GLOSSARY – Q-R

GLOSSARY – Q-R

by: Charles O’Dale

QUARTZ

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

Experimental results and phase boundary of the coesite-stishovite transition in SiO2. Solid squares and circles denote the stability conditions of coesite and stishovite respectively. The dashed line shows the phase boundary determined in this study. Open squares and circles denote coesite and stishovite reported by previous experimental study (Zhang et al., 1996).

Rock-forming silicate mineral SiO2. Quartz is a sensitive indicator of shock metamorphism.

Decorated PDFs – Planar deformation features (PDFs) decorated by fluid and/or mineral inclusions as the result of annealing.

Kentland quarry shocked quartz – deformation at impact. Quartz grain taken from a core sample examined by Smithsonian geologist Bevan French.

RADIOMETRIC DATING
Radiometric dating or radioactive dating is a technique used to date materials such as rocks or carbon, in which trace radioactive impurities were selectively incorporated when they were formed
[see – DATING– radiometric]

RAREFACTION WAVE
Tensional wave originating from the reflection of a compressional wave at a free surface or at a boundary of material of reduced impedance (impedance is the product of density and sound velocity). In impact cratering, rarefaction (tensional) waves play an important role. They are formed when the compressional shock wavesare reflected back downwards from the free surface of the impacted target. Combined with the rock mass flow behind the shock front, they determine the excavation flow field. Because the tensile strength of rocks is always much lower than the compressive strength, rarefaction waves are in general more destructive than the compressional shock waves (see Grady-Kipp fragments). On a smaller scale, rarefaction may lead to characteristic spallation effects in shocked rocks.

REE
The Rare Earth Elements (REE) are the 15 lanthanide series elements, plus yttrium. Scandium is found in most rare earth element deposits and is sometimes classified as a rare earth element.

The Rare Earth Elements are the 15 lanthanide series elements, plus yttrium. Scandium is found in most rare earth element deposits and is sometimes classified as a rare earth element. Image by Geology.com.

Heavy and light rare earth elements: The rare earth elements are often subdivided into “Heavy Rare Earths” and “Light Rare Earths.” Lanthanum, cerium, praseodymium, neodymium, promethium, and samarium are the “light rare earths.” Yttrium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium are the “heavy rare earths.” Although yttrium is lighter than the light rare earth elements, it is included in the heavy rare earth group because of its chemical and physical associations with heavy rare earths in natural deposits.

REFRACTORY
Any material that has a relatively high  condensation temperature.
[see – VOLATILE]

REIDITE (METAMORPHIC ZIRCON), ZrSiO
A high-pressure polymorph of zirconium orthosilicate that forms >30 GPa (during the crater compression stage) and is an important accessory mineral in studies of shock metamorphism as its formation conditions have been experimentally constrained. Naturally occurring reidite is rare. Reidite has been found in crater impacts: the Chesapeake Bay Crater in Virginia, Rock Elm Crater in Wisconsin. Laboratory and theoretical results show how zircon transforms into reidite and then back into zircon.
Structure and stability of ZrSiO4 under hydrostatic pressure, Marqués, et al APS PHYSICS  2006
[see – SHOCK METAMORPHISM]

RINGWOODITE, Mg2SiO4
Formed at high temperatures and pressures of the Earth’s mantle between 525 and 660 km (326 and 410 mi) depth. It is polymorphous with the olivine phase forsterite (a magnesium iron silicate).

RUBINITE, Ca3Ti3+2Si3O12

Rubinite was identified as tiny crystals in calcium-aluminum-rich inclusions (CAIs), and is among the first solids formed in the solar nebula. Researchers say the mineral either condensed from solar nebula gas or it crystallized from an 16O-rich Ca, Al, and Ti-rich melt under highly-reduced conditions about 4.6 billion years ago. Post-crystallization oxygen-isotope exchange occured either while still in the solar nebula and/or on the meteorite parent asteroid.

Type material is deposited in the meteorite collection of the Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA, section VCM3 (Vigarano), and the Division of Earth and Planetary Materials Sc