Crater, Meteor

Pressure—temperature diagrams for the coesite—quart2 equilibrium have been summari2ed (23). Coesite has been found ia nature ia the meteor crater ia Ari2ona. 

Stishovite. Stishovite was first prepared (68) ia the laboratory ia 1961 at 1200—1400°C and pressures >16 GPa (158,000 atm). It was subsequentiy discovered, along with natural coesite, ia the Ari2ona meteor crater. It has been suggested that these minerals are geological iadicators of meteorite impact stmctures. Stishovite (p = 4.35 g/cm ) is the densest known phase of silica. The stmcture, space group P42/nmn is similar to that of... 

Some wave phenomena, familiar to many people from the human senses, include the easy undulation of water waves from a dropped stone or the sharp shock of the sonic boom from high-speed aircraft. The great power and energy of shock events is apparent to the human observer as he stands on the rim of the Meteor Crater of Arizona. Human senses provide little insight into the transition from these directly sensed phenomena to the high-pressure, shock-compression effects in solids. This transition must come from development of the science of shock compression, based on the usual methods of scientific experimentation, theoretical modeling, and numerical simulation. 

Defects in Coesite and Stishovite. The high-pressure phase of Si02, coesite and stishovite were synthesized and irradiated by y-rays to check the ESR spectra and to find ones formed by impacts at meteor craters. 

Six different silica modifications were used vitreous silica, quartz, cristobalite, tridymite, coesite, and stishovite. Two of these—cristobalite and tridymite—were prepared from fine amorphous silica powder by tempering samples at 950°C. with 1% of a mineralizer (K2CO3 and KH2P04, respectively). Vitreous silica was obtained from fused rock crystals. All other samples were natural minerals pure specimens of Brazilian rock crystal were used as quartz coesite and stishovite were obtained as fine powders by isolation from Coconino sandstone of the Barringer Meteor Crater in Arizona (4). 

In view of this, the dissolution patterns of all other silica modifications were interpreted disregarding the condensation reaction. This seemed particularly adequate for stishovite, a high pressure, high temperature material, first produced in an autoclave by Stishov and Popova (17) in 1961 and detected at Meteor Crater one year later (6). The lattice... 

Spencer, L. f. Meteoric iron and silica-glass from the meteoric craters of Henbury (Central Australia) and Wabar (Arabia). Min. Mag. XXIII, No. 142, 387-404 (1933). 

Some meteoroids shatter upon impact, while others remain intact. Those that break when they hit the ground may be found in fragments around the site of impact. Some of these fragments may weigh a couple of tons All of the meteorites from a single fall are given the same name. Thus, the many hundreds of meteorites that have been found at Meteor Crater in Arizona are called by the name Canyon Diablo. 

Dense forms of Si02, called coesite and stishovite, were first made under drastic conditions (250-1300°C at 35-120 katm), but they were subsequently identified in meteor craters where the impact conditions were presumably similar stishovite has the rutile structure. Both are chemically more inert than normal Si02 to which they revert on heating. 

These events liave sporadically happened even in our history. The Meteor Crater of Arizona is tliought to liave been formed between 20,000 and -30,000 years ago [58], Explosion of Tunguska meteor in 1908 and collision of Comet Shoemaker-Levy 9 against Jupiter in 1994 are the representative great events in the history. 

Small amounts of iridium can be found in meteorites. The Barrin on Crater (also known as Meteor Crater), in northern Arizona, was created about 25,000 years ago by a meteorite the size of a large house. It hit the ground at 9 miles per second, and it created... 

In the last ten years, at least a dozen polymorphs of pure Si02 have been reported [6], Stishovite, another form of silica obtained at high temperatures and pressures, has, rather than a tetrahedral-based geometry, a rutile (Ti02) structure in which each Si atom is bonded to six O atoms and each O atom bridges three Si atoms [6], Stishovite (found in Meteor Crater, Arizona) is more dense and chemically more inert than normal silica but reverts to amorphous silica upon heating. 

Nishiizumi K, Kohl CP, Shoemaker EM, Arnold JR, Klein J, Fink D, Middleton R (1991b) In situ Be- A1 exposure ages at Meteor Crater, Arizona. Geochim Cosmochim Acta 55 2699-2703 Nishiizumi K, Lai D, Klein J, Middleton R, Arnold JR (1986) Production of °Be and A1 by cosmic rays in terrestrial quartz in situ and implications for erosion rates. Nature 319 134-136 Nishiizumi K, Winterer EL, Kohl CP, Klein J, Middleton R, Lai D, Arnold JR (1989) Cosmic ray production rates of °Be and A1 in quartz from glacially polished rocks. J Geophys Res 94 17907-17915... 

Meteorites are meteors that strike the earth s surface. A physical example of the impact of the meteorite on the earth s surface can be seen in Arizona, The Barringer Crater is a huge Meteor Crater. There many other such meteor craters found throughout the world. 

Shoemaker EM, Kieffer SW (1974) Guidebook to the geology of Meteor Crater, Arizona. Pub. 17. Center for Meteorite Studies, Arizona State University, Tempe, AZ Steed RHN, Drewry DJ (1982) Radio-echo sounging investigations of Wilkes Land, Antarctica. In Craddock C (ed) Antarctic geoscience. University of Wisconsin Press, Madison, WI... 

However, the diamond Dg is not unique out of the classical structure, showing all-hexagonal rings of sp carbon atoms in a cubic network (space group Fd3m), there is Lonsdaleite (Frondel and Marvin 1967 He et al. 2002) a rare stone of pure carbon discovered at Meteor Crater, Arizona, in 1967 and also several hypothetical diamond-like networks (Sunada 2008 Diudea et al. 2010). The Lonsdaleite hexagonal network (space group P6 lmmc) is illustrated in Fig. 11.2. 

Coesite was first synthesised at a pressure of about 3.4 GPa, in the temperature range of 500-800 °C in the laboratory by Coes (1953) and later discovered and identified from shocked Coconino sandstone of the Meteor Crater in Arizona (Chao et al. 1960, Bohn and StOber 1965), and from the Ries Crater in NordUngen, Bavaria. It has a specific gravity of 2.91510.015 and a hardness of about 8. It is biaxial positive with 2V about 64°. Its indices of refraction are a 1.5940, P 1.5955, and y 1.597010.0005. It is nearly insoluble in 5 % HF at room temperature. 

Coesite was discovered by Coes, in 1953 (59). It is made from amorphous silica in the same temperature range as for keatite, but at 10 times the pressure and with weakly acidic catalysts such as boric acid or ammonium chloride (59). It was found in nature in I960 at Meteor Crater, Arizona, apparently formed under the high temperature and pressure conditions of the impact. 

Similarly, stishovite was first made in the laboratory in 1961 by Stishov and Popova (60) and di,scovered in Meteor Crater by Chao, Shoemaker, and Madsen, in 1962 (61). A most interesting story of the isolation of substantial amounts of coesite and stishovite from the crater Is told by Bohn and Stober (62. 63). 

Earth-Moon Differences. Scientists believe that the moon s surface has a large number of craters formed by the impact of meteorites. In contrast, there are relatively few meteorite craters on the Earth, even though, based simply on its size, the Earth is likely to have been hit by as many or even more meteorites than the Moon. This notable difference is attributed to the Earth s atmosphere, which bums up incoming meteorites, particularly small ones (the Moon does not have an atmosphere). Larger meteorites can pass through the Earth s atmosphere, but their impact craters may have been filled in or washed away over millions of years. Only the more recent ones, such as Meteor Crater in northern Arizona, with a diameter of 4,000 feet and a depth of 600 feet, remain easily recognizable. 

Lonsdaleite is a crystalline form of carbon, and is named after the English crystallographer K.Y. Lonsdale (1903-71). It is polymorphous with the other hexagonal forms of crystalline carbon, chaoite and graphite (qq.v.), and the cubic form, diamond. Lonsdaleite was first described by Frondel and Marvin in 1967 from the Canyon Diablo meteorite (Meteor Crater, Arizona,... 

Coesite is a monoclinic sihca mineral that exists at pressures above a few GPa and transforms to stishovite above about 10 GPa. Coesite was first identified as a natnral phase at Meteor Crater, AZ [433] and coesite can assist in the recognition of meteorite impacts. The mineral occurs as a shock-induced phase in meteorites and tektites. It also is fonnd in some high-pressure metamorphic rocks as in the Dora Maira massif of the Western Alps. A TEM study found Burgers vectors [100], [001], and [110], which correspond to a and a-l-c [72]. Twinning occurs on (021) [434]. 

One impact crater that remains intact is Arizona s Meteor Crater, found 20 km west of Winslow, Arizona. This crater, measuring 200 m deep and 1.2 km in diameter, was formed about 50,000 years ago by a meteorite 45 m in diameter. The impact of this relatively small object had an energy roughly equivalent to 20 millions tons of TNT—about the same as a hydrogen bomb. However, most impact craters are not so obvious. 

Coesite, which can be produced synthetically (Coes [1953]), has been found naturally in rocks subjected to the impact of large meteorites and stishovite, the polymorph with the highest density (d 4.2), has also been recognized in a meteor crater (Stishov and Popova [1961]). The quartz-coesite stability relations have been investigated by MacDonald [1956] and Boyd and England [1960] at 400 to 800°C and 80,000 tons pressure. From the estimated thermal gradient of the earth, quartz should, therefore, invert to coesite at a depth of 60 to 100 km. 

---