahedral chains + octahedral strip Æ I-beams Diopside CaMgSi2O6 Monoclinic, due to symmetry of octahedra “Octahedral ducks” The symmetry of a pyroxene can depend on the facing direction of the octahedra. To remember this, we picture the octahedron as a duck, and signify as positive “ducks facing forwards” and as negative “ducks facing backwards.” How to make an orthorhombic pyroxene Pyroxene structure is a polytype Polytypism – one basic building unit put together in different ways Orthopyroxene (OPX) Stack I-beams in rows alternating directionality of octahedrons Pyroxene polytypes + monoclinic c·2/c + CPX + clinopyroxene, diopside + + + - orthorhombic Pbcn + protoenstatite - + + orthorhombic Pbcn + OPX - orthopyroxene - + + Pyroxene quadrilateral Other types of pyroxenes NaAlSi2O6 jadeite NaFeSi2O6 aegirine LiAlSi2O6 spodumene CaAl(Al, Si)O6 CaTs Calcium Tschermaks Fassaite=>50% CaTs MgAl(Al, Si)O6 MgTs Magnesium Tschermaks 2+ Augites can have substantial amounts of Ti in M1 site Æ Ti-CaTs (R TiAl2O6, where R can be any of a number of 2+ elements) 2+ Or with Cr, Cr-CaTs, distinctive green pyroxenes (R CrAlSiO6) Protoenstatite almost pure Mg pyroxene, like enstatite found in meteorites, enstatite chondrites boninites – discovered 1989, contain protoenstatite produced in subduction zones compositional variation in pyroxenes due to temperature when you look at pyroxenes in thin section, commonly you see evidence for exsolution – unmixing of components, chemical separation important in petrology because provides a record of temperature history In thin section, see blebs of unmixed mineral, esp. in plutonic rocks (slowly cooled igneous intrusives) or in high T igneous rocks where minerals grow directly from melt Can tell about rate of cooling @ low T, pigeonite phase disappears, reacts to form OPX and augite In plutonic rocks, get inverted pigeonite, OPX with augite lamelli Pigeonite Æ augite + OPX makes martensite, similar to phase transition in carbon-iron system .

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SPECTRAL CHARACTERIZATION of the ANCIENT SHERGOTTITES NORTHWEST AFRICA 7034 and 8159. KJ Orr1, LV Forman1, GK Benedix1
Ninth International Conference on Mars 2019 (LPI Contrib. No. 2089) 6177.pdf SPECTRAL CHARACTERIZATION OF THE ANCIENT SHERGOTTITES NORTHWEST AFRICA 7034 AND 8159. K. J. Orr1, L. V. Forman1, G. K. Benedix1, M. J. Hackett2, V. E. Hamilton3, and A. R. Santos. 1Space Science and Technology Centre (SSTC), School of Earth and Planetary Sciences, Curtin University, Perth, Western Australia, Australia ([email protected]), 2School of Molecular and Life Sciences, Curtin University, Perth, Western Australia, Australia, 3Southwest Research Institute, 1050 Walnut St. #300, Boulder, CO 80302 USA. 4USRA, 7178 Columbia Gateway Dr., Columbia, MD 21046. Introduction: Thermal infrared (TIR) spectros- Ga), the oldest confirmed shergottites recovered so far copy is a powerful remote sensing tool used to unravel [5]. As the only shergottites of Early Amazonian to No- the surface compositions of a target body. This tech- achian in age, they provide an invaluable opportunity to nique has been widely used in space missions, because understanding Mars’ early history. of its ability to detect and determine modal mineralogy Methods: Both samples (~0.5g chips) were ac- of the surface geology. It has been instrumental in de- quired from UNM and were made into epoxy mounts. veloping our understanding of Mars, as the majority of NWA 8159 was analyzed with a Tescan Integrated Min- missions sent to Mars have included an infrared spec- eral Analyzer (TIMA) to determine modal mineral trometer. These spectrometers can operate either in the abundancies and produce high-resolution mineral maps. visible (VIS) to near-infrared (NIR) or in the mid-infra- NWA 8159 was also analyzed using EBSD to charac- red (MIR).
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