Modeling of dioctahedral 2:1 phyllosilicates by means of transferable empirical potentials

 Dioctahedral 2:1 phyllosilicates with different interlayer charge have been studied theoretically by using transferable empirical interatomic potentials. The crystal structures of pyrophyllite, muscovite, margarite, beidellite, montmorillonite, and different smectites and illites have been simulated. The interatomic potentials were able to reproduce the experimental structure of phyllosilicates with high, medium and low interlayer charge. The calculated structures are in agreement with experiment for the main structural features of the crystal lattice. The effect of the cation substitution in the octahedral and tetrahedral sheets on the structural features has been also studied. Good linear relationships have been found, and the calculated effects are consistent with experimental results. Some unknown structural features of the crystal structures of clays are predicted in this work. Received: 8 March 2000 / Accepted: 19 September 2000     

Paleomagnetism of certain constituents of the Bay St. George sub-basin,western Newfoundland

Paleomagnetic studies have been made of certain constituents of the Bay St. George sub-basin. Specifically, results are reported from the Spout Falls Formation (Tournaisian), the Jeffreys Village Member of the Robinsons River Formation (Visean), and the Searston Formation (Namurian-Westphalian). The following magnetizations have been isolated: Spout Falls A (Tournaisian) with D = 343.5°, I = ?22.7°, k = 61.2, α₉₅ = 7.1° and the corresponding pole at 28.6°N, 139.5°E (4.5°, 8.5°); Spout Falls B (Kiaman) with D = 166.7°, I = 12.2°, k = 51.7, α₉₅ = 10.7° and the corresponding pole at 34.5°S, 42.7°W (5.5°, 10.9°); Jeffreys Village A (Visean) with D = 351.2°, I = ?27.3°, k = 54.0, α₉₅ = 7.6° and the corresponding pole at 26.5°N, 130.7°E (4.5°, 8.3°); Searston A (Namurian) with D = 161.7°, I = 11.7°, k = 107, α₉₅ = 7.4° and the corresponding pole at 33.9°S, 37.2°W (3.8°, 7.5°); and Searston C with D = 111.6°, I = ?13.8°, k = 28.8, α₉₅ = 14.5° and the corresponding pole at 19.6°S, 19.0°E (7.6°, 14.8°). After comparison with paleopoles of similar ages derived from eastern and western Newfoundland rocks, from constituents of the east coast basin and for interior North America, it is concluded that: (1) it is unlikely that any large scale relative motion took place since the Early Carboniferous between eastern and western Newfoundland; (2) it is unlikely that any north-south relative motion took place between the east coast basin and the Bay St. George sub-basin; and (3) the Bay St. George sub-basin results do not support the earlier proposed displaced terrane hypothesis of the northern Appalachians in as much as the motions during the Carboniferous are not supported. There is evidence of the northward motion of the Appalachians and North America as a whole during the Carboniferous. The magnetostratigraphic horizon marker in the Carboniferous separating a dominant normal and reversed magnetization on the older side and an entirely reversed (Kiaman) magnetization on the younger side may be placed in the Bay St. George sub-basin at the base of the Searston Formation.     

Discussion: Structural and stratigraphic problems of the Argylla Formation near Mary Kathleen,Queensland

Circumstantial evidence indicates that Gaussberg, an isolated, 370 m high volcanic cone on the Antarctic coast at 57°S, 89°E, is the product of subglacial eruption. The vesicular, highly potassic leucitite, of which Gaussberg is composed, has been dated by K‐Ar and fission‐track methods, the former being applied to leucite concentrates, the latter to glassy leucitite from the ropy‐textured, outer rind of a pillow‐like structure. The K‐Ar geochronology yields an average date of 56 000 ± 5000 years, jwhich is interpreted as defining the time of Gaussberg's formation. The fission‐track work yields a less precise date, which supports the K‐Ar age estimate. These new age determinations indicate that previously published K‐Ar age determinations of 20 Ma and 9 Ma for Gaussberg should be rejected.