GLOSSARY – H

GLOSSARY – H

by: Charles O’Dale

HIGH PRESSURE MINERALS:

Coesite is a form (polymorph) of silicon dioxide SiO2 that is formed when very high pressure (2–3 gigapascals), and moderately high temperature (700 °C or 1,300 °F), are applied to quartz.

Pressure–temperature fields for shock metamorphism of silicate rocks in comparison with endogenous metamorphism of the terrestrial crust. The upper curve of the field for shock metamorphism holds for porous silicate rocks, and the lower curve for dense crystalline silicate rocks based on data for quartz (Wackerle 1962) and porous sandstone (Ahrens and Gregson 1964; Kieffer et al. 1976a). The temperature estimates for the slightly dashed boundaries of the P-T field of shock metamorphism (upper part of ” melting ” and ” vaporization ” ; yellow to orange) are based on theoretical considerations by Davies (1972); the geotherm for continental crust is from Anderson (1981). Modified from St€ offler and Langenhorst (1994).

HUGONIOT EQUATIONS
Also termed Hugoniot-Rankine equations, describing the behavior of material subjected toshock waves.

HYDROTHERMAL
The origin and emergence of life under impact bombardment (Cockell 2006)
Craters formed by asteroids and comets offer a number of possibilities as sites for prebiotic chemistry, and they invite a literal application of Darwin’s ‘warm little pond’. Some of these attributes, such as prolonged circulation of heated water, are found in deep-ocean hydrothermal vent systems, previously proposed as sites for prebiotic chemistry. However, impact craters host important characteristics in a single location, which include the formation of diverse metal sulphides, clays and zeolites as secondary hydrothermal minerals (which can act as templates or catalysts for prebiotic syntheses), fracturing of rock during impact (creating a large surface area for reactions), the delivery of iron in the case of the impact of iron-containing meteorites (which might itself act as a substrate for prebiotic reactions), diverse impact energies resulting in different rates of hydrothermal cooling and thus organic syntheses, and the indiscriminate nature of impacts into every available lithology—generating large numbers of ‘experiments’ in the origin of life.
[see – CRATER CLASSIFICATION– hydrothermal]

HYPERVELOCITY
A velocity approximately over 3,000 meters per second (6,700 mph, 11,000 km/h, 10,000 ft/s, or Mach 8.8). In particular, hypervelocity is velocity so high that the strength of materials upon impact is very small compared to inertial stresses.