Compressibility and crystal structure of sillimanite, Al2SiO5, at high pressure     

H. Yang  R. M. Hazen  L. W. Finger  C. T. Prewitt  R. T. Downs 《Physics and Chemistry of Minerals》1997,25(1):39-47

The unit-cell dimensions and crystal structure of sillimanite at various pressures up to 5.29 GPa have been refined from single-crystal X-ray diffraction data. As pressure increases, a and b decrease linearly, whereas c decreases nonlinearly with a slightly positive curvature. The axial compression ratios at room pressure are β_a:β_b:β_c=1.22:1.63:1.00. Sillimanite exhibits the least compressibility along c, but the least thermal expansivity along a (Skinner et al. 1961; Winter and Ghose 1979). The bulk modulus of sillimanite is 171(1) GPa with K′=4 (3), larger than that of andalusite (151 GPa), but smaller than that of kyanite (193 GPa). The bulk moduli of the [Al1O₆], [Al2O₄], and [SiO₄] polyhedra are 162(8), 269(33), and 367(89) GPa, respectively. Comparison of high-pressure data for Al₂SiO₅ polymorphs reveals that the [SiO₄] tetrahedra are the most rigid units in all these polymorphic structures, whereas the [AlO₆] octahedra are most compressible. Furthermore, [AlO₆] octahedral compressibilities decrease from kyanite to sillimanite, to andalusite, the same order as their bulk moduli, suggesting that [AlO₆] octahedra control the compression of the Al₂SiO₅ polymorphs. The compression of the [Al1O₆] octahedron in sillimanite is anisotropic with the longest Al1-OD bond shortening by ～1.9% between room pressure and 5.29 GPa and the shortest Al1-OB bond by only 0.3%. The compression anisotropy of sillimanite is primarily a consequence of its topological anisotropy, coupled with the compression anisotropy of the Al-O bonds within the [Al1O₆] octahedron.  相似文献   

‘A citizen of all the different bits’: emotional scaling of identity     

Joni Tuomas Vainikka 《GeoJournal》2016,81(1):5-22

Scale is a debatable term in the humanities and social sciences. Conceptualized in human geography as spatial categories of thought, as the arenas where social processes occur, as bounded political-economic frames or as unhelpful binaries privileging either the local or the global, scale intersects a significant body of geographical research. The unfolding and intermeshing of topological connections that help to share moments and experiences are important sources for the differentiation, renewal and recalibration of individual identities, but they often work as co-components to scalar identifications. Engaging with the recent discussion on scale and the upsurge of emotional geographies, I seek to understand how people contextualize space through situated scalar perspectives and how they realign and recognize their identities through embodied emotions. The analysis of the empirical material, that comprises 23 focus groups with locally and universally-orientated civic organizations in Finland and England, focuses on the ways people use landscapes and communities as emotional signposts in their scalar identification. I argue that scale is a situated category, whose spectrum individuals negotiate through the performance of social discourses and cultural practices. In such negotiation, people scale their identity narratives to overcome or emphasize the distinctions between ‘us’ and ‘others’.  相似文献   

Meeting the Challenge of Systemic Change in Geography Education: Lucy Sprague Mitchell’s Young Geographers     

Roger M. Downs 《The Journal of geography》2016,115(1):3-11

The history of K–12 geography education has been characterized by recurrent high hopes and dashed expectations. There have, however, been moments when the trajectory of geography education might have changed to offer students the opportunity to develop a thorough working knowledge of geography. Lucy Sprague Mitchell’s geography program developed in the 1920s and 1930s was brilliant in its conception and execution but it did not lead to large-scale systemic change. Analyzing the origins, development, and outcome of her program suggests the conditions that facilitate and inhibit systemic change, and therefore perhaps we can learn to capitalize on comparable moments in the future.  相似文献   

Psychoanalytic Geographies          下载免费PDF全文

Roger M. Downs 《Geographical review》2016,106(4):e54-e57

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Comparative compressibilities of majorite-type garnets     

R. M. Hazen  R. T. Downs  P. G. Conrad  L. W. Finger  T. Gasparik 《Physics and Chemistry of Minerals》1994,21(5):344-349

Relative compressibilities of five silicate garnets were determined by single-crystal x-ray diffraction on crystals grouped in the same high-pressure mount. The specimens include a natural pyrope [(Mg_2.84Fe_0.10Ca_0,06) Al₂Si₃O₁₂], and four synthetic specimens with octahedrally-coordinated silicon: majorite [Mg₃(MgSi)Si₃O₁₂], calcium-bearing majorite [(Ca_0.49Mg_2.51)(MgSi)Si₃0₁₂], sodium majorite [(Na_1.88Mgp_0.12)(Mg_0.06Si_1.94)Si₃O₁₂], and an intermediate composition [(Na_0.37Mg_2.48)(Mg_0.13Al_1.07 Si₀₈₀) Si₃O₁₂]. Small differences in the compressibilities of these crystals are revealed because they are subjected simultaneously to the same pressure. Bulk-moduli of the garnets range from 164.8 ± 2.3 GPa for calcium majorite to 191.5 ± 2.5 GPa for sodium majorite, assuming K′=4. Two factors, molar volume and octahedral cation valence, appear to control garnet compression.  相似文献   

Bond length and radii variations in fluoride and oxide molecules and crystals     

J. S. Nicoll  G. V. Gibbs  M. B. Boisen Jr.  R. T. Downs  K. L. Bartelmehs 《Physics and Chemistry of Minerals》1994,20(8):617-624

Molecular orbital calculations completed on fluoride molecules containing first and second row cations have generated bond lengths, R, that match those observed for coordinated polyhedra in crystals to within ～0.04 Å, on average. The calculated bond lengths and those observed for fluoride crystals can be ranked with the expression R=Kp ^?0.22, where p=s/r, s is the Pauling strength of the bond, r is the row number of the cation and K=1.34. The exponent -0.22 (≈ -2/9) is the same as that observed for oxide, nitride and sulfide molecules and crystals. Bonded radii for the fluoride anion, obtained from theoretical electron density maps, increase linearly with bond length. Those calculated for the cations as well as for the fluoride anion match calculated promolecule radii to within ～0.03 Å, on average, suggesting that the electron density distributions in the vicinity of the minima along the bond paths possess a significant atomic component despite bond type. Bonded radii for Si and O ions provided by experimental electron density maps measured for the oxides coesite, danburite and stishovite match those calculated for a series of monosilicic acid molecules. The resulting radii increase with bond length and coordination number with the radius of the oxide ion increasing at a faster rate than that of the Si cation. The oxide ion within danburite exhibits several distinct radii, ranging between 0.9 and 1.2 Å, rather than a single radius with each exhibiting a different radius along each of the nonequivalent bonds with B, Si and Ca. Promolecule radii calculated for the coordinated polyhedra in danburite match procrystal radii obtained in a structure analysis to within 0.002 Å. The close agreement between these two sets of radii and experimentally determined bonded radii lends credence to Slater's statement that the difference between the electron density distribution observed for a crystal and that calculated for a procrystal (IAM) model of the crystal “would be small and subtle, and very hard to determine by examination of the total charge density.”  相似文献   

The paleomagnetism and magnetic polarity stratigraphy of the mammal-bearing section of Anza Borrego State Park, California     

N. D. Opdyke  E. H. Lindsay  N. M. Johnson  T. Downs 《Quaternary Research》1977,7(3):316-329

We sampled 150 sites in fine-grained Plio-Pleistocene sediments of the Palm Springs and Imperial Formations. Sampling was confined to 3000 m of stratigraphically continuous section containing abundant vertebrate fossil remains of the Vallecito Creek, Arroyo Seco, and Layer Cake local faunas of the Irvingtonian and Blancan Land Mammal Ages. The magnetic stability of these sediments was sufficient to delineate the magnetic stratigraphy, which ranges from below the Cochiti event at the base, to the Matuyama reversed magnetic epoch at the top of the section. Eight faunal events are placed relative to the magnetic polarity sequence. They are cf. Pliohippus extinction, Geomys appearance, cf. Equus appearance, Tremarctos appearance, Hypolagus extinction, cf. Odocoileus appearance, Smilodon and ? Euceratherium appearance. The latter two faunal events characterize Irvingtonian Land Mammal Age. The transition from the Blancan to Irvingtonian Land Mammal Age occurs in the lower Matuyama magnetic epoch close to the Pliocene-Pleistocene boundary. The appearance of European migrants during the lower Matuyama magnetic epoch indicates that the Bering land bridge was exposed for animal migration between Europe and North America at this time.  相似文献   

Virilization of the Broad Line Region in Active Galactic Nuclei—connection between shifts and widths of broad emission lines     

S. Jonić  J. Kovačević-Dojčinović  D. Ilić  L. Č. Popović 《Astrophysics and Space Science》2016,361(3):101

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Postglacial vegetation history of Mitkof Island, Alexander Archipelago, southeastern Alaska     

Thomas A. Ager  Paul E. Carrara  Victoria Anne  Joni Johnson 《Quaternary Research》2010,73(2):259-462

An AMS radiocarbon-dated pollen record from a peat deposit on Mitkof Island, southeastern Alaska provides a vegetation history spanning ∼12,900 cal yr BP to the present. Late Wisconsin glaciers covered the entire island; deglaciation occurred > 15,400 cal yr BP. The earliest known vegetation to develop on the island (∼12,900 cal yr BP) was pine woodland (Pinus contorta) with alder (Alnus), sedges (Cyperaceae) and ferns (Polypodiaceae type). By ∼12,240 cal yr BP, Sitka spruce (Picea sitchensis) began to colonize the island while pine woodland declined. By ∼11,200 cal yr BP, mountain hemlock (Tsuga mertensiana) began to spread across the island. Sitka spruce-mountain hemlock forests dominated the lowland landscapes of the island until ∼10,180 cal yr BP, when western hemlock (Tsuga heterophylla) began to colonize, and soon became the dominant tree species. Rising percentages of pine, sedge, and sphagnum after ∼7100 cal yr BP may reflect an expansion of peat bog habitats as regional climate began to shift to cooler, wetter conditions. A decline in alders at that time suggests that coastal forests had spread into the island's uplands, replacing large areas of alder thickets. Cedars (Chamaecyparis nootkatensis, Thuja plicata) appeared on Mitkof Island during the late Holocene.  相似文献   

Strategically Locating Wildlife Crossing Structures for Florida Panthers Using Maximal Covering Approaches     

Joni Downs  Mark Horner  Rebecca Loraamm  James Anderson  Hyun Kim  Dave Onorato 《Transactions in GIS》2014,18(1):46-65

Crossing structures are an effective method for mitigating habitat fragmentation and reducing wildlife‐vehicle collisions, although high construction costs limit the number that can be implemented in practice. Therefore, optimizing the placement of crossing structures in road networks is suggested as a strategic conservation planning method. This research explores two approaches for using the maximal covering location problem (MCLP) to determine optimal sites to install new wildlife crossing structures. The first approach is based on records of traffic mortality, while the second uses animal tracking data for the species of interest. The objective of the first is to cover the maximum number of collision sites, given a specified number of proposed structures to build, while the second covers as many animal tracking locations as possible under a similar scenario. These two approaches were used to locate potential wildlife crossing structures for endangered Florida panthers (Puma concolor coryi) in Collier, Lee, and Hendry Counties, Florida, a population whose survival is threatened by excessive traffic mortality. Historical traffic mortality records and an extensive radio‐tracking dataset were used in the analyses. Although the two approaches largely select different sites for crossing structures, both models highlight key locations in the landscape where these structures can remedy traffic mortality and habitat fragmentation. These applications demonstrate how the MCLP can serve as a useful conservation planning tool when traffic mortality or animal tracking data are available to researchers.  相似文献