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391.
Experimental studies were carried out to evaluate phase relationsinvolving titaniteFAl-titanite solid solutionin the system CaSiO3Al2SiO5TiO2CaF2. Theexperiments were conducted at 9001000°C and 1·14·0GPa. The average F/Al ratio in titanite solid solution in theexperimental run products is 1·01 ± 0·06,and XAl ranges from 0·33 ± 0·02 to 0·91± 0·05, consistent with the substitution [TiO2+]1[AlF2+]1.Analysis of the phase relations indicates that titanite solidsolutions coexisting with rutile are always low in XAl, whereasthe maximum XAl of titanite solid solution occurs with fluoriteand either anorthite or Al2SiO5. Reaction displacement experimentswere performed by adding fluorite to the assemblage anorthite+ rutile = titanite + kyanite. The reaction shifts from 1·60GPa to 1·15 ± 0·05 GPa at 900°C, from1·79 GPa to 1·375 ± 0·025 GPa at1000°C, and from 1·98 GPa to 1·575 ±0·025 GPa at 1100°C. The data show that the activityof CaTiSiO4O is very close to the ideal molecular activity model(XTi) at 1100°C, but shows a negative deviation at 1000°Cand 900°C. The results constrain 相似文献
392.
The formation of ooids 总被引:5,自引:0,他引:5
Field and laboratory studies suggest that different types of ooids form during quiet and agitated water conditions. Both types have been synthesized in the laboratory. Quiet water types exhibit a radial orientation of carbonate crystals, whereas in those formed in agitated conditions, a tangential orientation is prevalent. Successful laboratory formation of quiet water ooids was accomplished in supersaturated seawater solutions containing humic acids. Negative results were obtained from strictly inorganic solutions, and from those containing simple amino acids, single proteins, mixtures of proteins or mucopolysaccharides, soil and sediment extracts. Partly successful results were obtained using an organic extract from Bahamian ooids. The organic parameters most important in quiet water ooid formation are molecular weight, the presence of carboxyl groups and an ability to participate in hydrophobic/hydrophilic interactions, all of which are critical to membrane formation. Membranes form concentric shells which act as growth surfaces for carbonate and also induce the periodicity in carbonate precipitation. Ooids exhibiting a tangential orientation of batten-like crystals have been synthesized under conditions of agitation, supersaturation and without the intervention of organic processes during the precipitation. Complete growth may be divided into agitation, resting and sleeping stages In the agitation stage, quartz nuclei induce an inorganic, heterogeneous nucleation from a supersaturated solution, which finally ceases as a result of Mg2+ and possibly H+ poisoning of the carbonate surfaces. No further precipitation occurs until the crystal surfaces are reactivated by removal of Mg2+ and H+ during the resting stage. Following a series of agitation and resting stages, precipitation is inhibited by a degree of poisoning which is not totally removed during the resting stage. For further growth, a new substrate is required and is provided by the development of organic membranes around the grains. This occurs when the grains are buried in the subsurface, the period of organic growth constituting the sleeping stage. Only 2% of an ooid's life is spent growing in the agitated environment, while 95% of its life is spent accreting organic membranes in the subsurface. Our experimental work indicates that ooids of Bahamian type are inorganic precipitates. The tangential arrangement of battens is the result of suspension in an environment where the degree of turbulence is sufficient to induce grain to grain contact of sufficient strength and frequency to inhibit any crystal growth other than tangential. The role of organics is to provide a substrate for further growth after precipitation has slowed to a point when no further accretion is occurring. 相似文献
393.
PETER A. RIEZEBOS RUDOLF T. SLOTBOOM 《Boreas: An International Journal of Quaternary Research》1978,7(2):75-82
Pollen analysis and radiocarbon dating of a core section from the valley head of the Husterbaach near Hosingen (Oesling) provides a very detailed record of the successive vegetations from 1200 A. D. until today. This palynological record agrees largely with the consecutive events known from the agricultural history of that area. Particularly significant are the curves obtained of several cultivated and ruderal plants. In this section it could be demonstrated that the interruption of peat growth by deposition of minerogenic material corresponds with a comparatively high arable land/pasture ratio. As well as being intrinsically interesting for assessing which features of the present landscape are due to former human activities, a detailed knowledge of the Subatlantic evolution of the vegetation and land use in the Oesling is indispensable, as its geomorphological development in the immediate past has largely been influenced by man. 相似文献
394.
PETER H. BANHAM 《Boreas: An International Journal of Quaternary Research》1977,6(2):101-105
By analogy with hard rock myloniles it is suggested that the base of a glacial, litho-structural, stratigraphical unit should he taken at the lower limit of penetrative structures in (soft) bedrock. Reference is made to mylonites in the NW Basal Gneiss Complex of South Norway and to the Marly Drift of Norfolk, England. 相似文献
395.
396.
Hydrothermal synthesis and investigations of stability relationsof MgAl pumpellyite were conducted using high-pressurecold-seal apparatus over the temperature range 250600°C and 28 kb Pfluid. MgAl pumpellyite Ca4Al5MgSi6O21(OH)7was synthesized from partially crystalline gel mixtures of stoichiometriccomposition at 275410 °C, 69 kb Pfluid, andruns of 790 days. Pure monomineralic synthetic MgAlpumpellyite has refractive index nß = 1.624 (2) andcell dimensions = 8.825 (8) Á, b = 5.875 (5) Á,c = 19.10 (1) Á, and ß = 97.39 (7)°. The high temperature assemblage of the equivalent bulk compositionconsists of clinozoisite, hydrogrossular/grossular, aluminousseptechlorite/chlorite, quartz, and H2O. Hydrogrossular wassynthesized in the presence of quartz at 8 kb from 400500°C, and hydrogrossular + quartz are unstable with respectto grossular + H2O at 400 °C and 8 kb Pfluid. At 8 kb Pfluid,aluminous septechlorite forms at temperatures below 500 °Cwhereas aluminous 14 Á chlorite crystallizes at 500600°C. The equilibrium relations of MgAl pumpellyite were determinedusing subequal mixtures of synthetic MgAl pumpellyiteand its high temperature assemblage. The reaction 9 MgAlpumpellyite = 9 clinozoisite + 6 grossular + 2 chlorite + 4quartz + 19 H2O occurs at temperatures of 390 °C at 8 kb,368 °C at 5 kb, and near 325 °C at 2 kb Pfluid. Thereversal data yield an approximate value of 3141 joules/mole°K for the standard entropy of formation for the syntheticMgAl pumpellyite. The Schreinemakers' relations for pumpellyite, prehnite, clinozoisite,tremolite, grossular, and amesite in the presence of excessquartz and fluid were constructed in the pseudo-ternary systemCaOAl2O3MgO(SiO2H2O). The results, togetherwith reconnaissance experiments on the reaction 4 MgAlpumpellyite + 2 quartz = 8 prehnite + aluminous septechlorite+ 2 H2O, locate the invariant point [TR] at approximately 5.7kb Pfluid and 375 °C. The results of the present study arenot compatible with previous experimental data on the invariantpoint [GR]. The PT oriented phase relations are used to interpretsome natural parageneses developed in low-grade metabasalticrocks recrystallized under conditions of low co2. The high-temperaturestability relations of MgAl pumpellyite are useful todenote the onset of greenschist facies metamorphism in rocksof basaltic composition. 相似文献
397.
398.
BENT FREDSKILD PETER WAGNER 《Boreas: An International Journal of Quaternary Research》1974,3(3):105-108
Investigation of two Late-Wisconsin and two sub-recent core samples from the Greenland Ice Cap has shown the presence ol determinable pollen, wood fragments and other plant remains in the ice. 相似文献
399.
400.
CLARKE D. BARRIE; MACDONALD MICHAEL A.; REYNOLDS PETER H.; LONGSTAFFE FRED J. 《Journal of Petrology》1993,34(4):653-679
The South Mountain Batholith is a peraluminous granitic complexranging in composition from biotite granodiorite to muscovite-topazleucogranite. Leucogranitic rocks (with generally<2% biotite) form a minor part ({small tilde}15%) ofthe batholith, and are of two types: (1) associated leucogranitesoccurring as relatively small zones in fine-grained leucomonzogranites;and (2) independent leucogranites forming generallylarger bodies having no particular spatial association withother rock types. Mean chemical compositions of these two typesof leucogranite are as follows (associated, independent): Na2O(346,383),K2O(440,409),andP2O5 (026, 045)in wt.%;Li(149, 281), F(1199, 2712),Rb (393, 725), U (74, 44), Nb (128, 234),Ta (29, 71), and Zr (52, 31) in ppm. Rare earthelements also differ between the two types (associated, independent):REE (341 ppm, 199 ppm); and in the degree and variabilityof heavy REE fractionation (GdN/YbN=4622, 2007).In addition, associated leucogranite has REE compositions similarto those of its host rocks. Mean 18O values (associated +ll212,independent +ll405; relative to SMOW) are comparablewith the mean for the entire South Mountain Batholith (+l0807).Radiometric dating (40Ar/39Ar on muscovite) shows that bothtypes of leucogranite have identical ages of 3723 Ma, equivalentto ages determined by other techniques for granodiorite andmonzogranite samples elsewhere in the batholith. Field relationsand geochemistry suggest that the associated leucogranite resultsfrom an open-system interaction between a fluid and its hostleucomonzogranite, whereas the independent leucogranite bodiesare discrete intrusions of highly fractionated melts that underwentclosed-system, late-magmatic to post-magmatic fluid alteration.Where mineralized, the associated leucogranite characteristicallyhosts greisen-type or disseminated polymetallic mineralization,whereas the independent leucogranite hosts pegmatitic or disseminatedpolymetallic mineralization. 相似文献