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I. Shock metamorphism of the Coconino sandstone at Meteor Crater, Arizona. II. The specific heat of solids of geophysical interest

Citation

Kieffer, Susan W. (1971) I. Shock metamorphism of the Coconino sandstone at Meteor Crater, Arizona. II. The specific heat of solids of geophysical interest. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechETD:etd-06232004-134838

Abstract

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PART I: A study of the shocked Coconino sandstone from Meteor Crater, Arizona, was undertaken to examine the role of porosity in the compression of rocks and in the formation of high-pressure phases. A suite of shocked Coconino specimens collected at the crater is divided into five classes, arranged in order of decreasing quartz content. The amounts of coesite, stishovite (measured by quantitative x-ray diffraction) and glass vary systematically with decreasing quartz content. Coesite may comprise one-third by weight of some rocks, whereas the stishovite content does not exceed 1%. The five classes of rocks have distinct petrographic properties, correlated with the presence of regions containing coesite, stishovite or fused silica. Very few occurrences of diaplectic glass are observed, in striking contrast to its abundant occurrence in the non-porous rocks from the Ries Crater.

In the lowest stages of shock metamorphism (Class I), the quartz grains are fractured and the voids in the rock are filled with myriads of small chips derived from neighboring grains. The fracture patterns in the individual quartz grains are controlled by the details of the initial morphology of the colliding grains. In one weakly shocked rock, it was possible to map the general direction of shock passage by recording the apparent direction of collision of individual grains. The principal mechanism of energy deposition by a shock wave in a porous material is the reverberation of shock and rarefaction waves through grains due to collisions with other grains. A one-dimensional model of the impact process can predict the average pressure, volume and temperature of the rock if no phase changes occur, but cannot predict the observed nonuniformity of energy deposition.

In all rocks shocked to higher pressure than was necessary to close the voids, high-pressure and/or high-temperature phases are present. Locally high pressures enduring for microseconds and high temperatures enduring for milliseconds controlled the phases of SiO2 which formed in the rock. Collapsing pore walls became local hot spots into which initial deposition of energy was focused. Microcrystalline coesite in Class II rocks occurs in symplektic regions on quartz grain boundaries which were regions of initial stress and energy concentration, or in sheared zones within the grains. The occurrence and morphology of the coesite-rich regions can be explained only if the transformation from quartz to coesite proceeds slowly in the shock wave. In Class III rocks, microcrystalline coesite occurs in opaque regions which surround nearly isotropic cores of cryptocrystalline coesite. The cores are interpreted to be the products of the inversion of stishovite (or a glass with Si in six-fold coordination) which initially formed in the shock front in regions of grains shocked to pressures near 300 kb. Stishovite is preserved only in the opaque regions, which are believed to have been cooler than the cores.

In Class IV rocks, vesicular glass occurs in core regions surrounded by opaque regions containing coesite. The relation of the glass to the coesite and quartz suggests that the glass was formed by inversion of stishovite formed above 350 kb upon release to lower pressure.

Class V rocks are composed almost entirely of glass with vesicles uniformly distributed in the glass. These vesicles were probably formed by exsolution of water that had been dissolved in melted SiO2 during passage of the shock.

PART II: The use of Debye temperatures as parameters for material properties of silicate minerals is becoming common in geophysical studies. The elastic Debye temperature, [...] alone is, in general, insufficient to specify properties which depend on lattice vibrations. Two effects ignored by the Debye model are shown to be important: high frequency lattice vibrations and the dispersion relation. As an alternative to the Debye model, a somewhat more complicated model is proposed that is still reasonably convenient and is able to account much better than the Debye model does for the variation of specific heat of complex substances over a wide range of temperature. This model is designated the acoustic-optic model. The parameters required for this model are the maximum lattice vibrational frequency, the elastic Debye temperature, and the specific heat at a single (say, room) temperature. Adequate approximations to these parameters are generally available.

To consider heat capacity data and compare the data with either the Debye or acoustic-optic model, the calorimetric Debye temperature, [...] is considered. [...] is the value of Debye temperature that will reproduce the specific heat at constant volume at the temperature T. For silicates, a large increase at high temperatures in [...] above the elastic Debye temperature is due to the presence of oscillators between the elastic Debye frequency and a maximum vibrational frequency which exceeds the Debye frequency. Vibrations of these oscillators cause the spectral lines observed at infrared frequencies. The proposed model takes these oscillators into account by adding a constant valued continuum to an assumed low-frequency Debye spectrum.

In all substances considered, [...] at low temperatures initially decreases to values below [...]. This decrease is believed due to the dispersion relation, i.e., the nonlinear relation between the wave vector and the frequency.

In two substances of geophysical interest, NaCl and MgO, the maximum observed vibrational frequency is depressed below the observed elastic Debye frequency as a consequence of the dispersion relation. The acoustic-optic model is capable of predicting the specific heat of these two substances and the acoustic-optic spectra which result from application of the model to these substances describe qualitatively the spectra that result from the dispersion relation.

The nonlinearity of the dispersion relation dominates the specific heat behavior of NaCl and MgO and influences the low temperature behavior of all silicates with varying degrees of severity. The effect of the dispersion relation can be ignored for some silicates if specific heats only at high temperatures (T > 100[degrees]K) are considered. The effect cannot be ignored in the case of rutile or stishovite.

Item Type:Thesis (Dissertation (Ph.D.))
Degree Grantor:California Institute of Technology
Division:Geological and Planetary Sciences
Major Option:Geological and Planetary Sciences
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Shoemaker, Eugene Merle
Thesis Committee:
  • Unknown, Unknown
Defense Date:14 December 1970
Record Number:CaltechETD:etd-06232004-134838
Persistent URL:http://resolver.caltech.edu/CaltechETD:etd-06232004-134838
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:2695
Collection:CaltechTHESIS
Deposited By: Imported from ETD-db
Deposited On:24 Jun 2004
Last Modified:26 Dec 2012 02:53

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