1,721,007 research outputs found
Serpentine minerals discrimination by thermal analysis
No full textThis paper reports a complete set of TG, DTG, and DTA data, coupled with emitted gas analysis, for well-constrained, almost pure serpentine samples. Serpentine dehydroxylation takes place between 550 and 800 °C, with DTG and DTA peak temperatures progressively decreasing from antigorite (720 and 715 °C, respectively) to lizardite (708 and 714 °C), polygonal serpentine (685 and 691 °C), and chrysotile (650 and 654 °C). Antigorite has an additional diagnostic signal at ~740–760 °C, always absent in the other serpentines, and dependent on antigorite superperiodicity (T shift of ~20 °C from 36 to 49 Å modulation wavelength). A sharp exothermic peak occurs at extremely constant temperatures (~820 °C), independently from the starting serpentine structure. The high-T mineral assemblage is always represented by forsterite and enstatite.
Based on the observed relationships between serpentine structures and DTG/DTA dehydroxylation temperatures, thermal analysis may represent a useful tool for serpentine identification, particularly in the case of natural massive samples where different varieties are mutually intermixed. The accurate definition of serpentine mineralogy would have obvious implications in both geological-petrological and health-related issues
Exploring fault rocks at the nanoscale
No full textThis paper aims to demonstrate the potential role of micro/nanoscale investigations in the study of fault
rocks, given that a multiscale and multidisciplinary approach linking structural geology, petrology and
mineralogy from the outcrop to the unit-cell scale is essential to the comprehensive characterization of
geological materials. To explore fault rocks at the sub-micrometer scale, a fundamental contribution is
provided by the Transmission Electron Microscope (TEM), which offers a real-time combination of highresolution
images, diffraction patterns, and chemical data for extremely small volumes, providing
accurate mineralogical and nanostructural determinations.
This paper focuses on a few selected examples, that are particularly appropriate for TEM investigation,
in particular: 1) grain-size determinations on ultrafine-grained fault rocks; 2) detection and characterization
of poorly crystalline minerals (e.g., clays) and amorphous phases within fault rocks; 3) identification
of deformation-induced mineral reactions occurring within fault zones (e.g., dolomite
decarbonation and serpentine dehydration); and 4) observations of recurrent deformation nanotextures
in phyllosilicates. TEM investigation can provide an unexpected amount of data, much of which cannot
be obtained by conventional techniques, and it can therefore significantly contribute to an understanding
of the physico-chemical conditions for faulting and fault mechanics
Contrasting chemical compositions in associated lizardite and chrysotile in veins from Elba, Italy
No full textSerpentinitic veins consisting of associated chrysotile and well crystallized lizardite were extensively sampled in the Monte Fico quarries, Elba Island, Italy. The veins are mostly formed of euhedral lizardite-1T. After optical, IR and X-ray characterization, the specimens were studied by transmission electron microscopy. TEM provides evidence for the occurrence of chrysotile, polygonal serpentine and lizardite, with microstructural relations pointing to a continuous recrystallization from chrysotile to polygonal serpentine and to lizardite. No major fault structure occurs in lizardite. Chemical data indicate the existence of a compositional gap between lizardite [mean composition (Mg2.79Fe2+ 0.09 Fe3+ 0.07Al0.07)Σ=3.02(Si 1.91Al0.09)Σ=2.00 O5 (OH)3.90] and chrysotile [mean composition (Mg2.80Fe0.11Al0.06) Σ=2.97Si2.00 O5 (OH)4] with lizardite systematically enriched in aluminum and iron with respect to chrysotile. Therefore, the two minerals should no longer be considered as polymorphs
Vein antigorites from Elba Island, Italy
No full textAntigorite in veins from Elba Island, Italy, appears as light-green splintery fibers, discontinuously surrounding the massive dark serpentinite, extending along fractures and deformed by compressive shearing.
The chemical compositions cluster around Mg2.66Fe0.12Al0.02Si2.00O5(OH)(3.60) with the exact value depending upon the polysomatic structural modulations. Electron diffraction shows superperiodicities variable from 33 to 49 Angstrom and changing from vein to vein, with no relation with the metamorphic grade of the surrounding rocks.
Common polysomatic faulting, (001) twinning and parallel association with polygonal serpentine as well as with anomalous (thin wall tubes) chrysotile are the most evident microstructural features.
A complete set of X-ray diffraction, IR, NMR and thermal analyses results is presented for a selected number of specimens.
With respect to massive antigorite serpentinites, antigorite veins show periodicities shorter than expected; this feature is interpreted as due to shearing stress and/or favourable kynetics able to accelerate the sluggish chrysotile-antigorite transformations
Nanostructures and microinfrared behavior of black opal from Gracias, Honduras
No full textThis paper reports nanostructural and microinfrared data of a black opal from Gracias, Honduras. The opal is hosted in
a volcanoclastic rock, as the cement among glass shards and as the fi lling material of steam vesicles (100–400 μm in diameter). It
is amorphous (opal-A), inclusions-free and consist of silica spheres, 450 nm in diameter, arranged in ordered close-packed fcc lattice,
producing intense play of color effects. The observation of thin sections under the polarizing light microscope reveals that the
Honduran opal is characterized by pleochroism and sharp birefringence, together with striae, "twins" and zoning.
Electron microscopy revealed that all these features are due to sphere packing mistakes. "Twins" and striae are due to changes
in the close-packed planes stacking, whereas zoning is related to a preferential concentration of point defects (i.e., vacant silica
spheres) at the opal/glass boundary.
Microinfrared spectrometry indicates that the Honduran opal is closely associated with minor CO2, possibly infi ltrating among
silica spheres
The effect of chlorite interlayering on Ar-40-Ar-39 biotite dating: an Ar-40-Ar-39 laser-probe and TEM investigations of variably chloritised biotites
No full textBiotite is one of the most common minerals dated by the Ar-40-Ar-39 method. It frequently shows K contents below the expected stoichiometric value, suggesting the presence of low-K impurities. The most common low-K alteration product of biotite is chlorite. Therefore, it is important to understand the effects of chlorite interlayering on Ar-40-Ar-39 ages in order to correctly interpret Ar-40-Ar-39 data. This study examines the outcome of Ar-40-Ar-39 dating analyses on variably chloritised biotites from Ordovician intrusive rocks. The infrared (IR) laser-probe technique and different gas extraction methods were adopted. Incremental laser-heating data on bulk samples yielded hump-shaped age profiles with meaningless young and old age steps. Both the extent of anomalous old age steps and the degree of discordance of the age spectra were much more pronounced in the more chloritised biotite samples. In contrast, in situ data on rock chips and total-fusion ages on single biotite flakes yielded ages concordant with, or younger than, the inferred emplacement ages. Transmission electron microscopy (TEM) was used to texturally characterise biotite samples at the nanometre scale. It was also used to document the complex decomposition-transformation process affecting interlayered biotite-chlorite during in-vacuo IR-laser heating to temperatures ranging from similar to600 to >1,000 degreesC. TEM results suggest that hump-shaped age profiles result from an interplay between Ar-39(K) redistribution by recoil during sample irradiation and differential release of argon isotopes hosted in three main reservoirs. These reservoirs are (from least to most retentive): extended defects, chlorite and biotite. The final descending age segment is attributed to the progressive release of argon with increasing temperature from large biotite domains for which Ar-39(K) recoil loss was less important. Ar-40-Ar-39 data support previous findings, which suggest that Ar-40-Ar-39 ages when recoil effects are minimised, provide minimum estimates that approach the true biotite age, when the pristine domains are analysed. The most effective approach for obtaining meaningful Ar-40-Ar-39 ages was using individual total-fusion analyses on carefully selected, single flakes previously split along the basal cleavage by wet-grinding and corresponding to a sample mass of a few micrograms
Thermal decomposition of serpentine during coseismic faulting: nanostructures and mineral reactions
No full textThis paper reports a detailed characterization of an antigorite-bearing serpentinite, deformed at seismic
slip-rate (1.1 m/s) in a high-velocity friction apparatus. Micro/nanostructural investigation of the slip
zone (200 mm thick) revealed a zonal arrangement, with a close juxtaposition of horizons with significantly
different strength, respectively consisting of amorphous to poorly-crystalline phases (with bulk
anhydrous composition close to starting antigorite) and of highly-crystalline assemblages of forsterite
and disordered enstatite (200 nm in size and in polygonal-like nanotextures). The slip zone also hosts
micro/nanometre sized Cr-magnetite grains, aligned at low angle with respect to the slipping surface and
inherited from the host serpentinite.
Overall observations suggest that frictional heating at asperities on the slipping surface induced
a temperature increase up to 820e1200 C (in agreement with flash temperature theory), responsible for
serpentine complete dehydration and amorphization, followed by crystallization of forsterite and
enstatite (under post-deformation, static conditions). The results of this study may provide important
keys for the full comprehension of the mechanical behaviour and of the possible geodynamical role of
serpentinite-hosted faults through the seismic cycle
Asbestiform talc from a talc mine near Talcville, New York, U.S.A.: Composition, morphology, and genetic relationships with amphiboles
No full textWe analyzed the chemical composition and observed morphological relationships of mineral phases in a thin section and grain
mount of a talc-bearing amphibole schist collected at a former talc mine near Talcville, New York at scales ranging from hand sample to crystal lattice. The main mineral phases are tremolite, anthophyllite, and talc. Tremolite exists as unaltered crystals throughout the sample. Anthophyllite has an acicular habit and a series of fractures running perpendicular to crystal elongation (i.e., its c crystallographic axis). Talc occurs as a fine-grained mass in anthophyllite fractures, between acicular anthophyllite partings, and as pseudomorphs after anthophyllite, with some occurring with an asbestiform habit. Thus, the talc alterations occur both parallel and perpendicular to the elongate anthophyllite, with the latter producing very high-aspect ratio (i.e., greater than 20 times longer than wide) non-asbestiform anthophyllite particles. Based on wavelength-dispersive spectroscopy (WDS) analysis the three phases have near endmember formulas: tremolite [K0.03Na0.17Ca1.74Mn0.01Fe0.01Mg5.21Al0.03Si7.94O22(OH1.65F0.35)], anthophyllite [Na0.04Ca0.04 Mn0.04Fe0.02Mg6.96Al0.02Si7.95O22(OH1.82F0.18)], fracture-filling talc [Mg3.10Mn0.02Fe0.01Al0.01Si3.93O10(OH1.92F0.08)], and talc pseudomorph [Mg3.03Mn0.02Fe0.01Al0.01Si3.96O10(OH1.92F0.08)]. Microscopic examination in light and electron microscopes showed intergrowths of talc and anthophyllite, but it was only with the aid of high-resolution lattice imaging perpendicular to the c crystallographic axis where the formation mechanism of asbestiform talc could be ascertained
Growth and deformation mechanisms of talc along a natural fault: a micro/nanostructural investigation
No full textThis paper documents the occurrence of large
amounts of talc within a continental normal fault. The
talc-in reaction is deformation-enhanced and occurs by the
interaction between dolostones and silica-rich hydrothermal
fluids. In the high-strain, foliated fault core, talc forms
an interconnected network of oriented (001) lamellae,
200–300 nm thick, locally associated with minor tremolite
fibres, up to 300 nm in diameter. The talc structure is
affected by several strain-induced defects, among which
(001) interlayer delamination that produces talc ‘‘sublamellae’’
down to 10–30 nm thick. Micro/nanostructural
observations definitely point to a predominant deformation
mechanism of (001) frictional sliding, further enhanced by
pervasive delamination that gives rise to an almost infinite
number of possible sliding surfaces. These effects have
fundamental implications in fault mechanics, resulting in
significant fault weakening
Frictional strength and deformation microstructures of mineralogically controlled Serpentinites
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