Boundary Dissolution Research Articles

Structural and lithological guidance on speleogenesis in quartz–sandstone: Evidence of the arenisation process

A detailed petrographic, structural and morphometric investigation of different types of caves carved in the quartz–sandstones of the “tepui” table mountains in Venezuela has allowed identification of the main speleogenetic factors guiding cave pattern development and the formation of particular features commonly found in these caves, such as funnel-shaped pillars, pendants and floor bumps. Samples of fresh and weathered quartz–sandstone of the Mataui Formation (Roraima Supergroup) were characterised through WDS dispersive X-ray chemical analyses, picnometer measurements, EDAX analyses, SEM and thin-section microscopy. In all the caves two compositionally different strata were identified: almost pure quartz–sandstones, with content of silica over 95% and high primary porosity (around 4%), and phyllosilicate-rich quartz–sandstone, with contents of aluminium over 10% and low primary porosity (lower than 0.5%). Phyllosilicates are mainly pyrophyllite and kaolinite. SEM images on weathered samples showed clear evidence of dissolution on quartz grains to different degrees of development, depending on the alteration state of the samples. Grain boundary dissolution increases the rock porosity and gradually releases the quartz grains, suggesting that arenisation is a widespread and effective weathering process in these caves. The primary porosity and the degree of fracturing of the quartz–sandstone beds are the main factors controlling the intensity and distribution of the arenisation process. Weathering along iron hydroxide or silt layers, which represent inception horizons, or a strata-bounded fracture network, predisposes the formation of horizontal caves in specific stratigraphic positions. The loose sands produced by arenisation are removed by piping processes, gradually creating anastomosing open-fracture systems and forming braided mazes, geometric networks or main conduit patterns, depending on the local lithological and structural guidance on the weathering process. This study demonstrates that all the typical morphologies documented in these quartz–sandstone caves can be explained as a result of arenisation, which is guided by layers with particular petrographic characteristics (primary porosity, content of phyllosilicates and iron hydroxides), and different degrees of fracturing (strata-bounded fractures or continuous dilational joints).

Effect of varying conditions of exposure to an aggressive medium on the corrosion behavior of the 2050 Al–Cu–Li alloy

The corrosion behavior of aluminum alloy 2050 was studied as a function of varying conditions of exposure to an aqueous NaCl solution. Two metallurgical states were considered, i.e., an as-received alloy (NHT) and aged samples (HT). After continuous immersion tests, the NHT samples were susceptible to intergranular corrosion, whereas intragranular corrosion was observed for the HT samples. For the NHT samples, cyclic corrosion tests, with alternate immersion – emersion periods, induced subgrain boundary dissolution, and for the HT samples, grain and/or subgrain boundary sensibilization was induced. Hydrogen content measurements suggested a hydrogen enrichment of the alloy under cyclic corrosion tests.

Immigration & the Color Line at the Beginning of the 21st Century

The “color line” has long served as a metaphor for the starkness of black/white relations in the United States. Yet post-1965 increases in U.S. immigration have brought millions whose ethnoracial status seems neither black nor white, boosting ethnoracial diversity and potentially changing the color line. After reviewing past and current conceptualizations of America's racial divide(s), we ask what recent trends in intermarriage and multiracial identification – both indicators of ethnoracial boundary dissolution – reveal about ethnoracial color lines in today's immigrant America. We note that rises in intermarriage and multiracial identification have emerged more strongly among Asians and Latinos than blacks and in more diverse metropolitan areas. Moreover, these tendencies are larger than would be expected based solely on shifts in the relative sizes of ethnoracial groups, suggesting that immigrationgenerated diversity is associated with cultural change that is dissolving ethnoracial barriers – but more so for immigrant groups than blacks.

Grain boundary dissolution porosity in quartzofeldspathic ultramylonites: Implications for permeability enhancement and weakening of mid-crustal shear zones

Quartzofeldspathic ultramylonites from the Alpine Fault Zone, one of the world's major, active plate boundary-scale fault zones have quartz crystallographic preferred orientations (CPO) and abundant low-angle (<10° misorientation) boundaries, typical microstructures for dislocation creep-dominated deformation. Geometrically necessary dislocation density estimates indicate mean dislocation densities of ∼109 cm−2. A significant proportion (∼30%) of grain boundaries (>10° misorientation) are decorated by faceted pores, commonly with uniformly-oriented pyramidal shapes. Only grain boundaries with >10° misorientation angles in polymineralic aggregates are decorated by pores. Mean grain boundary pore densities are ∼5 × 108 cm−2. Grain boundary pores are dissolution pits generated during syn-deformational transient grain boundary permeability, nucleating on dislocation traces at dilatant grain boundary interfaces. They have not been removed by subsequent grain boundary closure or annealing. Pore decoration could have led to grain boundary pinning, triggering a switch in the dominant deformation mechanism to grain boundary sliding, which is supported by evidence of CPO destruction in matrix quartz. Pore-decorated grain boundaries have significantly reduced surface area available for adhesion and cohesion, which would reduce the tensile and shear strength of grain boundaries, and hence, the bulk rock. Grain boundary decoration also significantly decreased the mean distance between pores, potentially facilitating dynamic permeability. Consequently, these microstructures provide a new explanation for strain weakening and evidence of fluid flow along grain boundaries in mylonites at mid-crustal conditions.

The Diversity Paradox by Jennifer Lee and Frank D. Bean (review)

Reviewed by: The Diversity Paradox by Jennifer Lee and Frank D. Bean Kenneth Prewitt The Diversity Paradox. By Jennifer Lee and Frank D. Bean. New York: Russell Sage Foundation. 2010. This ambitious book poses questions key to race relations: a) is diversity resulting from immigration re-drawing America’s race boundaries? b) if so, how does biracial marriage and multiracial progeny fit into this picture? The authors base their analysis on national statistics (the American Community Survey) and in-depth interviews of 46 multiracial adults and 36 interracial couples with children, all Californian. The major finding: diversity is leading to the dissolution race boundaries. Diversity is defined as social-structural and cultural-attitudinal shifts that accompany demographic dynamics, that is, how many different ethnoracial groups there are, and their relative sizes and geographic distribution. The authors want to measure boundary dissolution net of demographic change; and, in this effort use qualitative data to uncover the mechanisms that give rise to national trends. The book summarizes the four paths that the 21st-century color line might take. The color line could continue to separate the white and nonwhite, that is, Asian and Latino immigrants and their children will be “racialized.” Alternatively, Asians and Latinos could follow the path of hyphenation, the Irish, Italian, and Slavic who escaped the 19th-century effort to racialize them, becoming white American even as they retain traces of their national origins. This second possibility is the much-discussed black/nonblack color line, African-Americans on one side and everyone else on other. The third is the tri-parte path, with Asians and Latinos hovering between the black and white races. This outcome replaces one with two boundaries, separating Asians and Latinos both from African-Americans and from European Whites. Finally is the post-racial possibility; boundaries fade, the color line disappears. In investigating which path the nation is on, Lee and Bean offer a rich stream of mid-level findings, fresh insights, clever analysis—altogether a sociological feast. [End Page 142] Space does not permit review of this feast, but even if the reader is unpersuaded on the general finding there is much to be learned from close reading. The authors write that for Asians and Latinos diversity is weakening the boundaries separating them from whites—evident in proud assertions of multiracialism, in the increase of biracial marriages, and in the acceptance by white America of educationally and economically successful Asians and Latinos. Whether less successful Asians and Latinos, among the latter especially the undocumented, are breaking through the color barrier is not fully examined. Then there is the paradox—black exceptionalism. The authors find “deep-seated cultural differences” (199) in how whites, Asians, and Latinos view each other and in how all three view African-Americans and correspondingly in how African-Americans view themselves. The authors qualify black exceptionalism by arguing that the black/nonblack color line is not completely immune to the openings presented by diversity. Diversity, Lee and Bean argue, erodes all race boundaries, but, paradoxically, “the new diversity blurs some color lines more than others” (19). For African-Americans there are countervailing factors that markedly weaken the effect of diversity. The principle countervailing factor invoked is group-threat, that is, as minority numbers grow relative to the white majority, the latter feel threatened. The authors do some clever metropolitan-level analysis to suggest why group-threat might explain black exceptionalism, and in lesser degree to some instances of Latino exceptionalism. The ingenious analysis notwithstanding, the authors acknowledge that the explanation of why the black/nonblack boundary is resistant to the transformation otherwise resulting from diversity is not dispositive. These general findings suffer from several data problems. One is self-inflicted. The authors use an “upper-bound” (57) definition of non-white, including Latinos who self-define as white as well as all multiracials, but their own qualitative data indicate that for many in these groups race is a “nonissue” (90). The upper-bound definition of non-white tilts the statistical data in the direction of finding that diversity weakens racial boundaries. A second data problem is beyond their control, but might have been noted. The qualitative data make clear the...

Particle Loosening on the Surface of Dense Hydroxyapatite in Water

This work describes dissolution and related mechanical weakening of phase-pure and dense hydroxyapatite (HA) in distilled water of pH 7.4. Phase-pure HA powder has been synthesized by a wet precipitation method. After uniaxial and cold isostatic pressing, and sintering at 1200°C, dense HA with 98% above of the theoretical density has been obtained. The results show that HA powder has stoichiometric composition with a Ca/P ratio 1.67 ± 0.02. Even after extended exposure for 10 h, no second phases, such as tricalcium phosphate (TCP) and calcium oxide can be observed. Although the HA is supposed to be stable in liquid environment, surface dissolution appears specifically at material’s grain boundaries after immersion for 7 days. Following further immersion to 14 days, grain boundary dissolution progresses interior to the bulk following these paths. This dissolving behavior generates HA particles, disintegrates dense microstructure and at least forms micron-scale cavity. Mechanical property of the HA has been also affected. Fracture toughness (KIc) of the HA sintered body is approximately 1.0 MPa•m1/2. It drastically decreases to almost half of the initial value due to the severe surface dissolution

Supergene gold transformation: Secondary and nano-particulate gold from southern New Zealand

Biogeochemical processes drive the transformation of gold (Au) in surface environments. In this study we assess the link between surface morphologies of Au grains and supergene transformation processes, with a focus on the formation of nano-particulate Au in temperate settings. Gold grains were collected from six localities across the South Island of New Zealand. Deposit styles vary from eluvial-, alluvial-, and beach placer deposits in areas of moderate to very high levels of precipitation. Gold grains were assessed using optical microscopy (OM), field emission scanning electron microscopy (FEG-SEM), focused ion beam-scanning electron microscopy (FIB-SEM) coupled with X-ray dispersive analysis (EDXA) and electron microprobe analyses (EPMA). Morphologies indicative of Au- and Ag dissolution, e.g., grain boundary dissolution, as well as abundant Au neoformation- and aggregation morphologies were observed on all grains. The latter include a variety of secondary Au morphotypes, in particular nano-particulate- and μ-crystalline forms as well as bacteriomorphic Au, sheet-Au and porous, branched Au networks. Pervasive dissolution features on grains from an outcropping quartz-vein system as well as extensive nano-particle formation on weathered quartz-vein- and placer grains from the west coast of New Zealand's South Island, which is subject to very heavy orographic precipitation, suggest that these climatic conditions enhance the transformation of Au grains. At these sites, Au nano-particles are most abundant in the polymorphic layer (i.e., a coating of biofilms, secondary Au, siliceous and carbonaceous materials on the surface of transforming Au grains), and in soil materials associated with the grains. Nano-particulate Au is also highly abundant in carbonaceous, likely exopolymeric, coatings on Au grains from Orepuki Beach, suggesting that Au dissolution in seawater and microbial biomineralization are important contributors to Au alteration in beach placer deposits. In conclusion, surface morphologies of Au grains from New Zealand are the result of supergene transformations occurring in current environments. The formation of nano-particulate Au, which was previously thought to be evaporation-driven, is in these high-rainfall environments likely due to other mechanisms, such as biomineralization.

Online news media in the Web 2.0 era: from boundary dissolution to journalistic transformation

This paper examines how online news media have changed as the Internet has moved from Web 1.0 to Web 2.0. Based on medium theory and boundary theory, online news media are conceptualized as being regulated by the “participatory open media code”. The code analysis attempts to examine the newly emerged form and structure of online news. In the new interactive media environment, it is found that online news media have changed significantly in the following areas: (1) the inclusion of user-generated content; (2) user participatory interactivity; (3)YouTubization; (4) instant reporting; (5) sharing and community networking; and (6) customization through new communication technologies. The trend of growing reader participation in social dialogue is apparent around the globe. The transformed online media have provided good platforms for citizen participation in news. Yet, there are still constraints against quality participation.

Longitudinal Effects of Prenatal Cocaine Use on Mother-Child Interactions at Ages 3 and 5 Years

To assess the effect of maternal prenatal and past-year cocaine use on mother-child interactions across preschool years. The sample is drawn from the Miami Prenatal Cocaine Study, a longitudinal follow-up of prenatal cocaine exposure (PCE) in a large cohort of African-American infants prospectively enrolled at birth. Analyses are based on the 366 children (168 PCE and 198 non-cocaine-exposed) in the care of their biological mothers and with completed mother-child interaction measures at the 3- and/or 5-year assessments. Videotaped interactions were coded using a modified Egeland Teaching Task scheme. Generalized linear models with a generalized estimating equations approach were used to evaluate the effect of PCE on the overall quality of maternal-child interaction, measured by the Egeland total score at both study visits, and on the individual Egeland subscales at the 5-year visit, while adjusting for other suspected influences on interactions. PCE dyads demonstrated less optimal overall mother-child interactions compared with non-cocaine-exposed dyads. The estimated PCE-associated difference did not shift appreciably with statistical adjustment for child sex, child age at examination, or other birth covariates. PCE dyads with past-year maternal cocaine use had significantly lower Egeland summary scores compared with children with neither exposure. In subscale analyses, PCE was most strongly associated with greater maternal intrusiveness and boundary dissolution at the 5-year visit. Prenatal and past-year maternal cocaine use seems to be associated with poorer quality in mother-child interaction during early childhood. These dynamics should be considered when examining the association between PCE and child cognitive, behavioral, and academic outcomes.

Slow Crack Growth by Grain Boundary Dissolution of Hydroxyapatite in Water

Mechanical degradation of hydroxyapatite ceramics due to the slow crack growth was observed in water. Microstructural crack on the surface of hydroxyapatite initiated by grain boundary dissolution in liquid environment resulting in particle loosening and microstructural-level degradation, followed by a drastic decrease of mechanical properties. In this study, slow crack growth by the dissolution behavior of hydroxyapatite ceramics was investigated based on microstructural observation by field emission microscopy. The crack growth and morphological change of hydroxyapatite surface, especially at the initial stage of dissolution and related surface roughness were observed with immersion time. The surface dissolution occurred from the initial stage of immersion showing increase in surface roughness. Following certain period of immersion time, the surface dissolution initiated at grain boundaries and generated many separated grains.

Intergenerational Transmission of Familial Boundary Dissolution: Observations and Psychosocial Outcomes in Adolescence

This study investigated the transmission of boundary dissolution (BD) in parent-child relationships from parental behaviors observed in early childhood to adolescent behaviors observed at age 13 and relations to adolescent psychosocial adaptation. The goals of the study are (a) to examine the developmental relation of early childhood BD to several measures of adolescent BD observed in 13-year-olds' behavior, (b) to compare two types of BD in adolescence and their associations with early childhood BD and subsequent psychosocial outcomes, and (c) to test the mediating role of adolescent BD between early childhood BD and later adolescent functioning. Participants were drawn from a study of 196 children followed since birth. Two forms of adolescent BD, role equalization and sexualized behavior, were observed during parent-adolescent structured interactions. Early BD childhood predicted observations of adolescent BD. Role equalization mediated relations to self-worth and externalizing problems. Implications regarding multiple types of adolescent BD are discussed.

Strengthening Couple Relationships for Optimal Child Development: Lessons from Research and Intervention - edited by Marc S. Schulz, Marsha Kline Pruett, Patricia K. Kerig, &amp; Ross D. Parke

Strengthening Couple Relationships for Optimal Child Development: Lessons from Research and Intervention. Marc S. Schulz, Marsha Kline Pruett, Patricia K. Kerig, & Ross D. Parke (Eds.). Washington, DC: American Psychological Association. 2010. 244 pp. ISBN 1433805472. $79.95 (cloth). Promoting strong families and optimizing positive development for children are paramount concerns in our society. Parents, researchers, clinicians, and policymakers work to understand and create environments that foster well-being, security, and achievement in children. We often fail, however, to emphasize in our efforts a context that for decades has been shown to be of fundamental importance to children's development, that is, the quality of parents' relationships with each other. Strengthening Couple Relationships for Optimal Child Development: Lessons from Research and Intervention focuses a lens on the profound impact that parents' relationships have on developing families and children. The editors of this volume (Marc Schulz, Marsha Kline Pruett, Patricia Kerig, and Ross Parke), together with prominent scholars in the fields of child development, couples research, family studies, and clinical intervention (including Bradbury, Conger, Cowan and Cowan, Cummings, Gottman, Levenson, and McHaIe, among others), articulate the ways in which couples' relationships enhance or compromise functioning in families and provide strategies for promoting optimal outcomes, particularly for children. The initial chapter, written by the book's editors, documents the historical path of couple and family research, including the shift from independent to interdependent models of family functioning. The remainder of the book comprises three parts. Part 1 (Chapters 2-5) highlights the implications of the quality of couple relationships for other dyads and individuals in the family. Chapter 2 includes a review of emotional security theory (EST) as a conceptual model for understanding the processes underlying the impact of marital conflict on children. The authors discuss specific aspects of EST that hold particular relevance for designing prevention and intervention programs, including their own psychoeducational program, to lessen the negative effects of parents' disputes for children. Parents' conflict, whether they handle it well or poorly, affects even very young children. The authors of Chapter 3 provide evidence to support a compelling technique for overcoming some of the methodological challenges of assessing how preschool-aged children perceive and process marital conflict, the Berkeley Puppet Interview. Chapter 4 examines ways in which marital relationship processes interrelate with parent- child and whole family relationships. The authors provide an insightful look into the construct of boundary dissolution and its implications for families and children of distressed couples. Chapter 5 provides a summary of the development of the construct of coparenting and highlights key findings that underscore its importance for children's care and upbringing. The author emphasizes the need to promote and support positive coparenting alliances in nuclear, fragile, and extended-kin family systems. Part 2 (Chapters 6-9) is concerned with the key domains and determinants of couple functioning and the promise of advances in methodology and statistical techniques for addressing more complex questions about the nature of effects and processes involved. Chapter 6 considers martial satisfaction across the transition to parenthood. The authors review research on group differences as well as findings from promising recent work considering trajectories of change, and they provide recommendations for future research, including continued focus on predictors of change and processes that might be amendable to intervention. The authors of Chapter 7 propose and provide partial validation for an integrative, developmental model for understanding how relationship quality changes during the transition to parenthood. …

Sintering and dissolution of bone ash-derived hydroxyapatite

Bone ash-derived hydroxyapatite (HA) ceramics were prepared by pressureless sintering and hot pressing, and their dissolution behavior was examined in buffered water. HA powder was obtained by soaking bone ash in a 0.1M NaOH solution at 80 °C, followed by calcination at 1000 °C to completely remove the organic material. The crystal structure of the HA powder with a particle size of approximately 1 μm was mainly hydroxyapatite with a minimal amount of α-tricalcium phosphate. To improve densification, the powder was hot-pressed at 1000 °C for 0.5 h under a pressure of 30 MPa in an Ar atmosphere. The sintered density of the hot-pressed HA was 95 % of the theoretical density, which is much higher than the 70% obtained for the pressureless-sintered compact. In the porous HA ceramics obtained by pressureless sintering, dissolution occurred adjacent to the pores rather than in the dense part, which increased the residual porosity. On the other hand, the hot-pressed HA showed grain boundary dissolution followed by particle loosening.

Improvement of the Stability of Hydroxyapatite Through Glass Ceramic Reinforcement

Hydroxyapatite has achieved significant application in orthopedic and dental implants due to its excellent biocompatibility. Sintered hydroxyapatites showed significant dissolution, however, after their immersion in water or simulated body fluid (SBF). This grain boundary dissolution, even in pure hydroxyapatites, resulted in grain separation at the surfaces, and finally, in fracture. In this study, hydroxyapatite ceramics containing apatite-wollastonite (AW) or calcium silicate (SG) glass ceramics as additives were prepared to prevent the dissolution. AW and SG glass ceramics were added at 0-7 wt% and powder-compacted uniaxially followed by firing at moisture conditions. The glass phase was incorporated into the hydroxyapatite to act as a sintering aid, followed by crystallization, to improve the mechanical properties without reducing the biocompatibility. As seen in the results of the dissolution test, a significant amount of damage was reduced even after more than 14 days. TEM and SEM showed no decomposition of HA to the secondary phase, and the fracture toughness increased, becoming even higher than that of the commercial hydroxyapatite.

Corrosion behaviour of nitrogen ion implanted AISI type 304L stainless steel in nitric acid medium

The effects of nitrogen ion implantation on corrosion behaviour of 304L stainless steel in 1 N HNO 3 medium were investigated using surface analytical and electrochemical techniques. Nitrogen ion was implanted at 70 keV in the dose range of 1 × 10 15, 1 × 10 16, 1 × 10 17 and 2.5 × 10 17 N +/cm 2, respectively. Grazing incidence X-ray diffraction results for unimplanted and up to dose of 1 × 10 16 N +/cm 2 showed co-existence of γ-Fe and α′-Fe and, at higher doses (1 × 10 17 and 2.5 × 10 17) preferential formation of chromium nitride was observed. X-ray photoelectron spectroscopy investigation confirmed the formation of chromium nitride at higher doses. Electrochemical corrosion investigation revealed nobler open circuit potential, decrease in corrosion current densities, passive current densities and increase in polarization resistance with increase in dose rate. Surface morphology analysis after polarization study using atomic force microscope showed grain boundary dissolution for unimplanted specimens and resistance to surface dissolution with increase in dose rate for implanted specimens.

Dissolution-resistance of glass-added hydroxyapatite composites

Hydroxyapatite (HA) has generated a great deal of interest as a promising implant material. However, its poor mechanical properties induced by severe dissolution in biological milieu limit medical applications and lead to clinical failure. In this study, HA ceramics with 30P2O5-30CaO-40Na2O glass (1.0 wt.% and 2.5 wt.%) were prepared to improve the resistance of monophase HA. The monophase HA sintered body showed microstructural degradation due to grain boundary dissolution in buffered water. However, the dissolution-resistance of HA/glass composites was significantly improved and showed no apparent evidence of dissolution. This suggests that a less soluble glass phase should be placed at grain boundaries to protect HA from dissolution.

AFM analysis of anisotropic dissolution in dense hydroxyapatite

Dense, polycrystalline hydroxyapatite (HA) ceramics prepared by microwave sintering process were immersed for 1–60 s in an acidic solution of pH 3, similar to the circumstance of osteoclastic resorption. Dissolution behavior on the surface of HA immersed in the solution was investigated. The surface topography was evaluated by using atomic force microscopy (AFM) with tapping mode. The erosion of grain boundaries in the dense HA was observed and the grain boundary dissolution occurred to a great extent forming a wide gap (50–100 nm) between grains. Notably, anisotropic dissolution was observed on the HA surface showing two topographically different regions; grains (Grain 1) with relatively intact surface, and grains (Grain 2) involving formation of subgrain boundaries and aligned crystallites in a certain direction. The difference in RMS surface roughness evidenced the distinct dissolution feature of ceramic grains. The surface roughness of Grain 1 increased from 1.45±0.30 to 3.11±0.63 nm, whereas the roughness of Grain 2 increased from 2.36±0.11 to 5.27±1.34 nm.

Book Review: Implications of Parent–Child Boundary Dissolution for Developmental Psychology: “Who Is the PARENT and Who Is the CHILD?” (2005). Patricia K. Kerig (Ed.), New York: The Haworth Maltreatment and Trauma Press, 297 pp., US $39.95. Reviewed by Thomas C. Flamboe

Book Review: Implications of Parent–Child Boundary Dissolution for Developmental Psychology: “Who Is the PARENT and Who Is the CHILD?” (2005). Patricia K. Kerig (Ed.), New York: The Haworth Maltreatment and Trauma Press, 297 pp., US $39.95. Reviewed by Thomas C. Flamboe

Influence of grain boundary structure on dissolution controlled pressure solution and retarding effects of grain boundary healing

It is widely accepted that the structure of a grain boundary undergoing pressure solution can have a strong influence on the rates at which diffusive transport in the grain boundary occurs. However, the influence of grain boundary structure on internal grain boundary dissolution rates has received little attention, despite evidence that dissolution controlled pressure solution in quartz is slower than expected assuming dissolution kinetics appropriate for free surfaces. In this paper, three hypothetical steady state grain boundary structures are defined and the influence of these structures on dissolution controlled pressure solution rates in an elastic solid are considered by deriving simple models based on internal grain boundary mass and energy balances. It is found that average dissolution rates in a rough grain boundary (island‐channel network) are slowed down by up to 13% compared to dissolution in a flat grain boundary containing a thin fluid film. This can only partly account for the discrepancy between models and experiments reported in the literature. In addition a model is derived providing a criterion or “yield stress” for pressure solution, below which the process is prevented by surface energy driven grain boundary healing (progressive reduction of the contact area filled by connected fluid). This “yield stress criterion” for pressure solution offers a further explanation for reduced rates or cessation of pressure solution at low effective stresses in nature and experiment. Using this criterion, limiting porosity depth curves are predicted for sandstones compacting by pressure solution, which show favorable agreement with porosity‐depth data for quartz sandstones.

Mechanism of Surface Dissolution in Dense Hydroxyapatite

In this study, it was demonstrated how second phases with small amount, which are hardly detected by XRD analysis, affect grain boundary dissolution and related mechanical properties of HA. All HA disks sintered at 1200 oC for 2 h in air with under moisture protection were phase pure and had Ca/P molar ratio of 1.67. Following certain period of exposure to the distilled water, the surface dissolution initiated at grain boundaries and particle loosening, subsequently resulting in decrease in mechanical properties of HA. In order to understand the dissolution mechanism, grain boundary structure of HA was identified by transmission electron microscopy (TEM) and high resolution TEM observation. From the analysis, it was found that the non-stoichiometric phase as α-tricalcium phosphate (TCP) transformed from β-TCP was existed at grain boundaries and caused surface dissolution of HA. From the XRD analysis, it was found that (211) and (112) planes of hydroxyapatite were susceptible to dissolution, whereas (300) plane was relatively stable.