Plastic shear significantly reduces the phase transformation (PT) pressure when compared to hydrostatic conditions. Here, a paradoxical result was obtained: PT of graphitelike hexagonal boron nitride (hBN) to superhard wurtzitic boron nitride under pressure and shear started at about the same pressure ( approximately 10 GPa) as under hydrostatic conditions. In situ x-ray diffraction measurement and modeling of the turbostratic stacking fault concentration (degree of disorder) and PT in hBN were performed. Under hydrostatic pressure, changes in the disorder were negligible. Under a complex compression and shear loading program, a strain-induced disorder was observed and quantitatively characterized. It is found that the strain-induced disorder suppresses PT which compensates the promotion effect of plastic shear. The existence of transformation-induced plasticity (TRIP) was also proved during strain-induced PT. The degree of disorder is proposed to be used as a physical measure of plastic straining. This allows us to quantitatively separate the conventional plasticity and transformation-induced plasticity. Surprisingly, it is found that TRIP exceeds the conventional plasticity by a factor of 20. The cascade structural changes were revealed, defined as the reoccurrence of interacting processes including PTs, disordering, conventional plasticity, and TRIP. In comparison with hydrostatic loading, for the same degree of disorder, plastic shear indeed reduces the PT pressure (by a factor of 3-4) while causing a complete irreversible PT. The analytical results based on coupled strain-controlled kinetic equations for disorder and PT confirm our conclusions.

40Ar/39Ar dating of fine-grained alunite that formed during cave genesis provides ages of formation for the Big Room level of Carlsbad Cavern [4.0 to 3.9 million years ago (Ma)], the upper level of Lechuguilla Cave (6.0 to 5.7 Ma), and three other hypogene caves (11.3 to 6.0 Ma) in the Guadalupe Mountains of New Mexico. Alunite ages increase and are strongly correlative with cave elevations, which indicates an 1100-meter decline in the water table, apparently related to tectonic uplift and tilting, from 11.3 Ma to the present. 40Ar/39Ar dating studies of the hypogene caves have the potential to help resolve late Cenozoic climatic, speleologic, and tectonic questions.

AugzugDie Kristallstrukturen von 2iVfi-Phengit und 23/i-Muskowit wurden bis zu R = 0,045 für Phongit und R = 0,035 für Muskowit verfeinert. Beide Minerale haben die Raumgruppe C2/c. Die Gitterkonstanten von Phengit sind: a = 5,2112 ± 0,0003 Ä, b = 9,0383 ± 0,0004 Ä, c = 19,9473 ± 0,0006 Ä, ß = 95,769 ± 0,005°, die für Muskowit: a = 5,1906 ± 0,0002 Ä, 6 = 9,0080 ± 0,0003 Ä, c = 20,0470 ± 0,0006 Ä, = 95,757 ± 0,002°. Im 2Mi-Muskowit sind die Kation-Sauerstoflf-Abstände in den Tetraedern Ti und T2 einander gleich, Ti-O = T2-O = 1,643 Ä, also die (Si, Al)-Lagen nicht geordnet. Die Differenz der entsprechenden Abstände im 2Afi-Phengit, Ti-O = 1,622 und T2-O = 1,633, weist auf eine mäßige Ordnung hin. Das Anionengerüst ist im 2Mi-Muskowit etwa doppelt so stark gestört wie im 2Mi-Phengit. AbstractThe crystal structures of 2 Mi phengite and 2 Mi muscovite have been refined by least-squares methods using single-crystal diffractometer data. Final discrepancy factors were unweighted R = 0.045 for 2Mi phengite and 0.035 for 2 Ml muscovite. Both specimens have the space group C2/c and the following cell dimensions at room temperature: 2 Ml phengite 2 Mi muscovite a 5.2112 ± 0.0003 Ä 5.1906 ± 0.0002 Ä b 9.0383 ± 0.0004 9.0080 ± 0.0003 c 19.9473 ± 0.0006 20.0470 ± 0.0006 ß 95.769 ± 0.005° 95.757 ± 0.002°.In 2Mi muscovite tetrahedral cation-oxygen distances Ti-0 = T2 -0= 1.643Ä show that there is no ordering of (Si,Al) over the tetrahedral sites. In the Brought to you by | New York University Bobst Library Technical Services Authenticated Download Date | 7/12/15 5:41 AM The crystal striictures of 2Mi phengite and 2.Vi muscovito 197 structure of 2J/i phongite, however, the corresponding values, Ti -0= 1.622 and Ta -0= 1.633Ä, indicate a very slight ordering. The framework of anions is distorted twice as mach in 2J/i inuseovite than in 2Mi phengite.