Black Sandstone Research Articles

Critical slowing down features of acoustic emission signals for predicting the failure of black sandstone under different freezing temperatures

Critical slowing down features of acoustic emission signals for predicting the failure of black sandstone under different freezing temperatures

An experimental study on the erosion of sandstone by self-excited oscillation cavitation waterjet in submerged environment

Self-excited oscillation cavitation waterjet (SOCW) has good prospects for application in deep sea mineral extraction of its cavitation characteristics. This study conducted the erosion experiments of three types of sandstone, red sandstone, white sandstone, and black sandstone by SOCW under a submerged environment. The erosion characteristics of three types of sandstones under the impact of SOCW at various erosion times, standoff distance, and pressure were compared. It was found that, a) Red sandstone with larger porosity is more likely to produce a large damage area and has the best rock breaking effect. Taking the standard working condition of this experiment as an example (t = 50s, Pj = 16 MPa, S/De = 4), the erosion depth of red sandstone can reach 1.9 times that of white sandstone and 5.5 times that of black sandstone; the erosion area can reach 19.6 times that of white sandstone and 6.2 times that of black sandstone; the erosion quality can reach 5.2 times that of white sandstone and 7.4 times that of black sandstone. b) With the increase of erosion time and erosion pressure, the erosion index of sandstone shows an increasing trend. However, in this study, there is an optimal standoff distance S/De = 4 to maximize the erosion depth of the sandstone. Again, there is an optimal standoff distance to maximize the erosion hole area, S/De = 10 for the red sandstone and white sandstone, and S/De = 6 for the black sandstone. c) The special phenomenon of the pressure difference formed after the collapse of the SOCW bubble in the submerged environment, which acts on the moisture content fine pore space inside the sandstone, the fine pore zone, was observed by scanning electron microscope (SEM). The microscopic characterization of the water wedge effect was observed by computed tomography (CT), and the trend of crack development is shown by the binarization processed images of the sandstone internal damage section. This result can be a theoretical basis to guide the working parameters of nozzles in deep sea mining, especially for the selection of standoff distance.

Investigations into Variations in Meso- and Macro-Physicomechanical Properties of Black Sandstone under High-Temperature Conditions Based on Nuclear Magnetic Resonance

The high-temperature environment has a significant deterioration effect on the meso- and macro-physicomechanical properties of rocks. Black sandstone in Chuxiong Yunnan was chosen as the test sample to perform a high-temperature test, a nuclear magnetic resonance (NMR) test, and a uniaxial compressive test (UCT). After high-temperature heating (25°C–1,200°C), the thermally damaged samples were subjected to NMR tests in both saturated and centrifugal states. The transverse relaxation time T2 spectrum distribution and T2 spectrum area were subsequently obtained. The temperature effect of the mesopore structure was investigated by analyzing the changes in porosity. The T2 spectrum cutoff value was determined, and the temperature effect of the movable fluid migration law for rocks was evaluated. The thermal damage constitutive relationship with porosity as the damage variable was determined, and the evolution law of mesopore structure parameters and macroscopic mechanical parameters was discussed. UCT results showed that the mechanical characteristics of black sandstone were obviously degraded by high temperature, and especially the uniaxial compressive strength (UCS) decreased sharply after the temperature exceeded 900°C. NMR results identified 900°C as an inflection point for the mesostructure change. For temperatures lower than 900°C, the improvement of pore connectivity was not obvious. For temperatures exceeding 900°C, the porosity increased rapidly. The porosity had a great influence on UCS, and there was a clear exponential function relationship between the two. The larger the porosity, the smaller the UCS. It showed that it was an effective angle to study the mesopore characteristics and mechanical properties of high-temperature rocks based on the NMR technique.

Mechanism study of preventing crack propagation of fractured rockunder dynamic loads

To deeply understanding dynamic fracture properties and preventing crack propagation of fractured rock mass under dynamic loads, impact experiments were conducted using TWSRC (tunnel with single radial crack) samples, and sandstone were selected as the raw material to manufacture fractured rock samples. The crack initiation, propagation and obstructing behavior were measured by using a drop hammer impact test device and crack propagation gauge measuring system. The mechanism of preventing crack propagation and failure behavior during dynamic fracturing process was focused, and then the corresponding numerical simulation was conducted by using the finite difference code, which can be used to accurately estimate the experiment result. The results indicate that the whole dynamic fracturing process of fractured rock under dynamic loads is composed of the cyclic process of crack initiation, high-speed crack propagation, slowly deceleration, preventing crack propagation. In addition, the period of crack obstruction was approximate the microsecond level. The ratio of transgranular (TG) fracture at the crack obstruction point of fractured rock was smaller than that of the crack initiation point, and the ratio of TG fracture of green sandstone during the dynamic fracturing process was larger than that of black sandstone. The fracture energy for crack initiation again after crack obstruction was much less than the fracture energy required for the initial initiation of pre-existing crack.

Energy Dissipation and Failure Characteristics of Layered Composite Rocks under Impact Load

Horizontal layered composite rock samples composed of white and black sandstones with large differences in physical and mechanical properties were tested to explore the dynamic characteristics of layered composite rocks under impact load. Using the split Hopkinson pressure bar test system, the dynamic compression tests of two incident states of stress waves, that is, stress waves from white sandstone to black sandstone (W⟶B) and from black sandstone to white sandstone (B⟶W), were designed and carried out under different impact velocities. Combining the ultrahigh‐speed photography system and digital photogrammetry for deformation measurement (DPDM), we obtained the stress wave propagation characteristics, failure characteristics, and particle size distribution characteristics of broken rocks of the composite rocks under the two conditions. The experimental results were compared and analyzed, while stresses and strength conditions at the interface of the composite rock samples were theoretically assessed, yielding the following main findings. The energy dissipation pattern of composite rock had an obvious strain rate effect. The reflected energy and fragmentation energy density of composite rock increased approximately as quadratic functions of the incident energy. Affected by the wave impedance matching relationship, the W⟶B and B⟶W samples were significantly different in terms of the stress wave shape, energy dissipation, average particle size, and fractal dimension of the broken rocks at low impact velocities. However, with an increase in the impact velocities, the two gradually shared the same behavior. When composite rock samples deformed and failed, the macrocracks mostly initiated from the white sandstone. When the crack tip stress of the white sandstone at the interface exceeded the strength of the weakened black sandstone, the crack continued to develop through the two‐phase rock interface due to the difference in Poisson’s ratios. The damage degrees and failure modes of the two parts of composite rocks were different: black sandstone was prone to tensile splitting with local shear failure, while white sandstone exhibited shear failure with local tensile splitting. The damage degree of white sandstone exceeded that of black sandstone.

Experimental System of Hydrodynamic Action of Dunhuang Mogao Grottoes under Temperature-Humidity-Mechanical (THM) Coupling

In order to thoroughly investigate the weathering mechanism of the murals in Dunhuang Mogao Grottoes, based on the independently developed hydrodynamic test system of the surrounding rocks under the combined action of temperature, humidity, and stress, this study performed the liquid water adsorption test and gaseous water adsorption test of the surrounding rocks of the support body of Dunhuang Mogao Grottoes, as well as the softening test of the strength of liquid water adsorbed by the rock samples. The experimental results are as follows: (a) Q–t curve and lnw–lnt curve of the three types of black sandstone rock samples obtained by analyzing the nonpressure water absorption experiment data indicate that the Q–t curve of the rock samples showed a similar trend with time, with the initial curve being steep and rapid increase in the water absorption; the double logarithmic curve of water absorption is convex and tends to be saturated. (b) The absorbing water softening test analysis was carried out for four types of siltstone specimens with intensity of pressure; bibulous rate was found to vary with time, and sample parameters such as compressive strength and elastic modulus decreased linearly with increasing water absorption at different times of uniaxial compression test. (c) Through the summer and winter and cave surrounding rock within the sample, the gas adsorption of water simulation was combined with the adsorption curve analysis, indicating that the adsorption characteristics of the surrounding rock and the correlation of larger sample depend on season. These research results provide a reliable theoretical basis and technical support for the protection of the murals in the Mogao Grottoes of Dunhuang and the prevention and control by salt damage.

Investigation on Fracture Properties of Single-Flawed Tunnel Model Under Medium-to-Low-Speed Impacts

Dynamic fractures occur frequently in geophysical processes and engineering applications. It is thus essential to study crack and failure behaviors, such as crack time-to-initiation, crack growth rate and arrest period under dynamic loading. In this study, impact experiments were implemented by utilizing the single-flawed tunnel specimens under drop-hammer impacts. Four brittle materials, i.e., green sandstone, red sandstone, black sandstone and polymethyl methacrylate, were selected to make single-flawed tunnel specimens. Strain gauges and crack extension gauges were employed to measure the crack extension parameters. The properties of crack growth rate, crack time-to-initiation and arrest period of these four brittle materials were discussed and analyzed. The corresponding numerical simulation was performed by using the commercial software AUTODYN. The numerical results of crack growth rate and crack time-to-initiation agreed with the impact test results. The commercial software ABAQUS was applied to compute the dynamic stress intensity factors. The results show that both the dynamic initiation fracture toughness and the crack growth rate increase with the elastic moduli of these four types of brittle materials under the same loading conditions, whereas the crack time-to-initiation decreases with the increase in elastic moduli of the brittle materials under the same loading conditions.

The characteristics of dynamic fracture toughness and energy release rate of rock under impact

To investigate the characteristics of rock fracture toughness and dynamic energy release rate, impact experiments were conducted in large single cleavage semicircle compression (LSCSC) specimens. Fracture toughness and energy release rate of three sandstones were investigated and a drop-weight device was applied. Crack length and crack propagation velocity were corrected by fractal method, and fracture surface morphology was obtained by scanning electron microscope. The dynamic stress intensity factor (DSIF) was acquired from the finite element model established in the ABAQUS. The DSIF history was acquired, and the initiation and propagation toughness were determined according to the measuring result of fracture time. It shows that, for a material, propagation toughness is related to its Young’s modulus, and the higher the Young’s modulus or wave impedance is, the larger the propagation toughness is. For black sandstone, the energy release rate increases rapidly with crack length, whereas for green and red sandstones, it increases slightly under a same speed impact.

Study of rock dynamic fracture toughness and crack propagation parameters of four brittle materials under blasting

The study on dynamic fracture toughness (DFT) is of considerable practical interest because it can be used to predict cracked rock structure stability under blasting. The measurement method of dynamic fracture toughness under blasting loads is less addressed until now. In order to study the measurement method of DFT and crack propagation behavior under blasting, a new configuration specimen, i.e. single internal cracked circular disc (SICCD) specimen was proposed in this paper. Four types of brittle materials, black sandstone, red sandstone, green sandstone and PMMA were selected to prepare the SICCD specimens. Crack propagation gauges (CPGs) were glued along crack propagation paths to measure crack initiation and propagation time and crack propagation speeds. An oscilloscope and an ultra-dynamic strain amplifier were employed in the blasting tests. X-ray diffraction (XRD) and scanning electron microscope (SEM) were used to analyze the ingredient and microstructure of the brittle materials respectively. Finite element code ABAQUS was applied in the calculation of dynamic stress intensity factors (DSIFs), and the curves of the DSIF versus time were obtained. By using these curves and the breaking time of CPG filaments, the critical DSIFs of mode I crack at initiation and propagation were obtained. The results show that the measurement method of the critical DSIF of brittle materials under blasting is feasible and applicable. During crack propagations, the crack propagation speed and dynamic energy release rate are not constant. Comparing the test results of these four brittle materials, the initiation toughness and crack propagation speed increase with the elastic modulus of the brittle material under same blasting conditions.

Investigation on the Linear Energy Storage and Dissipation Laws of Rock Materials Under Uniaxial Compression

To investigate the energy evolution characteristics of rock materials under uniaxial compression, the single-cyclic loading–unloading uniaxial compression tests of four rock materials (Qingshan granite, Yellow sandstone, Longdong limestone and Black sandstone) were conducted under five unloading stress levels. The stress–strain curves and failure characteristics of rock specimens under the single-cyclic loading–unloading uniaxial compression tests basically corresponded with those of under uniaxial compression, which indicates that single-cyclic loading–unloading has minimal effects on the variations in the loading–deformation response of rocks. The input energy density, elastic energy density and dissipated energy density of four rocks under five unloading stress levels were calculated using the graphical integration method, and variation characteristics of those three energy density parameters with different unloading stress levels were explored. The results show that all three energy density parameters above increased nonlinearly with increasing unloading stress level as quadratic polynomial functions. Meanwhile, both the elastic and dissipated energy density increased linearly when the input energy density increased, and the linear energy storage and dissipation laws for rock materials were observed. Furthermore, a linear relationship between the dissipated and elastic energy density was also proposed. Using the linear energy storage or dissipation law, the elastic and dissipated energy density at any stress levels can be calculated, and the internal elastic (or dissipated) energy density at peak compressive strength (the peak elastic and dissipated energy density for short) can be obtained. The ratio of the elastic energy density to dissipated energy density with increasing input energy density was investigated using a new method, and the results show that this ratio tends to be constant at the peak compressive strength of rock specimens.

Suggested Methods for Determining Dynamic Fracture Toughness and Numerical Investigation of Cracking Processes under Impacting Loads

Initiation toughness, propagation toughness, and arrest toughness can be considered as threshold values to predict the dynamic behavior of cracks in initiation, propagation, and arrest. For rock materials, initiation toughness has been studied extensively, whereas propagation toughness and arrest toughness have received less attention. In this study, a single cleavage semicircle compression (SCSC) specimen was proposed, and Ya’meng black sandstone was selected to make the SCSC specimens. Impact tests were conducted using the SCSC specimens and a split Hopkinson pressure bar (SHPB) test system. Crack initiation time, propagation time, and propagation speed were measured using crack propagation gauges (CPGs). A high-speed photography (HSP) system was also used to observe the dynamic behavior of crack propagations and arrests. Dynamic stress intensity factors (SIFs) were calculated using ANSYS code, and the crack propagation path was simulated using AUTODYN code. The simulation results generally agree with the test results. Dynamic initiation toughness, propagation toughness, and arrest toughness were determined using an experimental–numerical method. The results show that crack propagation speeds vary during crack propagations, and cracks may arrest for one or more periods during propagation. Dynamic initiation toughness is proportional to the loading rate applied to the specimens, and arrest toughness is less than both initiation toughness and the propagation toughness for the Ya’meng sandstone.

Weight Distribution Characteristics During the Process of Hole Enlargement When Drilling

The weight distribution between the pilot bit and reamer is the key factor to ensure the safety and performance of hole enlargement when drilling (HEWD) assembly when reaming. Based on the finite element method, a real-time model of HEWD assembly was proposed, in which it can simulate the interactions between the components. The characteristics of weight distribution and its effects on the dynamic of HEWD assembly were studied. The results indicate, in homogeneous formation reaming, the optimal rotary speed can improve the utilization of mechanical energy up to 97% and leads to a more reasonable weight distribution. The weight distribution will also be reasonable when the difference in strength between the upper soft and lower hard formation is less than 17 MPa. However, the reamer will be out of drilling when the difference exceeds 37 MPa. In addition, the weight distribution ratio is less than 0.3 when the difference in strength between the upper hard and lower soft formation is less than 32 MPa. But it will exceed 0.5 when the difference exceeds 37 MPa. The ROP keep on increasing 1.6 and 1.4 times when the HEWD assembly reams through the upper Quartzite and lower black sandstone formation, respectively. But the ROP drops by 52 and 11% when it reams through the upper black sandstone and lower Quartzite formation while the torque slightly increases. Based on the trend of ROP and surface torque, engineers can judge the working status of a HEWD assembly and make appropriate adjustments.

The effects of water on the strength of black sandstone in a brittle regime

The mechanical behavior of rock is significantly affected by water. To further study the influence of water on the mechanical behavior of rocks, uniaxial compressive loading experiments were performed on black sandstone specimens (collected at a site in Longchang, Sichuan Province, China) with different water contents. The water absorption was first tested to study the evolution of the water content in the specimens. Then, the mechanical behaviors of the specimens after different durations of soaking in water (from dry to saturation) were tested under uniaxial compression. The results showed that, in the stress-strain curves for the specimens, the initial slope and strength decrease with increasing soaking duration and that the curves are strongly nonlinear in the prepeak region. Furthermore, with increasing water content, the elastic modulus and uniaxial compressive strength decrease, while the Poisson's ratio increases. A comprehensive set of predictive empirical relationships is established between the mechanical properties and the water content/soaking duration for the black sandstone. As the soaking duration increases, the failure pattern gradually changes from tensile to shear-dominated failure. A novel observation is that, for two specimens with the same water content, the strength of a specimen with a homogeneous water distribution is lower than that of a specimen with a non-homogeneous water distribution. The results suggest that the water soaking duration, water distribution and loading-saturation sequence of a rock should all be considered in experiments to gain a deeper understanding of the mechanical responses of rocks with different water contents.

Sorption of long-lived radionuclides onto main types of rocks of the Novaya Zemlya Archipelago

The ability of black carbonaceous siltstones and silt sandstones, lime sandstones, gray limestones, carbon–silicon carbonate schists with pyrite, and other rocks that most widely occur on the Novaya Zemlya Archipelago to sorb 137Cs, 90Sr, 239+240Pu, and 241Am was studied. The distribution coefficients K d (cm3 g–1) are as follows: for 239+240Pu, 2.7 × 103–7.7 × 103; for 241Am, 2.5 × 103–1.8 × 104; and for 137Cs, 1.1 × 102–2.0 × 103. Strontium-85(90) it not noticeably sorbed (within the measurement uncertainty) by any of the rocks studied. 239+240Pu, 241Am, and 137Cs are strongly sorbed onto the rocks studied and are not noticeably desorbed from them with distilled water. The data obtained are required for predicting the migration of long-lived radionuclides generated by nuclear explosion with surface waters from test sites on the Novaya Zemlya Archipelago.

Heavy mineral concentrations in the sandstones of Amij Formation with particular emphasis on the mineral chemistry and petrographic characteristics of monazite, western desert of Iraq

The heavy minerals in the clastic unit of the Lower Jurassic Amij Formation exposed in the western desert of Iraq were studied. The uppermost part of the clastic unit contains thin, placer-like black sandstone horizons that are radioactive and abnormally rich in heavy minerals (0.6–56%), dominated by opaque (65%) and transparent (35%) heavy minerals. The minerals, in the order of decreasing abundance are pseudorutile, goethite, zircon, hematite, magnetite, monazite, rutile, leucoxene, tourmaline, ilmenite, chromite, and few others. Electron probe microanalysis (EPMA), microscopic and autoradiographic observations and analysis showed that the monazite is monazite-(Ce) type with an average composition of (Ce0.39Nd0.16La0.19Pr0.04Sm0.02Gd0.02Eu0.01Y0·04Th0·06U0·01Ca0·05Fe0.01)(P0·98Si0.03)O4.Monazite consists predominantly of REE-oxides (57.93%) and P2O5 (29.31%), with minor amounts of ThO2 (6.60%), Y2O3 (1.92%), UO2 (0.76%), CaO (1.14%), SiO2 (0.69%), and FeOt (0.17%). The dominant compositional substitution operating between REE and P were a mixture of the complex cheralite type substitution ([REE]−2 [Th][Ca]) and the coupled huttonite type substitution ([REE]−1 [P]−1 [Th][Si]). The chondrite-normalized REE distribution patterns of monazite show enrichment in LREE with positive Eu- and Pr-anomalies of 1.46 and 9.13, respectively. The median values of (La/Sm)CN and (La/Nd)CN ratios are 4.35 and 1.97, respectively. Zircon which is the dominant transparent mineral is Hf-rich that is composed of 30.61% SiO2, 57.58% ZrO2, 7.03% HfO2, 2.04% Y2O3, 0.56% ThO2, 0.19% UO2, and 0.19% Al2O3 corresponding to a formula (Zr0.909Hf0.065Th0·004U0·001Y0.031)Σ1.011(Si3·966Al0.028)Σ0.999O4. Rutile and tourmaline form 7% and 4% of the heavy minerals. Ilmenite which is one of the predominant heavy minerals forms 2.5% of the opaques because it is pervasively altered to Ti-Fe oxides. In addition of zircon and monazite, the chemical compositions of most of the other heavy minerals are also given in this study. The expected dominant source of heavy minerals and their host sandstones are most probably the felsic igneous and metamorphic complexes of the Arabian Shield, currently located ∼600 kms to the south of the studied area. The heavy minerals were carried from the source area by northward moving rivers and sorted out by ocean waves as black sand concentrations at the delta mouth along the southern beaches of the Neo-Tethys Ocean under passive margin tectonic setting. This border was apparently bordering the current western part of Iraq during Upper Jurassic period.

Chemical Characterization of a Mineral Deposit of Economic Interest

ABSTRACTChemical and structural characterization of four representative samples of an ore deposit located in the eastern of Hidalgo State was carried out. According with the results, it could be appreciate some areas showing silicified zones with abundant amounts of disseminated pyrites that are part of a rock unit from early Jurassic consisting in inter - bedded black shales and sandstones. Thus, the contents of base metal were greater than 30 ppm Zn and 9 ppm Cu. Chemical analysis of rock gave the following results; 82 ppm of Ba, 1.64 % Wt. Fe, 0.08 % Wt. Ti, 40.8 % Wt. Si, 20 ppm of Ce, 2.2 ppm Co, 30 ppm Cr, 2.7 ppm Cs, 0.9 ppm Er, 2.5 ppm Ga, 1.6 ppm Gd , 1.5 ppm Ge, 9 ppm La, 71 ppm Li, 104 ppm Mn, 10 ppm Nd , 17 ppm Rb, 2 ppm Se, 9 ppm Sr, 10 ppm Ta, 6 ppm Te, 28 ppm V, 9 ppm Y, and 0.7 ppm Yb, among others. Finally, the values found for precious metals, were; Au < 0.02 ppm, Pd <0.05 ppm, Pt <0.05 ppm. It was inferred that the low content of base metals in outcrop studied, are due to the alteration of the black shales. According to these results, we can consider a stratiform – type mineralization of Pb-Zn which could be prospective for SEDEX – Type deposit. By means of XRD, it was possible to identify; pyrite, chalcopyrite, pyrrhotite, and minor amounts of sphalerite and Co -Ni arsenide.

Dinoflagellate cysts from the Szlachtowa Formation (Jurassic) and adjacent deposits (Jurassic-Cretaceous) of the Grajcarek Unit at Szczawnica-Zabaniszcze (Pieniny Klippen Belt, Carpathians, Poland)

The Jurassic–Cretaceous fine-clastic dark deposits of the Grajcarek Unit (Pieniny Klippen Belt, Carpathians, Poland) show lithological similarities, which may lead to erroneous age correlations. Their well-exposed outcrop at Szczawnica-Zabaniszcze allows detailed sampling for their precise age determination. For this purpose, dinoflagellate cysts have been studied from a set of samples collected from the Szlachtowa, Opaleniec, Pieniny Limestone, Kapuśnica and Wronine formations exposed at that site (additional sample from the Pieniny Limestone Formation was studied). The Szlachtowa Formation, a very characteristic lithostratigraphic unit, which consists of black shale and sandstones extremely rich in mica flakes, occasionally referred to literature as the so-called “black flysch”, yielded rich and well-preserved assemblages. Their age is Middle Jurassic, spanning the Aalenian to Lower Bajocian. The exposures of the Szlachtowa Formation at Szczawnica-Zabaniszcze are located close to the outcrops of the Opaleniec Formation. The calcareous spotted shale from this lithostratigraphic unit yielded rich dinoflagellate cyst assemblage characteristic for Late Bathonian (Early Callovian?). The Opaleniec Formation tectonically contacts with very similar non-calcareous spotted shale of the Wronine Formation. Its age, based on infrequent dinoflagellate cysts is Aptian–Albian. Similar, Lower Aptian age was concluded for the Kapuśnica Formation. Late Barremian–earliest Aptian dinoflagellate cysts have been found in the underlying topmost part of the Pieniny Limestone Formation.

57 Fe Mössbauer and X-ray characterisation of sandstones

Sandstones from the Free State province in South Africa have been mined and processed mainly by small scale and artisanal miners in the rural areas. In the present investigation basic fire proof and water absorption tests, X-ray and γ-ray based characterisation techniques were used to study the sandstones. The collected samples were grouped according to their apparent colour in day light conditions and the elemental analysis showed the presence of a high amount of oxygen (>52%) and silicon (>38%) with Mn, Al, Fe and Ca as major elements in proportions related to the colour distribution of the various sandstones. The uniaxial compressive stress was found to be the highest (56 MPa) for the greyish sandstone and the lowest (8 MPa) for the white sandstone sample, also associated with the lowest (Al+Fe)/Si value of 0.082. The humidity test showed that the 6 % water absorption was lower than the recommended ASTM value of 8 %. The sandstone samples were also subjected to various high temperatures to simulate possible fire conditions and it was found that the non alteration of the mineral species might be one of the reasons why the sandstones are regarded as the most refractory amongst the building materials typically used. Mossbauer spectroscopy revealed that iron is present in all the sandstones, mainly as Fe3 + with the black sandstone showing an additional presence of 3 % Fe2 + indicating that a higher iron content coupled to higher silicon content, contributes to an increase in the uniaxial compressive strength.

Earliest microbially mediated pyrite oxidation in ~ 3.4 billion-year-old sediments

Pyrite (FeS2) oxidation in modern sedimentary environments is neither a purely chemical nor purely microbial process, but it is significantly enhanced by the activity of microorganisms that use reduced forms of iron and sulphur in their metabolisms. On the early Earth, where oxygen levels were thought to be <10−5 of the present atmospheric level and chemical oxidants scarce, such biological mediation may have been critical in the redox cycles of iron and sulphur. Here, we show that detrital sedimentary pyrite grains in a ~3.4 billion-year-old sandstone were colonised by microbial communities. The detrital pyrite comes from the basal quartz arenite member of the 3.43–3.35Ga Strelley Pool Formation (SPF) in the East Strelley greenstone belt of the Pilbara Craton, Western Australia. Rock chips and petrographic thin sections of black sandstones occurring on two ridges close to the SPF type locality of Strelley Pool were investigated using optical microscopy, SEM, TEM, laser Raman and NanoSIMS. The detrital pyrite grains exhibit laminated carbonaceous coatings of early Archean age, with localised enrichments of nitrogen that are interpreted as the in situ remains of biofilms growing on these nutrient-rich minerals. Pyrite surfaces contain spherical pits, chains of pits and channels that are morphologically distinct from abiotic alteration features. The pits and channels are widespread, have a clustered distribution typical of microbial colonisation, and are closely comparable to biologically mediated microstructures in the younger rock record and those created by extant Fe- and S-oxidising microbes in the laboratory. They are thus interpreted as trace fossils formed by the attachment of bacteria to the pyrite surfaces. A nano-layer and discreet nano-grains of secondary mineral precipitates, namely Fe-oxides belonging to the magnetite-maghaemite group, attest to pyrite oxidation. These are intimately associated with the biofilms and trace fossils, and are interpreted to represent the fossilised mineral products of biologically mediated pyrite oxidation. These data extend the geological range of microbes capable of metabolising reduced Fe and/or S compounds back to the early Archean and indicate that pyrite-rich sedimentary rocks provide promising targets in the search for extraterrestrial life.

Litostratigrafi dan sedimentasi Formasi Kebo dan Formasi Butak di Pegunungan Baturagung, Jawa Tengah Bagian Selatan

http://dx.doi.org/10.17014/ijog.vol3no4.20081Lithologically, the Nampurejo Pillow Lava, Kebo and Butak Formations, which are dominated by volcanic rocks, spread west - eastly, along the northern foot of the Baturagung Mountains. The Nampurejo Pillow Lava, which has an Early Oligocene age, is overlain by the Late Oligocene - Early Miocene Kebo and Butak Formations successively. The Nampurejo Pillow Lava consists of basaltic pillow-lavas showing pillow structures and they are intercalated by black sandstones. The Kebo Formation comprises alternating sandstone and pebbly sandstone with intercalations of siltstone, claystone, tuff, and shale. On the other hand, the Butak For- mation is composed of polymic breccia with intercalations of sandstone, pebbly sandstone, claystone, and siltstone/shale. The three units were deposited in a deep – shallow marine basin, which was filled by volcanic prod- ucts. Compared to the lower part of the Kebo Formation, volcanic activities during the deposition of the upper part of the Kebo Formation and the Butak Formation were more active.