Siliceous Scales Research Articles

Nanometer-sized, divalent-Mn, hydrous silicate domains in geothermal brine precipitates

X-ray diffraction, infrared spectroscopy, and X-ray absorption (XANES and EXAFS) spectroscopy were combined to characterize poorly crystalline Mn-rich silicate scale deposited from brine at a geothermal field in Indonesia. The precipitate has a vitreous pink-amber appearance, a nearly pure SiO 2 -MnO-H 2 O chemical composition, and a bulk Mn/Si atomic ratio of 0.63. X-ray microfluorescence indicated that the sample consists of Mn-free and Mn-containing silica domains, whose Mn/Si ratio (1.2 ′ 0.4) ranges between those of 2:1 and 1:1 phyllosilicates. The XRD pattern is characterized by a broad scattering band spanning from ∼6 to ∼2.5 A with weak modulations at 5.3, 3.5, and 2.72 A, and a faint band at 1.605 A. A surrogate was synthesized in an O 2 -free atmosphere by mixing and aging at 150 °C an MnCl 2 and a sodium meta-silicate solution for 3 hours. Its XRD pattern was similar to the scale sample, but the two reflections at 2.72 and 1.605 A were enhanced and the former was asymmetrical as in randomly stacked layered compounds. Diffraction data are consistent with a mixture of amorphous silica and a poorly ordered manganoan sheet silicate with a domain size of 40-50 A for the scale and 50-60 A for the surrogate material. The divalent oxidation state of manganese was confirmed by XANES spectroscopy, and the presumed existence of trioctahedral [Mn 2 + ] 3 (O,OH) clusters having a clay-like local structure was corroborated by the existence of a weak vibration band at 600-650 cm - 1 in infrared spectra. The connectivity of Mn octahedra was examined using EXAFS spectroscopy by comparing the local structure of Mn in the scale sample and in a large series of Mn compounds having different kinds of Mn octahedra and Si tetrahedra linkages. The manganoan scale has a polyhedral local structure resembling that of sheet silicates, in which metal octahedra are joined along edges and share corners with ditrigonal SiO 4 rings. Similar short-range ordering has been described in Fe- and Al- silicate scales, but the Mn-silicate scale has a lower domain size owing to the greater misfit between the lateral dimensions of the Mn and Si sheets. The strain induced by the shrinkage of the Mn sheet to fit the Si sheet(s) is alleviated by the nanometer size of the two-dimensional hydrous silicate domains.

Biogenic Saponite From an Active Submarine Hot Spring, Iceland

AbstractA study of the mineralogy, chemical composition and structure of poorly-crystalline saponite precipitated from a submarine hot spring in Eyjafjordur, northern Iceland is reported. Special emphasis was placed on the microstructures of the minerals and a possible connection with biological activity during their precipitation. The microstructures of the minerals were found to be very similar to specific clay minerals precipitated from geothermal vents in oceanic rift zones. The composition of the minerals was, however, found to be similar to magnesium silicate scales formed in geothermal installations in Iceland where geothermal waters were mixed with cold fresh waters. High contents of organic substances were found in the clay mineral samples as compared to geothermal precipitates from other localities. Microstructural features of the layer silicates in one of the samples suggest that a gelatinous substance was a precursor of the saponite clay. The organic matter content appears to be greater when the precipitates are more crystalline.

Poly(phenylenesulfide)-based coatings for carbon steel heat exchanger tubes in geothermal environments

To inhibit corrosion and fouling by calcium silicate and silica scales of the carbon steel heat exchanger tubes in geothermal power plants operating at brine temperature of 160°C, their internal surfaces were lined with high-temperature performance poly(phenylenesulfide) (PPS)-based coating systems. The systems included the PPS containing polytetrafluoroethylene (PTFE) as an anti-oxidant additive, silicon carbide (SiC) as a thermally conductive filler, and aluminum oxide-rich calcium aluminate (ACA) as an abrasive wear resistant filler. Then, the lined tubes underwent an eleven-month-long field exposure at the site of power plant. The results from the post-test analyses of the exposed liners revealed that these PPS coating systems had an excellent thermal stability, and satisfyingly withstood this brine temperature, and also greatly resisted the permeation of brine, demonstrating that they adequately protect the tubes against corrosion in a wet, harsh geothermal environment. Furthermore, modifying the surfaces of the PPS top layer with PTFE, retarded the hydrothermal oxidation of the liner, thereby reducing significantly the rate of scale deposition and creating surfaces unsusceptible to reactions with scales. Thus, all the scales deposited on the liner's surfaces were readily scoured off by hydroblasting at a very low pressure. By contrast, although the stainless steel tubes had a great protection against corrosion, the formation of passive oxide layers at their outermost surface sites was detrimental in that they became more susceptible to scale deposition and developed strong adherence to the scales. As a result, the high-pressure hydroblasting was required to remove this scale adhering to the tubes' surfaces.

A floristic and biogeographic survey of the Synurophyceae from Southeastern Australia

The Synurophyceae are freshwater, golden‐brown flagellates characterized by a cell covering of overlapping siliceous scales. Species‐level identification usually requires electron microscopy of the scales, and numerous studies have demonstrated that most species of Synurophyceae have a worldwide distribution. Although algal endemism is suggested as relatively common in Australia, floristic investigations of the Synurophyceae in Australia are few in number, and southeastern Australia, in particular, is poorly represented in these studies. This study examines the Synurophyceae in south central Victoria, northeastern New South Wales, and southern, coastal Queensland in Australia. The sampled habitats include mountainous and boggy areas, temperate and subtropical rainforests, as well as pools, small ponds, lakes and reservoirs. Biogeographic and taxonomic information is emphasized. Eighty‐six phytoplankton collections were made during June and July of 1996. Of these, 43 collections contain Synurophyceae. 18 Mallomonas taxa, 7 Synura taxa, Chrysodidymus synumideus, Tessellaria volvocina, and 3 unknown scale types were observed. Five taxa are reported for the first time in Australia and one is reported for the first time since its initial description in western Australia.

Apparent Global Ubiquity of Species in the Protist Genus Paraphysomonas

Evidence is presented for the ubiquity of protist species. Using the example of protists that leave traces (siliceous scales) of their recent population growth, we show that most - perhaps all species in the genus Paraphysomonas, are ubiquitous. Of the species recorded in surveys carried out worldwide, we have identified 78% of their number in 0.1 cm2 of sediment collected from a freshwater pond (total area 10(8) cm2) in England. Moreover, the pond appears to act like a microcosm of aquatic environments in general, for species that are globally rare or abundant, are likewise rare or abundant in the pond. We assume that the rate of neutral migration to the pond is greatest for the globally abundant species. As these species are probably capable of growth in a broad range of conditions, they will more frequently encounter the environment they require for population growth. Thus globally abundant species are also locally abundant in the pond - a pattern that will be amplified by periodic cyst production. Ubiquitous dispersal is probably driven by very high absolute abundance of individuals, and the water column of the pond was estimated to support >10(14) Paraphysomonas individuals. Ubiquity will dampen rates of speciation, and the evidence presented here indicates that global species richness of Paraphysomonas is indeed modest - perhaps close to what is already known.

Symbiomonas scintillans gen. et sp. nov. and Picophagus flagellatus gen. et sp. nov. (Heterokonta): Two New Heterotrophic Flagellates of Picoplanktonic Size

Two new oceanic free-living heterotrophic Heterokonta species with picoplanktonic size (< 2 microm) are described. Symbiomonas scintillans Guillou et Chrétiennot-Dinet gen. et sp. nov. was isolated from samples collected both in the equatorial Pacific Ocean and the Mediterranean Sea. This new species possesses ultrastructural features of the bicosoecids, such as the absence of a helix in the flagellar transitional region (found in Cafeteria roenbergensis and in a few bicosoecids), and a flagellar root system very similar to that of C. roenbergensis, Acronema sippewissettensis, and Bicosoeca maris. This new species is characterized by a single flagellum with mastigonemes, the presence of endosymbiotic bacteria located close to the nucleus, the absence of a lorica and a R3 root composed of a 6+3+x microtubular structure. Phylogenetical analyses of nuclear-encoded SSU rDNA gene sequences indicate that this species is close to the bicosoecids C. roenbergensis and Siluania monomastiga. Picophagus flagellatus Guillou et Chrétiennot-Dinet gen. et sp. nov. was collected in the equatorial Pacific Ocean. Cells are naked and possess two flagella. This species is characterized by the lack of a transitional helix and lateral filaments on the flagellar tubular hairs, the absence of siliceous scales, two unequal flagella, R1 + R3 roots, and the absence of a rhizoplast. SSU rDNA analyses place this strain at the base of the Chrysophyceae/Synurophyceae lineages.

Ubiquitous dispersal of microbial species

The biosphere supports astronomical numbers of free-living microorganisms that belong to an indeterminate number of species. One view1,2,3 is that the abundance of microorganisms drives their dispersal, making them ubiquitous and resulting in a moderate global richness of species. But ubiquity is hard to demonstrate, not only because active species have a rapid turnover, but also because most species in a habitat at any moment in time are relatively rare or in some cryptic state4. Here we use microbes that leave traces of their recent population growth in the form of siliceous scale structures to show that all species in the chrysomonad flagellate genus Paraphysomonas are probably ubiquitous.

Aluminum silicate scale formation and inhibition (2): scale solubilities and laboratory and field inhibition tests

The solubilities of pure silica, iron-silicate and aluminum-silicate scales were measured in water from 25–250°C in a laboratory pressure reactor. Iron- and aluminum-silicate scales are significantly less soluble than pure amorphous silica. Aluminum- and iron-rich silica scales at equilibrium conditions are predicted to deposit from near-neutral, low salinity brines at temperatures that are 25 and 75°C above the saturation point for pure amorphous silica, respectively. This laboratory study demonstrates that higher brine injection temperatures are required to mitigate aluminum- and iron-rich silica scaling compared with pure amorphous silica. In a laboratory scale test, pure amorphous silica and aluminum-rich silica deposition rates have been measured at high degrees of supersaturation in the presence of potential inhibitors. Scale deposition was best inhibited by brine pH modification techniques. A commercially available dispersant successfully inhibited amorphous silica scaling, but exacerbated aluminum silicate scaling. Scale inhibition was also achieved in the presence of aluminum complexing\\sequestering agents in a patent-pending process. Screening of these potential silica- and aluminum-silicate scale inhibitors in the laboratory will focus the efforts of ongoing field pilot scale testing. The effect of complexing\\sequestering agents on scale inhibition was monitored in preliminary field pilot tests. These complexing agents achieved 25% to 80% aluminum silicate scale inhibition.

Aluminum silicate scale formation and inhibition: Scale characterization and laboratory experiments

Amorphous silica scale enriched in aluminum is deposited from a variety of geothermal brines. Scale deposits examined in the present study are formed by tetrahedrally-coordinated aluminum substitution within an amorphous silica framework. There is no evidence that aluminum in these scales derives from distinct aluminum minerals, such as gibbsite, or from aluminum silicate minerals transported in brine from the reservoir. The formation of aluminum-rich amorphous silica scale is dependent on brine pH, temperature and aluminum concentration. Silica and aluminum exhibit concentration minima in brines at near-neutral pH. Aluminum-rich silica scales dissolve more slowly than pure amorphous silica in water and brine. Lowering brine pH below 5 or increasing it above 9 retards the kinetics of silica polymerization and the formation of aluminum-rich silica. Laboratory studies demonstrate that sequestering agents such as citric acid, acetic acid, and ethylenediaminetetraacetic acid (EDTA) may inhibit aluminum silicate scale formation.

Scale Biogenesis in Synurophycean Protists: Phylogenetic Implications

The Synurophyceae is a class of golden-brown, freshwater, photosynthetic flagellates with a world-wide distribution. A well-developed taxonomy exists where genera and species are distinguished by colony or cell morphology or by the siliceous scales that cover the cells. However, phylogenetic relationships within the class are poorly understood, and incongruous taxonomic concepts occur. This study reviews scale morphology from field-collected samples and controlled culturing experiments as well as from studies of scale biogenesis. The information is used to identify homologous silicification surfaces among taxa and to document the diversity of the resulting scale structures. Thirty-two character states are coded into 11 characters in a cladistic analysis of 13 pivotal taxa. Colonial species are emphasized. One most-parsimonious phylogenetic tree is found (HI = 0, CI = 1). Synura lapponica is shown to be most closely related to Tessellaria volvocina. S. sphagnicola emerges at the base of the tree. Mallomonas caudata and the S. petersenii clade emerge from within Sectio Synura. Chrysodidymus synuroideus appears as an ancestral taxon in the Synura spinosa-like clade (i.e., Series Spinosae). The poorly understood developmental bases for some characters, especially secondary scale structures, are identified and may help focus future research.

Scale Biogenesis in Synurophycean Protists: Phylogenetic Implications

Abstract The Synurophyceae is a class of golden-brown, freshwater, photosynthetic flagellates with a world-wide distribution. A well-developed taxonomy exists where genera and species are distinguished by colony or cell morphology or by the siliceous scales that cover the cells. However, phylogenetic relationships within the class are poorly understood, and incongruous taxonomic concepts occur. This study reviews scale morphology from field-collected samples and controlled culturing experiments as well as from studies of scale biogenesis. The information is used to identify homologous silicification surfaces among taxa and to document the diversity of the resulting scale structures. Thirty-two character states are coded into 11 characters in a cladistic analysis of 13 pivotal taxa. Colonial species are emphasized. One most-parsimonious phylogenetic tree is found (HI = 0, CI = 1). Synura lapponica is shown to be most closely related to Tessellaria volvocina. S. sphagnicola emerges at the base of the tree. Mallomonas caudata and the S. petersenii clade emerge from within Sectio Synura. Chrysodidymus synuroideus appears as an ancestral taxon in the Synura spinosa-like clade (i.e., Series Spinosae). The poorly understood developmental bases for some characters, especially secondary scale structures, are identified and may help focus future research.

Species of Thaumatomastix (Thaumatomastigidae, Protista incertae sedis) from the Arctic sea ice biota (North-East Water Polynya, NE Greenland)

The sea ice biota of polar regions contains numerous heterotrophic flagellates very few of which have been properly identified. The whole mount technique for transmission electron microscopy enables the identification of loricate and scaly forms. A survey of Arctic ice samples (North-East Water Polynya, NE Greenland) revealed the presence of ca. 12 taxa belonging to the phagotrophic genus Thaumatomastix (Protista incertae sedis). Species of Thaumatomastix possess siliceous body scales and one naked and one scale-covered flagellum. The presence in both Arctic samples and sea ice material previously examined from the Antarctic indicates that this genus is most likely ubiquitous in polar sea ice and may be an important component in sea ice biota microbial activities.

SILICEOUS SCALE PRODUCTION IN CHRYSOPHYTE AND SYNUROPHYTE ALGAE. I. EFFECTS OF SILICA‐LIMITED GROWTH ON CELL SILICA CONTENT, SCALE MORPHOLOGY, AND THE CONSTRUCTION OF THE SCALE LAYER OFSYNURA PETERSENII1

ABSTRACTThe cells of synurophyte flagellates (algal class Synurophyceae, formerly included in the Chrysophyceae) are enclosed within a regularly imbricate layer of ornamented siliceous scales. Scale morphology is of critical taxonomic importance within this group of algae, and the scales are valuable indicator microfossils in paleolimnological studies. The data presented here demonstrate that scale morphology and the integrity of the scale layer can exhibit extreme variability in culture as a function of the cellular quota of silica under silica‐limited growth. Silica‐limited, steady‐state populations of the colonial flagellateSynura peterseniiKorsh. were maintained over a range of specific growth rates (μ= 0.11–0.69 days−1) and silica cell quotas (Qsi= 0.13–2.40 pmoles Si · cell1). Scale morphology and the organization of the scale layer became increasingly aberrant as silica stress increased. Under severe stress, scale deposition was completely suppressed so that cells appeared scale‐free. This depression of scale deposition was reversible; populations of silica‐starved, scale‐free cells rapidly regenerated new scale layers when placed in batch culture and spiked with dissolved silica. During recovery from silica stress, cell division was repressed for 24 h while mean cell silica quota increased 25‐fold. The first new scales appeared within 2 h after the silica addition, and development of the new scale layer proceeded in an approximately synchronous manner, residting in normal scale layers on virtually all cells after 48 h of recovery in Sirich medium. Silica content of silica‐repleteSynuracells is comparable to freshwater diatoms of siynilar size, butSynurahas much greater potential quota variability than diatoms and no apparent threshold silica requirement. Silica‐limited growth kinetics and competition between diatoms andSynurafor silica are discussed.The results suggest that morphological variability of siliceous scales in natural populations of synurophyte flagellates may result from silica stress and that the experimental approach developed here has great potential value as a means for circumscribing ecotypic variation in scale morphology. Results also demonstrate that scale production can be uncoupled from cell division, suggesting that cell cycle regulation of silica biomineralization in the Synurophyceae may be fundamentally different from that of diatoms (algal class Bacillariophyceae). This experimental system has application in the future study of the intracellular membrane systems and the regulatory processes involved in silica biomineralization.

High molecular mass glycoproteins associated with the siliceous scales and bristles of Mallomonas splendens (Synurophyceae) may be involved in cell surface development and maintenance

The elaborate scale case of Mallomonas splendens (Synurophyceae) consists of an overlapping arrangement of siliceous scales. In addition, siliceous bristles are attached to specialized base plate scales located at both the anterior and posterior ends of the cells. We have generated monoclonal antibodies against molecules associated with the scale case of M. splendens. One of these antibodies, designated MsS.H9, labelled a proteinaceous epitope of high-molecular-mass cell surface glycoproteins. Immunofluorescence and immunoelectron microscopy demonstrated that only two regions of M. splendens scale cases were labelled by MsS.H9, namely, the upper surface of the scales that contact neighboring scales and the bases of the bristles. Immunoelectron microscopy using thin sections of M. splendens cells showed these labelling sites corresponded to the amorphous material at the sites of scale-to-scale overlap and to a fibrillar complex located at scale-to-bristle attachment sites. Scales and bristles of M. splendens are formed within the cell, in silica deposition vesicles. Immunolabelling of cell sections containing developing scales and bristles showed that MsS.H9 labelling sites were present very early in the formation of these cell surface components. MsS.H9 labelling was also found associated with developing flagellar hairs whereas no labelling was detected on these structures after their deployment onto the flagellum. The location of MsS.H9 labelling sites strongly suggests that the molecule(s) recognized by the antibody plays a role in the adhesion of the individual components making up the scale case of M. splendens.

Scale prevention method by brine acidification with biochemical reactors

Siliceous scale deposition often causes serious problems in geothermal power stations. In order to solve these problems, a number of scale prevention methods have been proposed to date, one of which is brine acidification. Although it is well known that silica deposition is prevented by keeping the pH of the geothermal brine acidic, the pH adjustment of the brine by mineral acid injection has not been commercially applied so far in geothermal power stations. A new method is proposed in which sulfuric acid can be biochemically manufactured from hydrogen sulfide in the gas exhausted from geothermal power stations. The applicability of this method has been experimentally confirmed in field tests with a biochemical reactor. It is believed that the method will provide an economical and environmentally-friendly acidification process to prevent siliceous scale deposition and will also contribute to the reduction of hazardous hydrogen sulfide emission from geothermal power stations.

Mineralogy and Origin of Well-Bore Scales in an Active Steamflood: Tar Zone, Fault Block IIA, Wilmington Field, California: ABSTRACT

Precipitation of well-bore scale is a significant problem in the active steam drive project in the Tar Zone of Fault Block IIA, Wilmington Field. This present study is part of the US Department of Energy Class III program to improve oil recovery in slope and basin reservoirs. The aim of the study is to design the least expensive treatment to minimize scale precipitation. As the first step in this program, it is necessary to precisely identify the mineralogy and origin of the scales. Three different families of well-bore scale occur. (1) Carbonate scale, dolomite, high and low Mg-calcite; Mg content increases with decreasing well bore temperature. Calcium originates from connate waters: Mg originates from waterflood seawater Mn and some Mg originate from framework grains. Routine acidizing with 15% HCI readily removes this scale. (2) Sulfate scale, barite and anhydrite, is not common. Sulfates can be interlayered with carbonates and were precipitated at relatively low temperatures. Sulfates have small attachment areas and active well-bore brushing removes them with or without acid treatment. (3) Hydrous Magnesium Silicate scale occurs sparingly. The actual mineral form is difficult to identify because of poorly developed crystal structure. This mineral represents precipitation at the lowest temperaturemore » of all scales in the field. It is non-soluble in acid and must be mechanically removed.« less

Effects of caffeine treatment on the fine structure, silica secretion, and shell morphology of the testate amoeba Assulina muscorum

Assulina muscorum secretes morphologically altered shells when cultured in a medium with 5 mM caffeine. The siliceous scales, normally distributed in a regular overlapping pattern, are disorganized, thicker and wider than normal, and occasionally have incompletely silicified surfaces that appear irregular in profile in transmission electron microscopic ultrathin sections. The shape of the silica deposition vesicles (SDVs) in the cytoplasm is altered and they are less regularly arranged. The swollen appearances of the SDVs, and of nearby Golgi tubules, give additional evidence that caffeine affects the fine structural morphology of membranous secretory organelles and can disrupt their normal depositional activity. In addition to the greater thickness and width of the siliceous scales in caffeine-treated cells, the length and width of the shell are larger compared to controls, but the aspect ratio (length / width) is smaller. The latter is attributed to a larger increase in width relative to the increase in length of the caffeine-reated cells. Since some of the scales are deposited with the long axis laterally on the shell surface, in addition to being greater in width, this raises the interesting question of whether the morphology of the SDVs and the siliceous products influences the size and morphogenesis of the shell. Further research is needed to clarify the interaction of the SDVs with the cytoplasmic cytoskeletal system during shell morphogenesis. © 1995 Wiley-Liss, Inc.

Crystal Chemistry of Hydrous Iron Silicate Scale Deposits at the Salton Sea Geothermal Field

AbstractThe crystal chemistry of Fe-Si scales deposited from geothermal brines at Salton Sea, California, was studied by powder X-ray diffraction and spectroscopic techniques including infrared, 57Fe Mössbauer, 27Al and 29Si nuclear magnetic resonance (NMR), and Fe and Si K-edge extended X-ray absorption fine structure (EXAFS). Scales precipitated at near 250°C from dissolved ferrous iron and silicic acid are composed of hisingerite. This phase is shown to possess the same local structure as nontronite and is a poorly-crystallized precursor of the ferric smectite. A clear distinction can be made at the local scale between hisingerite and 2-line ferrihydrite because, even in their most disordered states, the former possesses a two-dimensional and the latter a three-dimensional anionic framework. At temperature near 100°C Fe-Si scales are a mix of Al-containing opal and hydrous ferrous silicate, whose local structure resembles minnesotaite and greenalite. This hydrous ferrous silicate is very well ordered at the local scale with an average Fe coordination about Fe atoms of 6 ± 1. The difference in crystallinity between the ferrous and ferric silicate scales was related to variations of growth rates of clay particles precipitated from ferrous and ferric salt solutions. The low crystallinity of the ferric smectite suggests that the oxidation of ferrous iron occurs before polymerization with silica.

The fine structure of the euglyphid testate amoeba Assulina muscorum (Rhizopoda: Euglyphidae) with observations of growth rate in culture, morphometries, and siliceous scale deposition

Summary Scanning and transmission electron microscopic analysis of the shell and cytoplasm of Assulina muscorum Greef, 1888 show that the siliceous scales of the test are characteristic of other euglyphid species and originate in the posterior portion of the cytoplasm near Golgi bodies. The scales are deposited within vesicles, mature as they move anteriorly, and become stored within enclosing vesicles in the vicinity of pigment granules. A frothy layer of vacuolated cytoplasm, containing digestive vacuoles with bacteria, extends anteriorly toward the aperture, and occasionally includes small amoebae (c. 2–3 μm) that appear to be commensals. Reserve bodies, possibly lipid, are more abundant in the posterior cytoplasm surrounding the nucleus. The nucleus varies in shape from approximately circular in sections to elongate or lobate and contains a centrally located nucleolus. In overall view, the fine structure of A. muscorum resembles similar terrestrial euglyphid species such as Tracbeleuglypha dentata . Membrane-bound, electron-opaque granules with less dense cylindrical cores may be secretory granules, and appear to be different from those reported in other euglyphid species. The doubling time in laboratory culture is 3.8 to 4.3 days, and the natural growth coefficient is 0.19 to 0.17 depending on the kind of growth medium.

Fibrous Monolithic Ceramics: III, Mechanical Properties and Oxidation Behavior of the Silicon Carbide/Boron Nitride System

Fibrous monolithic ceramics were fabricated in the SiC/BN system. The microstructure consists of high‐aspect‐ratio polycrystalline cells of SiC separated by cell boundaries of BN. The fibrous monolith with aligned cells fails noncatastrophically in flexure, with delamination cracking along the BN cell boundaries. Indentations cause controlled damage on the surface, but no strength‐degrading flaws. The flexural strength is in the range 300–375 MPa, and the estimated shear strength is ∼14 MPa. The SiC/BN fibrous monoliths also show excellent resistance to high‐temperature oxidation in air. Noncatastrophic fracture behavior is observed at room temperature after heat treatments between 1200° and 1500°C. The SiC cells on the surface are oxidized to form a protective silicate scale, which prevent deterioration of the BN cell boundaries.