T. E. BUNCH ALVIN J. COHEN SHOCK DEFORMATION OF QUARTZ FROM TWO METEORITE CRATERS Abstract: Quartz, in association with coesite and fracturing under shock loading and is similar to silica glass from two meteorite-impact sites, ex- ruptures developed in quartz by static and shock- hibits prominent cleavage. Individual quartz grains loading experiments. The degree of cleavage de- have a shattered appearance and are largely co- velopment and the texture of the shattered quartz herent with little or no rotation of cleaved seg- is different from quartz deformed by other geo- ments. Cleavage was probably induced by brittle logic processes. during the propagation of a crack, it is a ductile Introduction fracture. If the fracture is characterized by lack During a petrographic investigation of rock of gross plastic deformation, it is termed a materials from meteorite craters it was found brittle fracture. Parting in crystals is separation that quartz displayed certain textures distinct along planes that are not true cleavage planes. from quartz of other geologic environments. Cleavage is the fracturing of a mineral body Quartz in association with coesite and silica along crystallographic planes, usually of low glass from two meteorite craters, Meteor bond density, so that the observable break is a Crater, Arizona, and Wabar Crater, Saudi smooth planar surface. Cleavage is always con- Arabia, shows well-developed cleavage. Milton sistent with the crystal symmetry and takes and others (1962, Astrogeologic Studies Semi- place only parallel to atomic planes. Although annual Prog. Rept.) described regular frac- cleavage does not normally occur in quartz, a tures, termed either parting or cleavage, in number of occurrences of cleavage in quartz deformed rocks from the Scooter high-explo- have been reported. sive cratering experiment. Englund and Roen Fairbairn (1939, p. 359-364) calculated the (1962) reported intensely deformed quartz that crystallographic planes in quartz that involved displayed parallel fracture patterns from the the least breaking of Si-O bonds. From these Middlesboro Basin, Kentucky, cryptoexplosion calculations he snowed qualitatively that the structure. This discussion illustrates the ex- best cleavage directions in order of decreasing tensive development of cleavage in quartz. facility should be parallel to r {1011}, z {01 Tl}, It is difficult to find a widely accepted usage wflOTOj, T{0001}, a{\m], *{1121}, and of the terms fracture, parting, and cleavage. *{5151}. Bloss and Gibbs (1963, p. 835-836) In describing breaks in a solid body that has determined the number of Si-O bonds which been under an unknown stress system (i.e. de- would have to be broken per unit area for sets formation by natural processes, not by ex- perimental devices), it is of particular interest of rational planes in the (0001) : (10TO) and to determine if these breaks are controlled by (0001) : (1120) zones. Of the first set of planes the crystal structure or if they occur at random. rflOTlj cuts the fewest bonds; in the second In a geological sense, the three above terms can set £{1122} cuts the fewest bonds, with r be used to describe a break in a mineral that cutting fewer than £. has undergone stress. In the present work, ruptures in quartz that The following is an account of the way these have resulted from shock deformation and terms will be used in this paper. Fracture or which parallel reported cleavage directions in rupture is the separation or fragmentation of a quartz are, in order of decreasing abundance, solid body into two or more parts under the r or z, c, m or a, and £. We refer to these action of stress. If fracture occurs with ap- crystallographically controlled ruptures as preciable plastic deformation prior to and cleavages. Geological Society of America Bulletin, v. 75, p. 1263-1266, 2 pis., December 1964 1263 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/75/12/1263/3442551/i0016-7606-75-12-1263.pdf by guest on 01 October 2021 1264 BUNCH AND COHEN—SHOCK DEFORMATION OF QUARTZ FROM TWO CRATERS The nomenclature of quartz crystallographic and nonglassy in hand specimen. The former is planes used in this paper is: extremely friable and has a sintered appearance. r—the positive rhombohedron {1011} It is composed predominantly of quartz with g—the negative rhombohedron {01T1} minor amounts of silica glass, iron oxides, and m—the unit prism {10TO} coesite. The association of silica glass, coesite, a—the second order prism {1120} and quartz is shown in Figure 1 of Plate 1. c—the basal pinacoid {0001} The slightly deformed sandstone from the rim £—the second order trigonal pyramid {1122} of the crater is grayish white and loosely s—the second order trigonal dipyramid cemented and is composed of quartz, iron {1121} oxides, and a trace of plagioclase. x—the trigonal trapezohedron {5151} Quartz grains in thin sections of the quartz- Thin sections from four different specimens glass-coesite material are highly fractured with were studied on a 5-axis universal stage with a little development of cataclastic texture (PI. 1, fig. 2). A total of 1758 observations of planar polarizing microscope. The orientation of the fractures were made on grains selected at cleavage planes and the c axis of each grain random. Of these observations, 685 are cleavage were plotted on a stereonet and the angles planes that are parallel to r or 2 and are the most between the c axis and the poles of the cleavage frequent in abundance. Other cleavage planes planes were then measured and used to de- observed in order of decreasing abundance are termine crystallographic directions. c (510), £ (215), m or a (156), and s (55). Unde- This method cannot distinguish between termined planar fractures total 137. Cleavage correlative forms such as r{ 10T1} and z{01Tl} in individual quartz grains can be seen in Fig- or mjlOTO} and a{1120}. Thus, the symbol r ures 1, 2, and 3 of Plate 2. Under crossed nicols is used to indicate the specific form and the extinction is extremely mottled. The extinc- correlative forms. tion pattern is unlike any observed by the writers in quartz from any other location, with Meteor Crater Quartz the exception of Wabar Crater. It is probably Meteor Crater, Arizona, is 183 m deep and the result of microfracturing or slight plastic 1220 m in diameter and is encircled by a rim flow. Curved fractures are structurally con- that rises 30-60 m above the surrounding trolled combinations of several cleavage planes. terrain. The exposed rocks in the crater range In Figure 3 of Plate 2, the dominant cleavages from Permian Coconino Sandstone to Triassic in the grain illustrated are r and c. The long Moenkopi Formation in age. Beneath the curved fractures are combinations of r and c raised rim lies a complex sequence of Quater- cleavages, predominantly, which form contin- nary alluvium that unconformably overlies uous curved fractures. Moenkopi and Kaibab strata. The alluvium is Quartz is slightly fractured and the extinc- composed, in part, of material from underlying tion pattern is somewhat mottled in the un- strata, meteorite fragments, and "fused rock" damaged sandstone, but it is comparable to (Shoemaker, 1960, p. 420-421). Rogers (1930) tectonically deformed quartz. In comparison described a "unique" occurrence of lechate- some of the cleaved quartz in the deformed lierite (silica glass) in the Meteor Crater specimens has experienced a small change in sandstone. Chao and others (1960, p. 220-222) shape. A few grains are elongated in the direc- reported the first natural occurrence of the tion of the c axis and show cleavage parallel to high-pressure SiC>2 polymorph, coesite, from the basal plane. Other changes in grain shape the sheared, partly fused, Coconino Sandstone are caused by fragmentation around the grain of Meteor Crater. Because the existence of exterior and by internal plastic flow after coesite indicates pressures in excess of 20 kb, fracturing. In general the change in grain shapes they suggested that the presence of coesite has not been extensive, nor is it a predominant affords a criterion for the recognition of impact feature in the majority of the grains studied. craters. Chao and others (1962, p. 419-421) The subhedral quartz in Figure 1 of Plate 1 still discovered the very high-pressure tetragonal retains several well-developed crystal faces. The SiC>2 polymorph, stishovite, in coesite-bearing behavior of the cleaved quartz during deforma- rocks from Meteor Crater. tion appears to have remained elastic until For the present investigation thin sections rupture occurred. In thin sections no evidence were made of the fractured, glassy, coesite- of shear faults have been observed, nor is there bearing Coconino Sandstone and also of Coco- any apparent recrystallization of quartz. nino Sandstone that appeared to be unfractured Silica glass occurs interstitially between Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/75/12/1263/3442551/i0016-7606-75-12-1263.pdf by guest on 01 October 2021 Figure 1. Photomicrograph of a quartz grain showing crystal faces r, m, and c with cleavages that are parallel to these faces. Coesite, iron oxide, and silica glass are indicated at the periphery of the grain. C axis direction (arrow) and cleavages indicated in the photograph. Plane-polarized light, X 80 Figure 2. Photomicrograph (plane-polarized light) of partially fused, shattered Coconino Sandstone. Dark intergranular areas are iron oxides and amor- phous material, X 40 QUARTZ FROM COCONINO SANDSTONE, METEOR CRATER, ARIZONA BUNCH AND COHEN, PLATE 1 Geological Society of America Bulletin, volume 75 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/75/12/1263/3442551/i0016-7606-75-12-1263.pdf by guest on 01 October 2021 Figure 1.