Shock metamorphism of silicate glasses

The changes in refractive index caused by shock compression have been determined for tektite, soda-lime, and silica glasses shocked to pressures up to 460 kb. For shock compression below 80 kb for fused silica and 40 kb for tektite and soda-lime glasses, compression is reversible as the refractive indices are within 0.0025 of the starting values. Index increases of 0.01, 0.04, and 0.06 are observed for soda-lime, tektite, and silica glasses shocked to pressures of 80, 130, and 140 kb respectively. For soda-lime glass subjected to shock pressures between 80 and 230 kb there is a decrease in the postshock refractive index to n=1.5211 at 230 kb. For fused silica shocked to pressures of 140 to 460 kb, refractive index drops from 1.52 to 1.47. The reasons for these decreases in index are not obvious. New values for postshock temperatures for fused silica based on release adiabat data, e.g. ∼1000°C for a shock state at 250 kb, suggest that the decreases in refractive index are caused by a combination of decompression along release adiabats and reconstructive transformation from a shock-induced stishovitelike phase to a low-density glass. Postshock densities calculated from the refractive index data agree closely with those calculated from the release adiabat data

Effects of Shock Pressures on Calcic Plagioclase

Samples of single crystal calcic plagioclase (labradorite, An63, from Chihuahua, Mexico) have been shock-loaded to pressures up to 496 kbar. Optical and electron microscopic studies of the recovered samples show the effects of increasing shock pressures on this mineral. At pressures up to 287 kbar, the recovered specimens are still essentially crystalline, with only a trace amount of optically unresolvable glass present at 287 kbar. Samples recovered after shock-loading to pressures between 300 and 400 kbar are almost 100% diaplectic glasses; that is formed by shock transformation presumably in the solid-state. Above about 400 kbar, glasses with refractive indices similar to thermally fused glass were produced. The general behavior of the index of refraction with shock pressures agrees closely with previous work, however, the absence of planar features is striking. At pressures less than 300 kbar, the most prominent physical feature is the pervasive irregular fracturing caused by the shock crushing, although some (001) and (010) cleavages are observed. No fine-scale shock deformation structures, i.e. planar features, were noted in any of the specimens. We conclude, in contrast to previous studies of shocked rocks that planar features are not necessarily definitive shock indicators, in contrast to diaplectic glass (e.g., maskelynite) and high-pressure phases, but are rather likely indicative of the local heterogeneous dynamic stress experienced by plagioclase grains within shocked rocks

Shock compression of a recrystallized anorthositic rock from Apollo 15

Hugoniot measurements on 15,418, a recrystallized and brecciated gabbroic anorthosite, yield a value of the Hugoniot elastic limit (HEL) varying from - 45 to 70 kbar as the final shock pressure is varied from 70 to 280 kbar. Above the HEL and to 150 kbar the pressure-density Hugoniot is closely described by a hydrostatic equation of state constructed from ultrasonic data for single-crystal plagioclase and pyroxene. Above - 150 kbar, the Hugoniot states indicate that a series of one or more shock-induced phase changes are occurring in the plagioclase and pyroxene. From Hugoniot data for both the single-crystal minerals and the Frederick diabase, we infer that the shock-induced high-pressure phases in 15,418 probably consist of a 3.71 g/cm^3 density, high-pressure structure for plagioclase (An_(93)) and a 4.70 g/cm^3 perovskite-type structure (En_(64)) for pyroxene. Using the Kelly Truesdell mixture theory we separately calculated the entropy production in each phase, and predict incipient and complete melting in the plagioclase occurs upon release from ~ 500 and ~ 600 kbar. For the pyroxene component, incipient and complete melting occurs upon release from 700 and 850 kbar. The onset of shock-induced vaporization will occur upon release from ~ 1300 kbar and would require the impact of an iron meteoroid traveling at a velocity of ~8 km/sec

A Spectrographic Interpretation of the Shock-Produced Color Change in Rhodonite (MnSiO_3): The Shock-Induced Reduction of Mn(lll) to Mn(II).

Samples of rhodonite (MnSiO_3-pyroxenoid from Franklin, New Jersey) have been shockloaded to pressures up to 496 kilobars. Optical spectral studies of four recovered samples show a decreasing Mn^(3+) content upon recovery from successively higher shock pressures; after shock-loading to 496 kbar, the Mn^(3+) has essentially disappeared. No corresponding change in the optical spectrum results from heating rhodonite to 1250°C for 3.5 hours in a reducing atmosphere. Rhodonite heated to 1360° under the same conditions melts incongruently to manganese-rich glass and silica with disappearance of the 540 nm Mn^(3+) absorption band. The color change in the shocked rhodonite arises from irreversible reduction of Mn3+ during high shock pressures and possible high shock temperatures. It is suggested that Mn^(3+) is reduced to Mn^(2+) by water present in the sample during the shock event

New Structural Templates for Clinically Validated and Novel Targets in Antimicrobial Drug Research and Development.

The development of bacterial resistance against current antibiotic drugs necessitates a continuous renewal of the arsenal of efficacious drugs. This imperative has not been met by the output of antibiotic research and development of the past decades for various reasons, including the declining efforts of large pharma companies in this area. Moreover, the majority of novel antibiotics are chemical derivatives of existing structures that represent mostly step innovations, implying that the available chemical space may be exhausted. This review negates this impression by showcasing recent achievements in lead finding and optimization of antibiotics that have novel or unexplored chemical structures. Not surprisingly, many of the novel structural templates like teixobactins, lysocin, griselimycin, or the albicidin/cystobactamid pair were discovered from natural sources. Additional compounds were obtained from the screening of synthetic libraries and chemical synthesis, including the gyrase-inhibiting NTBI's and spiropyrimidinetrione, the tarocin and targocil inhibitors of wall teichoic acid synthesis, or the boronates and diazabicyclo[3.2.1]octane as novel β-lactamase inhibitors. A motif that is common to most clinically validated antibiotics is that they address hotspots in complex biosynthetic machineries, whose functioning is essential for the bacterial cell. Therefore, an introduction to the biological targets-cell wall synthesis, topoisomerases, the DNA sliding clamp, and membrane-bound electron transport-is given for each of the leads presented here