Amorphous Opal Research Articles

Chemical status of zinc in plant phytoliths: Impact of burning and (paleo)environmental implications

Phytoliths are microscopic structures made of amorphous opal (opal-A), an amorphous hydrated silica, dispersed within plant tissues and persisting after the decay of the plant. Silicon is known to alleviate metal toxicity in plants, but the role of phytoliths in metal sequestration and detoxification is unclear. Dry ashing, the most common protocol for phytolith extraction, was previously shown to lead to sequestration of metals by the phytoliths; however, the mechanisms of this process remained elusive. The purpose of this study was to evaluate whether the association between metals and phytoliths results from dry ashing or pre-exists in plant tissues. Thus, we compared phytoliths extracted by dry ashing at 700 °C and plant leaves before and after dry ashing. A combination of ICP-MS, XRD, SEM-EDX and Zn-K-edge EXAFS spectroscopy was used to assess elemental concentrations, morphology and crystallography of silica, and chemical status of Zn. Results demonstrated a phase transition from amorphous opal (opal-A) to opal-CT and α-cristobalite, and the sequestration of metal in phytoliths during dry ashing. For Zn, Mn and Pb, a linear relationship was found between the concentration in phytoliths and in leaves. In the phytoliths, Zn was sequestered in silica in tetrahedral configuration. We hypothesize that this association results form a solid-state reaction during ashing, involving a redistribution of Zn from the organic material to the silica, possibly promoted by the release of structural water from amorphous opal throughout the heating procedure. This study improves our understanding of the impact of high temperature treatments on plant biomass and phytoliths. It suggests that Zn toxicity alleviation in plants by silicon does not rely on its sequestration by phytoliths. In natural settings, wild fire events and biomass burning may lead to metal sequestration in low-soluble form, which should be considered in modeling of biogeochemical cycles and in paleoenvironmental studies.

Aeolian driven oxidant and hydrogen generation in Martian regolith: The role of mineralogy and abrasion temperature

The surface of Mars is a dynamic, cold environment where aeolian abrasion leads to the fracturing of silicate minerals which can produce oxidants upon exposure to water. Here we report results of a series of laboratory experiments where the abrasion of sand sized (125 – 300 μm) quartz, labradorite, forsterite and opal were conducted under a simulated Martian atmosphere at a range of temperatures common to Mars' surface (193 to 273 K). Our results suggest that abrasion rates are controlled by temperature; an observation that may have potential for providing insight into Martian paleo-temperatures. On the addition of water, detectable H2O2 was generated in all abraded experiments with crystalline quartz, labradorite and forsterite, but not amorphous opal – supporting previous inferences that mineral crystal structure plays a role in oxidant production. Dissolved Fe concentrations also indicated a strong additional control on net H2O2 production by Fenton reactions. Detectable H2 was similarly measured in abraded experiments with crystalline minerals and not for amorphous opal. Labradorite and forsterite generated minimal H2 and only in more abraded samples, likely due to the reaction of Si• with water. In quartz experiments H2 was only present in samples where a black magnetic trace mineral was also present, and where H2O2 concentrations had been reduced to close to detection. In the quartz samples we infer a mechanism of H2 generation via the previously proposed model of spinel-surface-promoted-electron transfer to water. The presence of H2O2 may exert an additional control on net H2 production rates either directly (via reaction of H2 with OH• and H2O2) or indirectly (by the oxidation of H2 generating sites on mineral surfaces). Overall, our data supports previous inferences that aeolian abrasion can produce additional oxidants within the Martian regolith that can increase the degradation of organic molecules. We further suggest that the apparent control of H2O2 concentrations on net H2 generation in our experiments may help explain some previous apparently contradictory evidence for mineral-water H2 generation at low temperatures.

The Occurrence and Genesis of Silicified Coals in Tanjung Redeb, Embalut, and Loa Kulu, East Kalimantan, Indonesia

DOI:10.17014/ijog.9.1.27-43The occurrence and genesis of silicified coals in Tanjung Redeb, Embalut, and Loa Kulu have been described and reconstructed by means of petrography and geochemistry. The silicified coals are black, hard, and contain dispersed fine-grained silica minerals. Silica (SiO2) contents vary from 58 to 76%, and LOI (mostly represents organic matter) ranges from 23 to 40% of the bulk silicified coal samples based on the ICP-INAA analysis. Microscopic examination shows the presence of quartz, chalcedony, and amorphous opal. Silicification occurs both as layers and nodules within the coal seams. The layer type is usually thin layers (<25 cm) near the top and bottom of the seams, and they can spread horizontally up to 3 m. Nodules occur randomly in the coal seams having a diameter from several centimeters to 1 m. Further microscopic investigation shows that preserved cellular structure is more commonly observed in the layered silicified coal samples. This may suggest that the layered silicification occurred relatively early in the coal seam diagenesis, while the nodule silicification took place during later diagenesis. The coexistence of silica polymorphs in some samples may indicate the presence of multiple silicification stages during coalification.

Mineralogical and Spectral Characteristics of Purple Boulder Opal from Australia

Purple boulder opal is a relatively new gem variety in the market. At present, the research that related to purple boulder opal is limited to the mineral components and the genesis of its colour. There is no in-depth analysis on the chemical compositions, spectral characteristics and causes of play-of-colour effect. The mineralogical and spectral characteristics of purple boulder opal from Australia are comprehensively studied in this paper. The results showed that the main mineral component of purple boulder opal is amorphous opal, and it contains a small amount of α-cristobalite. The results of EMPA analysis showed that the main component of opal matrix is SiO2, which is more than 96%. The UV-Vis spectrum showed that there are absorption peaks of strong absorption intensity at 407 nm and 480 nm, and totally absorbed above 522 nm. Based on the results of LA-ICP-MS and UV-Vis absorption spectra, the saturation of the purple colour of purple boulder opal is positively correlated with Fe content. SEM results showed that the diameter of SiO2 spheres in the play-of-colour effect area of the purple boulder opal is about 182-240 nm, and the octahedral space around the sphere is about 63-150 nm. Only violet light with wavelength of 400-450 nm can pass through the sphere to form the pure purple opal.

The effect of sediment lithology on oxygen isotope composition and phase transformation of marine biogenic opal

Biogenic amorphous opal (biogenic opal-A; SiO2·nH2O) in the form of diatoms, radiolaria, and sponge spicules, matures to more stable thermodynamic silica phases of opal-CT and microquartz chert upon accumulation and burial in deep sea sediments. Both temperature and lithology of the sediments (i.e., amounts of clay relative to calcium carbonate) influence the time and depth of maturation. Given that δ18O of silica reflects the δ18O of the water and temperature in which silica forms, it is possible to trace pathways of maturation by measuring δ18O of the different silica phases. We measured δ18O of biogenic opal-A, opal-CT, and microquartz chert from carbonate rich sediments to characterize the effects of lithology on the silica maturation. We have identified a set of cores with calcium carbonate rich and clay poor lithologies from ODP sites 1049–1053 in which these phases and phase transformations occur. The mineralogical phases are characterized by X-ray diffraction (XRD) and the purity of the samples with Scanning Electron Microscope/Energy Dispersive X-ray spectroscopy (SEM/EDS). The δ18O of biogenic opal-A is ~41 to 45‰, the δ18O of opal-A' (a transition silica phase from opal-A to opal-CT) is ~38 to 43‰, the δ18O of opal-CT is ~37 to 45‰, and δ18O of two microquartz cherts is ~38‰. δ18O of opal-A' and opal-CT indicate maturation at low temperature and with shallow depths in the sediments out of silica equilibrium with local porewater δ18O and temperature. In contrast, δ18O of two microquartz cherts reflect formation in isotope equilibrium with local porewater δ18O and temperature. Such scenarios of silica phase transformations appear to be unique to geographical locations with a low geothermal gradient and where sediments are composed of high calcium carbonate to clay ratio, either leading to a shallow and young maturation of biogenic opal-A or a delay by as much as several hundreds of meters and tens of millions of years. Heavy δ18O of porcellanites (siliceous sedimentary rocks with opal-CT mineralogy) and potentially also cherts through geologic time must reflect formation at shallow depths within the sediments, a maturation scenario influenced by simultaneous depositions of marine calcium carbonates.

ФАЗОВЫЙ СОСТАВ КРЕМНЕЗЕМА КРАСНОСЛОБОДСКОГО МЕСТОРОЖДЕНИЯ ТАМБОВСКОЙ ОБЛАСТИ

The study of the chemical and phase compositions of tripoli from Krasnoslobodskoye deposit in the Tambov region was carried out using modern equipment. Using the X-ray fluorescence analysis, the main elemental composition of the rock under study was established, determined mainly by the compounds of Si, Al, Fe, K, Mg, Ca, Ti, P, S, Na. It is shown that the proportion of silicon dioxide in the sample under study exceeds its content in tripoli from many Russian deposits and neighboring countries. The X-ray phase analysis with subsequent interpretation of the diffraction pattern showed that tripoli contains minerals of amorphous opal, microcrystalline - and -cristobalites as structural forms of silica, as well as small amounts of quartz, illite and montmorillonite impurities.

The Geochemistry of Rare Earth Elements in Natural Waters and Secondary Mineral Sediments of Thermal Fields of Kamchatka

The distribution of rare-earth elements in surface and underground waters, water-bearing rocks, and secondary mineral neoformations of thermal waters of the Paratunka and Vilyuchinsk hydrothermal systems of Kamchatka has been studied. It is revealed that their content, distribution, and fractionation differ with respect to the geochemical type of water and the pH–Eh of the aquifer. All waters are characterized by the domination of the transportation of dissolved rare earth elements, which exceeds their transportation with colloids by 1000 times. It has been shown that neoformations of carbonates, sulfates, and silicates, including calcite, gypsum, and amorphous opal, are widespread in the discharge areas of the studied thermal waters. The redistribution intensity of rare earth elements in the rock–water–secondary deposits system is very low in waters of the sulfate type, where the elements are bonded to sulfates. At the same time, the precipitation of rare earth elements as newly formed mineral phases is most intensive in water with carbonate complexes.

Geochemistry of rees in the natural waters and the secondary phases from thermal fields of Kamchatka

A study of the distribution of rare-earth elements in surface and underground waters, waterbearing rocks, as well as secondary mineral formed by the thermal waters of the Paratunka and Viluchinsky hydrothermal systems of Kamchatka was carried out. It was found that the REE content, their distribution and fractionation differ depending on the geochemical type of water, as well as the pH–Eh of the aquifer. All waters are characterized by the predominance of the dissolved form of the REE water transfer, which exceeds the transfer of REE with colloids by a factor of a thousand. It has been shown that in the discharge areas of the studied thermal waters carbonates, sulfates and silicates, including calcite, gypsum, and amorphous opal, are widespread. The intensity of the REE redistribution process in the rock-water-secondary deposits system is very weak in waters where REEs are associated with sulfates. At the same time, REEs bound in water with carbonate complexes most actively accumulate in newly formed mineral phases.

Diagenesis of Xing’anling Oil Reservoir in Sudeerte Area

In order to explore the types of diagenesis in Sudeerte area, through core and thin slice identification, it is clear that the Xing’anling oil layer is mainly affected by compaction, cementation, andrecrystallization. The compaction is mainly early compaction, late pressure dissolution is not obvious; cementation presents two forms of crystalline quartz and amorphous opal; recrystallization is manifested as the conversion process of opal t-ochalcedony quartz.

Structural analysis and transformation of biosilica during lignocellulose fractionation of rice straw

This study focused on the interfacial combination of silica and lignin-polysaccharides during lignocellulose fractionation of rice straw using scanning electric microscopy (SEM), x-ray photoelectron spectroscopy (XPS), and x-ray diffraction (XRD) techniques. Silica was deposited on the exterior surface of rice straw in the form of Si(OR)x, SiOSi, SiOx, and SiC respectively, while not in the form of elemental Si. After delignification and alkalization, silica was deposited on the surface of polysaccharide with a more regular pattern. It is due to that silica reacted with polysaccharide to form SiOx and Si(OR)x during the decomposition of lignin and hemicellulose, leading to the remained framework structure of crystalline cellulose. After the decomposition of polysaccharide, silica was embedded into lignin fragments, and shown as an amorphous opal. For silica-lignin, SiOSi was gradually transferred to Si(OR)x instead of SiC. The capacities of the fractionated components to bind or store silica were in order of lignin > polysaccharides > lipids. Moreover, the fractionated components of rice straw were associated with the different ultrastructural forms of the silica.

Experimental constraints on phreatic eruption processes at Whakaari (White Island volcano)

Vigorous hydrothermal activity interspersed by sequences of phreatic and phreatomagmatic eruptions occur at Whakaari (White Island volcano), New Zealand. Here, we investigate the influence of sample type (hydrothermally altered cemented ash tuffs and unconsolidated ash/lapilli) and fragmentation mechanism (steam flashing versus gas expansion) on fragmentation and ejection velocities as well as on particle-size and shape. Our rapid decompression experiments show that fragmentation and ejection speeds of two ash tuffs, cemented by alunite and amorphous opal, increase with increasing porosity and that both are significantly enhanced in the presence of steam flashing. Ejection speeds of unconsolidated samples are higher than ejection speeds of cemented tuffs, as less energy is consumed by fragmentation. Fragmentation dominated by steam flashing results in increased fragmentation energy and a higher proportion of fine particles. Particle shape analyses before and after fragmentation reveal that both steam flashing and pure gas expansion produce platy or bladed particles from fracturing parallel to the decompression front. Neither fragmentation mechanisms nor sample type show a significant influence on the shape. Our results emphasize that, under identical pressure and temperature conditions, eruptions accompanied by the process of liquid water flashing to steam are significantly more violent than those driven simply by gas expansion. Therefore, phase changes during decompression and cementation are both important considerations for hazard assessment and modeling of eruptions in hydrothermally active environments.

Generation of hydrothermal Fe‐Si oxyhydroxide deposit on the Southwest Indian Ridge and its implication for the origin of ancient banded iron formations

AbstractModern hydrothermal Fe‐Si oxyhydroxide deposits are now known to be analogues to ancient siliceous iron formations. In this study, samples of Fe‐Si oxyhydroxide deposits were collected from hydrothermal field on the Southwest Indian Ridge. An investigation of mineralization in these deposits was carried out based on a series of mineralogical and morphological methods. X‐ray diffraction and selected area electron diffraction analysis show that amorphous opal and poorly crystalline ferrihydrite are the major minerals. Furthermore, some typical filament structures detected by scanning electronic microscopy examinations, probably indicating the presence of Fe‐oxidizing bacteria (FeOB), are pervasive with the main constituents being Fe, Si, P, and C. We thus believe that chemolithoautotrophic FeOB play a significant role in the formation of Fe oxyhydroxide which can effectively oxidize reduced Fe(II) sourced from hydrothermal fluids. Precipitation of amorphous silica, in contrast, is only a passive process with the Fe oxyhydroxide acting as a template. The distinct microlaminae structure alternating between the Fe‐rich and Si‐rich bands was observed in our samples for the first time in modern seafloor hydrothermal systems. We propose that its formation was due to the episodic temperature variation of the hydrothermal fluid which controls the biogenic Fe oxyhydroxide formation and passive precipitation of silica in this system. Our results might provide a clue for the formation mechanism of ancient banded iron formations.

Investigation on the gemological, physical and compositional properties of some opals from Slovakia ("Hungarian" opals)

The physical and compositional properties of some “Hungarian” opals have been investigated through several methodologies such as optical analysis, specific gravity, refractive indices, AvaSpec spectroscopy, XRPD, IR and Raman spectroscopy, LA-ICP-MS. The “Hungarian” opal deposits, now in Slovakia, represented the largest and most significant gem opal deposits in Europe from the Roman Times to XIXth century. The analysed opals include brown semiopals, white opals and hyalite. In general, chromophore elements like V, Cr, Cu, Ti, Co are present in very low concentrations and do not influence the physical properties of the opals. Elements like Na, Mg, Al, K, Ca, Sc, Cr and Fe have been detected, in variable amounts, in all the investigated samples. The semiopals can be used as ornamental and semiprecious material, are mostly made up by CT opal close to pure tridymite and lower amounts of goethite, show high trace elements contents while the REE pattern show a clear Eu negative anomaly. These opals probably formed during an hydrothermal, or late magmatic stage. The white opals are made up by amorphous opal (A), show appreciable play of color and iridescence and are surely the most relevant for gemological purposes. The most pure white opals show low contents of trace elements and a rather homogeneous composition, that can be used as geographical marker. Conversely some opals affected by contamination from host rocks show much higher trace element contents. White opals probably formed during a low temperature tectonic event. The samples of hyalite are yellowish, opaque and greasy. They are made up by CT opal show relevant content of B and As while the other trace element are quite low, and are probably of volcanic origin.

Growth model of a hydrothermal low-temperature Si-rich chimney: Example from the CDE hydrothermal field, Lau Basin

The CDE hydrothermal field was first discovered during a Chinese cruise to the East Lau Basin Spreading Centre in 2007. Apart from significant amounts of loose Fe-Si-Mn (oxyhydr) oxide (referred to as oxide below) precipitates, a small Si-rich oxide chimney was also recovered on this cruise. In this study, we report on the mineralogical and geochemical analyses of this chimney and a model for its growth that has been developed. Based on the mineralogy and O isotope results, the chimney walls can be divided into four growth generations (layers) from the inner to the outer layers: amorphous opal and barite layer (precipitation temperature 68.5°C based on oxygen isotope determinations), a rod-like amorphous layer (precipitation temperature 39.6°C), a filamentous Fe-Si oxide layer, and an outer Fe-Mn oxide layer. Investigations based on SEM and EDS showed that neutrophilic Fe-oxidizing bacteria play an important role in the formation of this chimney, particularly in the outer two generations. In the first stage, the metabolic activity of the microbes results in the pervasive precipitation of the filamentous Fe-rich oxides inside a ring formed by some amorphous opal and barite; therefore, a loose porous layer forms. In the second stage, amorphous opal then precipitates inside this wall as a result of conductive cooling and gradually controls the mixing between the hydrothermal fluids and ambient seawaters. In the third stage, barite and some amorphous opal form from the higher temperature fluids at the summit of the chimney growth history. In the last stage, the chimney wall becomes thicker and denser and the exchange of hydrothermal fluids and seawater ceases. As a result, a Fe-Mn oxide layer precipitates onto the outer surface of the chimney wall as neutrophilic Fe-oxidizing bacteria reoccupy the surface of the chimney. This mineral sequence and the resultant growth generations are confirmed by the chemical characteristics of the chimney wall. Sr isotopes extracted from the Fe oxides of the four-generation wall generally show a decreasing trend of the 87Sr/86Sr ratios from the second layer to the inner layer (from 0.707008 to 0.705877) except for the outer layer (0.706502). The Sr isotope and chondrite normalized REE patterns of the corresponding bulk samples from the chimney wall also display a similar trend. Our study shows that the biogenic filament network plays a key role in the formation of the chimney in contrast to previous growth models of higher temperature chimneys, which often ignore the influence of biogenic factors.

Quantification and spatial distribution of dose rate relevant elements in silex used for luminescence dating

Thermoluminescence (TL) is routinely used to date heated lithic artefacts which mostly consist of silex (a mixture of amorphous opal and microcrystalline chalcedony). Analytical investigations of bulk samples confirmed that these materials contain considerable concentrations of radioactive elements, generating an internal dose rate contribution. Common dosimetric models assume the latter to be homogeneous throughout the sample. If this assumption would prove invalid, this will result in systematic errors in the calculated age, especially in the course of so called ‘hot spots’ of α-emitters (and associated local changes in α-sensitivity) and the dose response characteristics of α-radiation. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses of 22 silex samples are presented here, quantifying element concentrations at several tens analytical spots per sample. Along with radioactive elements (K, Rb, U, Th), another 21 major, minor and trace elements were measured in order to allow characterization of the impurities present in most of the samples. The dataset provides a detailed picture of the spatial distribution of radionuclides and hence of the uniformity of the internal α- and β-dose rate. It is shown that the silex itself mostly contains low amounts of K (<0.1 wt.%), U (<1.0 μg g−1) and Th (<0.4 μg g−1), and dosimetrically negligible Rb concentrations. Systematically higher concentrations are obtained by ICP-MS measurements of the bulk samples. This matches with the finding that impurities (veins, inclusions) often yield significantly elevated radionuclide concentrations, up to two orders of magnitude higher than the silex values. These veins and inclusions, for example Ca or Mg carbonates and Fe–Mn-oxy-hydroxides, lead to steep gradients mainly in the internal α-radiation field. Alternative approaches are required to account for the non-uniform internal dose rate and improve the reliability of TL dates of problematic samples.

Detrital grain size distribution of the surface sediments and sedimentary processes in Lake Kizaki, central Japan

This study presents that the grain-size distribution obtained by the hydrometer method is affected by diatom frustules in bulk sediments finer than 6 phi in median size. This fact implies that detrital grains need to be extracted from bulk muddy sediments to clarify sedimentary processes of detrital particles. This paper reveals that detrital grain size distribution of the surface sediments and sedimentary processes in Lake Kizaki, central Japan. Detrital grains were extracted by removing diatom frustules composed of amorphous opal, and were measured by laser grain size analyzer. Surface sediments can be divided into four groups. The Group I and II sediments are classified into clayey silt, and distribute in the offshore zone. The Group III sediments are of sandy silt or sand-silt-clay, and form transitional zone between the offshore and following nearshore. The Group IV sediments are of silty sand or sand, and their distributions almost correspond to nearshore zone. Boundaries between the Group III and Group IV cross the counter line toward offshore at the river mouth, implying that flood event plays an important role of sedimentation in Lake Kizaki. Sandy and gravelly sediments of the nearshore zone can be regarded as residual sediments by wave action. Sediments composed of transitional zone transported by waves and spread from the river mouth during flooding event and normal periods transported toward offshore, and then settle down quickly according to Stokes’ law. Moreover, fine sediments composed of the offshore zone remained suspended and appeared to be controlled by surface current.

Formation of Fe–Mn–Si oxide and nontronite deposits in hydrothermal fields on the Valu Fa Ridge, Lau Basin

Hydrothermal Fe–Mn–Si oxides and nontronite are pervasive in the Hine Hina, Vai Lili and Mariner hydrothermal fields along the central Valu Fa Ridge, Lau Basin. Morphometric and mineralogical analyses reveal that the iron-rich filaments are the most important constituents of these Fe–Mn–Si oxide deposits. Both the morphologies and chemical composition of the filaments indicate that neutrophilic Fe-oxidizing bacteria have played a key role in the formation of these deposits. A key process of the formation of these deposits is the creation of a complicated filamentous network in which a series of metabolic activities and passive sorption and nucleation processes occur. The precipitation of dissolved Si in unsaturated and saturated states leads to a “two-generation” growth model in the hydrothermal vents. The precipitation of amorphous opal occurs in a relatively narrow temperature range (41.1–42.9 °C) based on oxygen isotope analyses, indicating a fast precipitation rate of opal-A when conductive cooling of the hydrothermal fluid occurrs. Patchy nontronite in the Mariner fields is a product of the direct precipitation from hydrothermal fluids at a temperature of ∼87.9 °C, whereas the scattered nontronite at the Hine Hina field is the product of the replacement of hydrothermal Fe–Si oxides at a temperature of ∼46.2 °C.

The Gümüldür Fire Opal: Mode of Occurrence and Mineralogical Aspects

Five types of silica polymorphs have been identified in dacitic volcanics from the Gümüldür region of western Anatolia, Turkey. Two of them are micro-quartz and disordered cristobalite (opal-C) that occur within the groundmass of the dacitic volcanic rocks. The others are pore-filling opal nodules including mainly massy opal-CT (opal-CTM) and rarely lussatite (opal-CTL) and amorphous opal (opal-A). Red and orange opal nodules are very similar to the gemologically well-known fire opals. Results of structural, chemical and thermal studies of the Gümüldür opals reveal their origin and mode of occurrence. Opal-CTM has gel-like or nano-grain texture and opal-CTL has a fibrous, chalcedony-like texture while opal-C in the glassy groundmass of the rock shows a lepidospheric texture. Dissolved silicon was possibly precipitated directly from a Si-rich solution to form opal at lower temperatures and pressures. Our detailed mineralogical examinations show that the colours of the red and orange body coloured massy opals seem to be related to increasing iron concentration. The Gümüldür red and orange fire opals have an importance as gemstones. They could be cut as cabochon with medium dome as ovals rounds. The polished surfaces of the opals display a resinous to sub-vitreous luster, with fewer cracks and fractures than the well-known fire opals of western Turkey.

Microstructure investigations of Kamchatka geyserites

This paper discusses the results of examining solid geyserite specimens from two present-day Kamchatkan hydrothermal systems: from geysers of the Academy of Sciences and from the Eastern Thermal Field in the caldera of the Uzon Volcano. These studies were performed using methods of IR spectroscopy, XR spectral analysis, and electron microscopy. Three types of geysers were discriminated: underwater, subaerial, and those that originated under the ground surface. All geyserite varieties consist of amorphous opal, and differences between them are caused by the shape, dimensions, and distribution character of silica aggregates. Most examined geyserites enclose silicified microfossil remnants.

Lateral Diagenesis in Monterey Shale, Santa Barbara Coast, California: ABSTRACT

Lateral differences in Monterey strata along the Santa Barbara coast indicate increased diagenesis toward the west. Westward, silica alters from biogenic amorphous opal (in diatom frustules) to diagenetic opal-CT, and then to diagenetic quartz. Continuous exposure for 50 km, simple homoclinal structure, paleogeographic setting, and detailed analysis of 14 stratigraphic sections together show that sediments were laterally equivalent, as originally deposited. Distinctive stratigraphic differences in sediment composition, informally divided into five members, are also laterally age-constant. Because silica phases differ in rocks of the same age, same depositional environment, and with identical bulk chemical composition, the differences must reflect post-depositional conditions. Overburden thicknesses and thermal changes in organic matter indicate that diagenesis increased westward owing to greater burial temperature. Study of sample sets taken laterally shows that both silica phase changes occurred by rapid solution-precipitation accompanied by significant compaction, and by little movement of silica between beds. Distinctive field characteristics (hardness, brittleness, bulk density, and luster) changed mainly during opal-CT formation. Although these changes affected rocks of nearly all compositions, details varied--even in carbonate-rich rocks--with the proportion of detrital material to biogenic or diagenetic silica. As this proportion increased, opal-CT formation was retarded, accompanying compaction decreased, ordering of opal-CT increased, and quartz formation was promoted. Nearly identical timing relation in associated calcareous, dolomitic, and carbonate-free rocks show that silica diagenesis was unaffected by carbonate except in rocks containing at least 10 times more silica than detrital material. End_of_Article - Last_Page 473------------