Mg-rich Phyllosilicates Research Articles

Formation of Natural Magnesium Silica Hydrate (M-S-H) and Magnesium Alumina Silica Hydrate (M-A-S-H) Cement.

Occurrences of natural magnesium alumina silicate hydrate (M-(A)-S-H) cement are present in Feragen and Leka, in eastern and western Trøndelag Norway, respectively. Both occurrences are in the subarctic climate zone and form in glacial till and moraine material deposited on ultramafic rock during the Weichselian glaciation. Weathering of serpentinized peridotite dissolves brucite and results in an alkaline fluid with a relatively high pH which subsequently reacts with the felsic minerals of the till (quartz, plagioclase, K-feldspar) to form a cement consisting of an amorphous material or a mixture of nanocrystalline Mg-rich phyllosilicates, including illite. The presence of plagioclase in the till results in the enrichment of alumina in the cement, i.e., forms M-A-S-H instead of the M-S-H cement. Dissolution of quartz results in numerous etch pits and negative quartz crystals filled with M-A-S-H cement. Where the quartz dissolution is faster than the cement precipitation, a honeycomb-like texture is formed. Compositionally, the cemented till (tillite) contains more MgO and has a higher loss of ignition than the till, suggesting that the cement is formed by a MgO fluid that previously reacted with the peridotite. The M-(A)-S-H cemented till represents a new type of duricrust, coined magsilcrete. The study of natural Mg cement provides information on peridotites as a Mg source for Mg cement and as a feedstock for CO2 sequestration.

Near-ultraviolet absorption distribution of primitive asteroids from spectrophotometric surveys

Context. Hydrated minerals, such as phyllosilicates, on asteroids can provide constraints on the temperature or compositional distribution of the early Solar System. Previous studies pointed out the possibility that absorption in the near-ultraviolet (NUV, 0.35–0.5 μm) wavelength region is a proxy for hydrated minerals in primitive asteroids. However, the radial distribution of the NUV absorption among primitive asteroids was not revisited after the Eight Color Asteroid Survey (ECAS). Aims. Our objectives are first to evaluate the possibility of using the NUV absorption as diagnostics of hydrated minerals based on the recent datasets of primitive asteroids and hydrated carbonaceous chondrites, and second to investigate the reflectance spectrophotometry of the primitive asteroids in the NUV as functions of heliocentric distance and size. Methods. The NUV and visible reflectance spectrophotometry of more than 9000 primitive asteroids was investigated using two spectrophotometric surveys, ECAS and the Sloan Digital Sky Survey (SDSS), which cover wavelengths down to 0.32 μm and 0.36 μm, respectively. We classified asteroids from the main asteroid belt, the Cybele and Hilda zones, and Jupiter Trojans based on Tholen’s taxonomy and described the statistical distribution of primitive asteroid types. We also examined the relationship of the NUV, 0.7 μm, and 2.7 μm absorptions among primitive asteroids and hydrous carbonaceous chondrites CI and CM. Results. We find strong correlations between the NUV and the OH-band (2.7 μm) absorptions for primitive asteroids and hydrated meteorites, suggesting the NUV absorption can be indicative of hydrated silicates. Moreover, there is a great difference in the NUV absorption between the large asteroids (diameter d > 50 km) and small asteroids (d < 10 km) in the taxonomic distribution. The taxonomic distribution of asteroids differs between the inner main belt and middle-outer main belt. Notably, the C types are dominating large members through the main belt and the F types are dominating small asteroids of the inner main belt. The asteroids beyond the main belt consist mostly of P and D types, although P types are common everywhere in the main belt. The peculiar distribution of F types might indicate a different formation reservoir or a displacement process of F types in the early Solar System. The strongest absorptions of the NUV and 0.7 μm band were observed in G types, which likely comprise CM-like Fe-rich phyllosilicates. On the other hand, according to a recent sample return from an F-type asteroid (162173) Ryugu, the F types with the OH-band at 2.7 μm and the shallow NUV absorption could comprise CI-like Mg-rich phyllosilicates.

Fluid-rock interaction, skarn genesis, and hydrothermal alteration within an upper crustal fault zone (Island of Elba, Italy)

The Terranera magnetite-hematite-pyrite deposit of the Island of Elba (Italy) is an historical skarn deposit hosted by a fault zone of regional importance (Zuccale Fault) and by its hanging wall rocks.We combine field observations with petrographic data, electron probe microanalyses (EPMA), XRPD data, fluid inclusion microthermometry, and element imaging by Laser Ablation-Inductively Coupled Plasma-Time of Flight Mass Spectrometry (LA-ICP-TOFMS) to define the ore-forming process at Terranera. We show that in this location the fault is made of four levels of mineralized fault rocks having distinct mineral compositions. In these levels, a mineral association made of diopside, clinozoisite, and other Mg-rich minerals is replaced by magnetite, hematite, pyrite, Mg-hornblende, clinochlore, and other Mg-rich phyllosilicates. This paragenesis is overprinted by goethite and clay minerals. Chlorite-quartz geothermometry and fluid inclusion microthermometry show that ore precipitation occurred at 350–180 °C from fluids of distinct bulk salinities, but goethite and clay mineral overprinting progressed at lower T.We propose that Terranera is a magnesian Fe skarn formed due to the interaction between distinct hydrothermal fluids and a dolomitic protolith, which was preserved within the fault zone. These fluids mixed and cooled during protolith metasomatism, causing ore precipitation due to oxidation and desulfidation. A very similar process was described in a large deposit of Elba (Rio Marina). Argillic alteration was widespread within the fault but met permanently intermediate sulfidation conditions. Trace element composition of hematite shows that Terranera has features that overlap those of skarn and epithermal deposits. In particular, elements that are typical of epithermal deposits (Sb, Ga, Ge, As) occur at mass fractions (50–200 μg/g) that are either unreported or not typical of hematite from skarn deposits. These features identify Terranera as formed in an ore environment that was transitional between that of a skarn and of an epithermal deposit. These features are shared by other historical deposits located at Elba and in the massive pyritic ore district of south Tuscany (e.g., Gavorrano, Fenice Capanne). This indicates that a similar environment might have occurred during the Neogene beyond Elba, in a much larger ore district of south Tuscany.

Askival: An altered feldspathic cumulate sample in Gale crater.

Askival is a light-toned, coarsely crystalline float rock, which was identified near the base of Vera Rubin Ridge in Gale crater. We have studied Askival, principally with the ChemCam instrument but also using APXS compositional data and MAHLI images. Askival and an earlier identified sample, Bindi, represent two rare examples of feldspathic cumulate float rocks in Gale crater with >65% relict plagioclase. Bindi appears unaltered whereas Askival shows textural and compositional signatures of silicification, along with alkali remobilization and hydration. Askival likely experienced multiple stages of alteration, occurring first through acidic hydrolysis of metal cations, followed by deposition of silica and possible phyllosilicates at low T and neutral-alkaline pH. Through laser-inducedbreakdownspectroscopy compositional analyses and normative calculations, we suggest that an assemblage of Fe-Mg silicates including amphibole and pyroxene, Fe phases, and possibly Mg-rich phyllosilicate are present. Thermodynamic modeling of the more pristine Bindi composition predicts that amphibole and feldspar are stable within an upper crustal setting. This is consistent with the presence of amphibole in the parent igneous rocks of Askival and suggests that the paucity of amphiboles in other known Martian samples reflects the lack of representative samples of the Martian crust rather than their absence on Mars.

Identification of Phyllosilicates in the Antarctic Environment Using ASTER Satellite Data: Case Study from the Mesa Range, Campbell and Priestley Glaciers, Northern Victoria Land

In Antarctica, spectral mapping of altered minerals is very challenging due to the remoteness and inaccessibility of poorly exposed outcrops. This investigation evaluates the capability of Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) satellite remote sensing imagery for mapping and discrimination of phyllosilicate mineral groups in the Antarctic environment of northern Victoria Land. The Mixture-Tuned Matched-Filtering (MTMF) and Constrained Energy Minimization (CEM) algorithms were used to detect the sub-pixel abundance of Al-rich, Fe3+-rich, Fe2+-rich and Mg-rich phyllosilicates using the visible and near-infrared (VNIR), short-wave infrared (SWIR) and thermal-infrared (TIR) bands of ASTER. Results indicate that Al-rich phyllosilicates are strongly detected in the exposed outcrops of the Granite Harbour granitoids, Wilson Metamorphic Complex and the Beacon Supergroup. The presence of the smectite mineral group derived from the Jurassic basaltic rocks (Ferrar Dolerite and Kirkpatrick Basalts) by weathering and decomposition processes implicates Fe3+-rich and Fe2+-rich phyllosilicates. Biotite (Fe2+-rich phyllosilicate) is detected associated with the Granite Harbour granitoids, Wilson Metamorphic Complex and Melbourne Volcanics. Mg-rich phyllosilicates are mostly mapped in the scree, glacial drift, moraine and crevasse fields derived from weathering and decomposition of the Kirkpatrick Basalt and Ferrar Dolerite. Chlorite (Mg-rich phyllosilicate) was generally mapped in the exposures of Granite Harbour granodiorite and granite and partially identified in the Ferrar Dolerite, the Kirkpatrick Basalt, the Priestley Formation and Priestley Schist and the scree, glacial drift and moraine. Statistical results indicate that Al-rich phyllosilicates class pixels are strongly discriminated, while the pixels attributed to Fe3+-rich class, Fe2+-rich and Mg-rich phyllosilicates classes contain some spectral mixing due to their subtle spectral differences in the VNIR+SWIR bands of ASTER. Results derived from TIR bands of ASTER show that a high level of confusion is associated with mafic phyllosilicates pixels (Fe3+-rich, Fe2+-rich and Mg-rich classes), whereas felsic phyllosilicates (Al-rich class) pixels are well mapped. Ground truth with detailed geological data, petrographic study and X-ray diffraction (XRD) analysis verified the remote sensing results. Consequently, ASTER image-map of phyllosilicate minerals is generated for the Mesa Range, Campbell and Priestley Glaciers, northern Victoria Land of Antarctica.

Laboratory studies on the 3 μm spectral features of Mg-rich phyllosilicates with temperature variations in support of the interpretation of small asteroid surface spectra

Recent orbital data revealed the presence of hydrated minerals on the surfaces of asteroids, mainly through the identification and the study of the 3-μm spectral absorption band (Hamilton et al., 2019; Kitazato et al., 2019). The presence of an absorption feature around 3-μm on planetary bodies' surfaces is indicative of the presence of OH-bearing minerals. This band has been widely detected on carbonaceous chondrites but its appearance and its shape are diverse indicating different composition and/or the occurrence of subsequent alteration events. In this work, we present the results of laboratory experiments performed at the Planetary Spectroscopy Laboratory (PSL) of the German Aerospace Center (DLR) to study the spectral behaviour of the 3-μm spectral features in the Mg-OH minerals with thermal variation. It has been suggested that thermal alteration processes, can darken the surfaces of carbonaceous chondrites, thus decreasing the appearance and visibility of the spectral features around 3 μm. Thermal alteration processes are consistent with the scenario currently proposed to explain the formation of 162173 Ryugu asteroid (Sugita et al., 2019). The Near Infrared Spectrometer (NIRS3) on the Hayabusa2 mission detected a weak and narrow absorption feature centred at 2.72 μm across the entire observed surface of the C-type asteroid (Kitazato et al., 2019). However, the collected spectra from the Ryugu surface show no other absorption features in the 3-μm region. To investigate this point further and analyze the variation of the spectral features around 3-μm with thermal alteration, we studied the Mg-rich phyllosilicates serpentine and saponite in two different situations: 1) thermal alteration at increasing temperature - the samples were heated at steps of 100°C, starting from 100°C up to 700°C, for 4 hours each; 2) long time heating at constant temperature - samples were kept constantly at ∼250°C for 1 month (1st step), then cooled down and measured in reflectance. This long heating process has been repeated at the same temperature of 250°C for 2 months (2nd step). The results obtained show an important variation of phyllosilicates spectral bands with temperature and provide useful data for the interpretation of past and future mission small bodies collected surface spectra.

Variation of mineralogy and organic material during the early stages of aqueous activity recorded in Antarctic micrometeorites

Micrometeorites (MMs) recovered from surface snow near the Dome Fuji Station, Antarctica are almost free from terrestrial weathering and contain very primitive materials, and are suitable for investigation of the evolution and interaction of inorganic and organic materials in the early solar system. We carried out a comprehensive study on seven porous and fluffy MMs [four Chondritic porous (CP) MMs and three fluffy fine-grained (Fluffy Fg) MMs] and one fine-grained type 1 (Fg C1) MM for comparison with scanning electron microscope, transmission electron microscope, X-ray absorption near-edge structure analysis, and secondary ion mass spectrometer.They show a variety of early aqueous activities. Four out of the seven CP MMs contain glass with embedded metal and sulfide (GEMS) and enstatite whiskers/platelets and do not have hydrated minerals. Despite the same mineralogy, organic chemistry of the CP MMs shows diversity. Two of them contain considerable amounts of organic materials with high carboxyl functionality, and one of them contains nitrile (CN) and/or nitrogen heterocyclic groups with D and 15N enrichments, suggesting formation in the molecular cloud or a very low temperature region of the outer solar system. Another two CP MMs are poorer in organic materials than the above-mentioned MMs. Organic material in one of them is richer in aromatic C than the CP MMs mentioned above, being indistinguishable from those of hydrated carbonaceous chondrites. In addition, bulk chemical compositions of GEMS in the latter organic poor CP MMs are more homogeneous and have higher Fe/(Si+Mg+Fe) ratios than those of GEMS in the former organic-rich CP MMs. Functional group of the organic materials and amorphous silicate in GEMS in the organic-poor CP MMs may have transformed in the earliest stage of aqueous alteration, which did not form hydrated minerals.Three Fluffy Fg MMs contain abundant phyllosilicates, showing a clear evidence of aqueous alteration. Phyllosilicates in thee MMs are richer in Fe than those in hydrated IDPs, typical fine-grained hydrated (Fg C1) MMs, and hydrated carbonaceous chondrites. One of the Fluffy Fg MMs contains amorphous silicate, which is richer in Fe than GEMS and contains little or no nanophase Fe metal but contains Fe sulfide. Because the chemical compositions of the amorphous silicate are within the compositional field of GEMS in CP IDPs, the amorphous silicate may be alteration products of GEMS. The entire compositional field of GEMS in the CP MMs and the amorphous silicate in the Fluffy Fg MM matches that of the previously reported total compositional range of GEMS in IDPs.One Fluffy Fg MM contains Mg-rich phyllosilicate along with Fe-rich phyllosilicate and Mg-Fe carbonate. Mg-rich phyllosilicate and Mg-Fe carbonate may have been formed through the reaction of Fe-rich phyllosilicate, Mg-rich olivine and pyroxene, and water with C-bearing chemical species.These data indicate that CP MMs and Fluffy Fg MMs recovered from Antarctic surface snow contain materials that throw a light on the earliest stages of aqueous alteration on very primitive solar system bodies. Because mineralogy and isotopic and structural features of organic materials in D10IB009 are comparable with isotopically primitive IDPs, its parent body could be comets or icy asteroids showing mass ejection (active asteroids). By contrast, organic-poor CP MMs may have experienced the earliest stage of aqueous alteration and Fluffy Fg MMs experienced weak aqueous alteration. The precursor materials of the parent bodies of Fluffy Fg MMs probably contained abundant GEMS or GEMS-like materials like CP IDPs, which is common to fine-grained matrices of very primitive carbonaceous chondrites such as CR3s. However, highly porous nature of organic-poor CP MMs and Fluffy Fg MMs suggests that parent bodies of these MMs must have been much more porous than the parent bodies of primitive carbonaceous chondrites. Given no phyllosilicate among the returned samples of 81P/Wild 2 comet, the MMs may have been derived from porous icy asteroids such as active asteroids as well as P- and D-type asteroids rather than comets.

Effect of the irradiation dose on the luminescence emission of a Mg-rich phyllosilicate

This paper reports on the thermoluminescence and cathodoluminescence behaviour of a well-characterized sepiolite (Si12Mg8O30(OH)4(OH2)4·8H2O). Both natural and induced thermoluminescence curves display a complex emission suggesting a continuum in the trap distribution involving multiorder kinetics. UV–IR cathodoluminescence spectral emission shows five wavebands at 330, 400 and 440 (associated with structural defects, [AlO4] or non-bridging oxygen hole centers) and 520 and 770 nm (linked to point defects). This daily-use material, that is of interest since could be employed for personal dosimetry in the case of radiation accident or radiological terrorism where conventional monitoring has not been established, were never investigated before.

Aqueous alteration on asteroids: Linking the mineralogy and spectroscopy of CM and CI chondrites

CM/CI meteorites range in degree of aqueous alteration suggesting differences in initially accreted materials including water ice and possible spatial heterogeneities within their parent bodies. As alteration progresses, the total abundance and magnesium content of phyllosilicates increases. In this paper we present the results of a coordinated spectral–mineralogical study of a well-characterized suite of CM/CI meteorites that range from 60 to 90% alteration. By acquiring spectra the same meteorite powders as Howard et al. (Howard, K.T., Benedix, G.K., Bland, P.A., Cressey, G. [2009]. Geochim. Cosmochim. Acta 73, 4576–4589; Howard, K.T., Benedix, G.K., Bland, P.A., Cressey, G. [2011]. Geochim. Cosmochim. Acta 75, 2735–2751) and Bland et al. (Bland, P.A., Cressey, G., Menzies, O.N. [2004]. Meteorit. Planet. Sci. 39(1), 3–16), we are able for the first time to directly correlate mineralogy with features in reflectance spectra. At visible/near-infrared wavelength, the presence of a 0.7-μm charge transfer band is indicative of aqueous alteration. However, not all altered CM/CI meteorites exhibit this feature; thus the lack of a 0.7μm absorption band in asteroids does not necessarily signify a lack of aqueous alteration. Furthermore, the position and depth of 0.7-μm charge transfer band shows no correlation with the mineralogical changes associated with aqueous alteration. Similarly, the near-infrared slope, which is not directly related to the mineralogic progression associated with increasing alteration, is not unambiguously related to degree of alteration in the CM/CI meteorites studied. However, the mid-infrared reflectance spectra of CM/CI meteorites contain a broad absorption feature in the 10–13-μm region, which is a convolution of vibrational features due to Mg-rich phyllosilicates and unaltered olivine. The overall feature continuously changes with total phyllosilicate abundance from a shorter wavenumber/longer wavelength peak (815cm−1, 12.3-μm) for less altered meteorites to a longer wavenumber/shorter wavelength (875cm−1, 11.4-μm) peak in the highly altered meteorites, with roughly equal spectral contributions producing a doublet in intermediately altered meteorites, Using the results from the mid-infrared analyses of meteorite spectra, it is possible to estimate the degree of alteration on dark primitive asteroids. We find Asteroid (24) Themis to have a 1000–700cm−1 (10–13-μm) peak at longer wavelengths suggesting Themis has a complex surface mineralogy with approximately 70vol.% phyllosilicates and 25vol.% anhydrous silicates.

What the ancient phyllosilicates at Mawrth Vallis can tell us about possible habitability on early Mars

Phyllosilicate deposits on Mars provide an opportunity to evaluate aqueous activity and the possibility that habitable environments may have existed during the Noachian period there. Analysis of hyperspectral visible/near-infrared (VNIR) Mars Reconnaissance Orbiter (MRO) Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) images has shown thick, complex profiles of phyllosilicates at Mawrth Vallis, Mars that are consistent with long-term aqueous activity and active chemistry. The ancient phyllosilicates in places such as this could have served as reaction centers for organic molecules. Previous experiments even suggest that phyllosilicates could have played a role in the origin of life. Regardless of whether life formed on early Mars or not, evaluating the type and thickness of clay-bearing units on Mars provides insights into plausible aqueous processes and chemical conditions both during the time of formation of the phyllosilicates, but also the subsequent period following their formation. The phyllosilicate outcrops at Mawrth Vallis extend across a broad (∼1000km) region and exhibit a consistent general trend of Al-phyllosilicates and amorphous Al/Si species at the top of the clay profile and Fe/Mg-phyllosilicates on the bottom. This implies either a change in water chemistry, a change in material being altered, or an alteration profile where the upper clays were leached and altered more significantly than those below. A change in iron in the phyllosilicate units is also observed such that an Fe2+-bearing unit is frequently observed between the Fe3+- and Mg-rich phyllosilicates below and the Al/Si-rich materials above. Abrupt changes in chemistry like this are often indicative of biogeochemical activity on Earth. Possible microbe-clay interactions are considered in comparison with the CRISM observations. This study evaluates CRISM spectra from four images of different outcrops across the Mawrth Vallis region and evaluates the observed phyllosilicates and clay components in terms of plausible aqueous and microbial processes and the potential for retention of biosignatures, if present.

Petrographic investigation of melt and matrix relationships in Chicxulub crater Yaxcopoil-1 brecciated melt rock and melt rock-bearing suevite (846–885 m, units 4 and 5)

The drill core from the International Continental Drill Program’s Chicxulub Scientific Drilling Project Yaxcopoil-1 (Yax-1) borehole, in the annular trough of the Chicxulub crater, exhibits from 794 to 895m a continuous sequence of impactites consisting of reworked fallout, fallout suevite, and brecciated impact melt rock. These impact breccias exhibit a complex history of deposition, fracturing, matrix emplacement and hydrothermal alteration. Detailed investigation of the mineralogy and chemistry of these breccias has led to a better understanding of the complex events involved in their formation. We find that the paragenesis of the brecciated impact melt rock (unit 5, 861–885m) involved fracturing of melt rock and early K-metasomatism during a hydrothermal alteration episode as suggested by earlier work. However, the present work has identified the role of multiple episodes of precipitation of Mg-rich phyllosilicates and formation and dissolution of accessory minerals in a relatively high temperature (>300°C) hydrothermal event. The earliest matrix formation event involved precipitation of Mg-rich phyllosilicate, accessory quartz, calcite, apatite, and andradite garnet from a hydrothermal fluid with a brine or seawater component. The fluid could have partly incorporated elements and shock-metamorphosed mineral phases derived from sedimentary lithologies, including calcite and dolomite that underwent complex phase transformations such as melting, decomposition and possible back-reactions. The discovery of andradite garnet in the matrix confirms the presence of an early high temperature hydrothermal event previously identified by mineralogical, stable isotope and fluid inclusion studies. The mineral assemblage, including Mg-rich saponite, suggests the involvement of seawater by comparison with similar alteration assemblages in hydrothermal systems involving seawater around the world. The presence of a later, low-temperature phase of the hydrothermal system, with different fluid chemistry is indicated by the partial dissolution of andradite garnet, and continued precipitation of matrix phyllosilicate minerals, but without the accessory quartz, abundant calcite or andradite, and without an accompanying K-metasomatism event.

Assessing spectral evidence of aqueous activity in two putative martian paleolakes

We evaluate the evidence for the presence of mineral spectral signatures indicative of the past presence of water at two putative paleolakes on Mars using observations by the Mars Reconnaissance Orbiter (MRO) Compact Reconnaissance Image Spectrometer for Mars (CRISM). CRISM spectra of both sites are consistent with laboratory spectra of Mg-rich phyllosilicates. Our analysis represents the first detailed evaluation of these locations. The spatial occurrence and association with topographic features within the craters is distinctly different for the two sites. The occurrence of these minerals supports the conclusion that water was once active in the areas sampled by these craters. The distribution of the phyllosilicates in Luqa does not provide distinctive evidence for the presence of a previous standing body of water and is consistent with either impact emplacement or post-impact alteration. For Cankuzo, the phyllosilicate distribution provides evidence of a layer in the crater wall indicative of aqueous activity, but does not require a paleolake.

Modal mineralogy of CM2 chondrites by X-ray diffraction (PSD-XRD). Part 1: Total phyllosilicate abundance and the degree of aqueous alteration

CM carbonaceous chondrites are samples of incompletely serpentinized primitive asteroids. Using position sensitive detector X-ray diffraction (PSD-XRD) and a pattern stripping technique, we quantify the modal mineralogy of CM2 chondrites: Mighei; Murray; Murchison; Nogoya and Cold Bokkeveld. There is a narrow range in the combined modal volume (vol%) of the most abundant phases Mg-serpentine (25–33%) and Fe-cronstedtite (43–50%). Cold Bokkeveld is anomalous in containing more Mg-serpentine (49–59%) than Fe-cronstedtite (19–27%). Even including Cold Bokkeveld, the range in modal total phyllosilicate is 73–79% (average = 75%). Total phyllosilicate abundance provides a non-ambiguous measure of the degree of aqueous alteration and indicates that these meteorites have all experienced essentially the same degree of aqueous alteration. This reflects pervasive hydration of matrix across CM2 samples. Apparent differences in the alteration of chondrules observed in petrographic studies represent various stages in the progression towards complete hydration of all components but are not manifest in significant differences in modal mineralogy. For all samples there is a limited range in olivine (6.9%) and pyroxene (5%) abundances. Modal abundances of the remaining identified phases also show a limited range: calcite (0–1.3%); gypsum (0–1.6%); magnetite (1.1–2.4%); pentlandite (0–2.1%) and pyrrhotite (1–3.8%). As expected, we observe a strong negative correlation in the modal abundance of anhydrous Fe–Mg silicates (olivine + pyroxene) and total phyllosilicate (Mg-serpentine + Fe-cronstedtite) consistent with the idea that phyllosilicate is forming by aqueous alteration of the anhydrous components. The negative correlation in the modal abundance between Mg–serpentine and Fe-cronstedtite indicates: (a) mineralogic transformation of Fe-cronstedtite to Mg-serpentine by fluid driven recrystallisation or (b) that these meteorites had different initial abundances of olivine and pyroxene. The observed positive correlation in the relative proportion of Mg-serpentine with increasing total phyllosilicate abundance reflects the evolution of increasingly Mg-rich phyllosilicate during aqueous alteration. Fe-cronstedtite is the dominant phyllosilicate, while CM chondrule olivines are forsteritic and will form Mg-serpentine during aqueous alteration. This implies that matrix olivine was more Fe-rich than chondrule olivine prior to aqueous alteration.

Reflectance and emission spectroscopy study of four groups of phyllosilicates: smectites, kaolinite-serpentines, chlorites and micas

AbstractCoordinated visible/near-infrared reflectance/mid-infrared reflectance and emissivity spectra of four groups of phyllosilicates were undertaken to provide insights into the differences within and among groups of smectites, kaolinite-serpentines, chlorites and micas. Identification and characterization of phyllosilicates via remote sensing on Earth and Mars can be achieved using the OH combination bands in the 2.2–2.5 μm region and the tetrahedral SiO4 vibrations from ~8.8–12 μm (~1140–830 cm–1) and ~20–25 μm (500–400 cm–1). The sharp and well resolved OH combination bands in the 2.2–2.5 μm region provide unique fingerprints for specific minerals. Al-rich phyllosilicates exhibit OH combination bands near 2.2 μm, while these bands are observed near 2.29–2.31, 2.33–2.34 μm and near 2.35–2.37 μm for Fe3+-rich, Mg-rich and Fe2+-rich phyllosilicates, respectively. When a tetrahedral substitution of Al or Fe3+ for Si occurs, the position of the Si(Al,Fe)O4 stretching mode absorption shifts. Depending on the size of the cation, the Si(Al,Fe)O4 bending mode near 500 cm–1 is split into multiple bands that may be distinguished via hyperspectral remote sensing techniques. The tetrahedral SiO4 vibrations are also influenced by the octahedral cations, such that Al-rich, Fe-rich and Mg-rich phyllosilicates can be discriminated in reflectance and emissivity spectra based on diagnostic positions of the stretching and bending bands. Differences among formation conditions for these four groups of phyllosilicates are also discussed. Hyperspectral remote sensing can be used to identify specific phyllosilicates using electronic and vibrational features and thus provide constraints on the chemistry and formation conditions of soils.

The composition of carbonaceous chondrite matrix

Matrix compositions of 32 carbonaceous chondrites have been analyzed by an electron microprobe defocussed-beam technique. Except in those chondrites that show evidence of metamorphism, matrices are compositionally similar and have correlation coefficients of +0.96 or greater. Weight per cent Mg/Si in matrices is constant (0.82 ± 0.05) but less than ratios derived from bulk analyses. Matrices in metamorphosed meteorites are Mg-depleted relative to those of other chondrites. Al Rais and Renazzo (anomalous by any classification scheme) have Mg-enriched matrices. Average matrix compositions cluster into chemical subgroups similar to those based on bulk chemical and petrographie criteria [C1, C2, C3(0), C3(V)]. C1 matrices are particularly variable in composition from point to point within the same meteorite, but points within individual breccia clasts appear to be more compositionally uniform. Cl matrices are depleted in Na, S, and Ca relative to solar and C2 matrix values, probably as a result of leaching. Matrix Ca/A1 ratios are highly variable and generally fall below the accepted meteoritic value. The only strong interelement correlation is for Fe, Ni, and S in C2 matrices, suggesting mixing of variable proportions of two components: Mg-rich phyllosilicate and a Ni-bearing chalcophile phase. The amount of magnetite associated with C2 matrix appears to vary systematically with matrix composition. Isotopic, chemical, and mineralogical constraints suggest that matrix, although appreciably altered in some meteorites, is chiefly a solar system condensation product which contains an admixture of unprocessed interstellar dust.