Natural coesite, the high density monoclinic polymorph of silica, is found in shocked and melted sandstone at Meteor Crater, Arizona. Structural, morphological, and optical data have been obtained from synthetic coesite crystals and aggregates. Natural coesite occurs as microcrystalline anhedral grains which are unsuitable for the above determinations. In this study the following new data have been obtained for natural and synthetic coesite.
(1) The calculated and observed d spacings and their intensities for both natural and synthetic coesite have been reinvestigated through a 28 range from 100 to 750 for CuKo
(2) Nearly all grains of synthetic coesite observed in this study were anhedral. Those showing crystal development, the most common morphological forms, in decreasing frequency are:
(1) pseudohexagonal platelets parallel to (010) elongated parallel to c, (ii) lath-shaped crystals parallel to (100) and elongated parallel to c, and (iii) exactly pseudohexagonal platelets parallel to (010).
(3) The optical constants for synthetic coesite determinedby the immersion method for white light are: Nx = 1.596, Ny = 1.597, Nz = 1.600; (+)2V = 60 calculated £ran the indices of refraction. The optical orientation is x = b, and z^c = 00- 30.
(4) The solubility of natural coesite in 10, 5, and 1 percent solutions of hydrofluoric acid is much less than either lechatelierite (silica glass) or quartz.
(5)Coesite can be separated from shocked sandstone by repeated hydrofluoric acid treatments. No separation was achieved by heavy liquid techniques.
(6)The X-ray diffractometer method is found to be an effective method for quantitative analysis of coesite in shocked and melted sandstone. Optical methods are not possible because of the fine grain size and low birefringence of coesite. Chemical techniques are not applicable because of the exact chemical identity of coesite to other silica polymorphs.
At Meteor Crater, Arizona, coesite is found in shocked and melted Coconino Sandstone which constitutes (i) part of the layer of mixed debris (fallout layer) under the crater floor, (ii) part of the underlying breccia lens, and (iii) sparse disseminations in the Pleistocene and Recent alluvium (reworked mixed debris). The coesite content in the layer of mixed debris averages less than 1 percent.
Quartz grains comprise from 20 to 70 percent of the shocked sandstone, and are within a matrix of lechatelierite (n = 1.46 - 1.47). Two distinct types of fractures are present in shocked sandstone:
(1)A megascopic single set of parallel, discontinuous, wavy tension fractures which are from 0.05 to 0.4 mm in width, 0.1 to several millimeters in height, and several centimeters in length.
(2) Microscopic, closely spaced planar fractures, less than0.001 mm in width, occur in individual quartz and shocked-silica glass grains (quartz grains which have been converted to silica glass by shock without thermal melting). These fractures vary from sharp, through-going to irregular-discontinuous. They occur in sets which are parallel to certain cleavage planes.
The coesite content in the shocked sandstone of selected samples from the layer of mixed debris varies from 5 to 19 percent, whereas in completely melted sandstone (lechatelierite) it varies from 3 to 5 percent. In shocked sandstone coesite occurs as small, 1 to 50 microns, anhedral grains adjacent to quartz grains and also in the lechatelierite matrix. Coesite is sometimes found within planar fractures that are present in highly fractured quartz and shocked-silica glass grains. In addition stishovite, low tridymite, and possibly low cristobalite are associated with coesite.
The distribution of coesite in the Pleistocene and Recent alluvium on the rim of the crater was studied by means of a combined hydrofluoric acid - X-ray technique. No coesite was found in the Pleistocene or Recent alluvium on the crater rim. It is concluded that any coesite present 1s below the limit of sensitivity (0.04 to 0.06 percent) of the method used.
Level of Degree
Department of Earth and Planetary Sciences
First Committee Member (Chair)
Wolfgang Eugene Elston
Second Committee Member
Third Committee Member
J. Paul Fitzsimmons
Burgoyne, Alfred A.. "Mineralogy Of Coesite And Petrographic Association And Distribution At Meteor Crater, Arizona.." (1967). https://digitalrepository.unm.edu/eps_etds/308