E. C. T. Chao

First natural occurrence of coesite

Coesite, the high-pressure polymorph of SiO2, hitherto known only as a synthetic compound, is identified as an abundant mineral in sheared Coconino sandstone at Meteor Crater, Arizona. This natural occurrence has important bearing on the recognition of meteorite impact craters in quartz-bearing geologic formations.

Metallic Spherules in Tektites from Isabela, Philippine Islands

Iron-nickel spherules, as much as 0.5 mm in diameter, have been found completely embedded in some philippinites. The spherules consist mainly of kamacite with unidentified pink inclusions. The meteoritic origin of these spherules seems reasonable, suggesting that the tektites containing them were formed by asteroidal or meteoritic impact.

New data on the nickel-iron spherules from Southeast Asian tektites and their implications

Abstract This paper presents new occurrences and mineralogic, petrographic, and chemical data on metallic spherules found in Philippine tektites from the Ortigas site of Mandaluyong near Manila and in indochinites from Dalat, South Viet Nam. Most of the metallic spherules contain kamacite, schreibersite and troilite. Schreibersite and troilite occur interstitially or in blebs among round or elongate fine-grained matrix kamacite. Where abundant, schreibersite coalesces and forms a network throughout the entire spherule. Troilite generally occurs in small round droplets within the schreibersite. The amount of schreibersite varies from less than 5 to about 35 per cent by volume and the troilite may constitute as much as 5 per cent of the spherules. The composition of the phases present in the spherules was determined by the electron probe. In 9 analysed spherules from philippinites of the Ortigas site, the kamacite ranges from 1.6 to 4.5 per cent nickel. The average nickel content of kamacite in 3 analysed spherules from indochinites ranges from 4.7 to 12.9 per cent. The average nickel content in 4 schreibersite grains analysed from a single spherule ranges from 12.1 to 15.8 per cent. The spherules in the tektites are nearly identical with the meteoritic spheroids from Meteor Crater, Arizona, in texture, mineral assemblage and nickel content as shown by the new data. We conclude that these spherules were formed as molten droplets of an impacting meteoritic body which was instrumental in producing the tektite glass.

Coesite from Wabar Crater, near Al Hadida, Arabia

The third natural occurrence of coesite, the high pressure polymorph of silica, is found at the Wabar meteorite crater, Arabia. The Wabar crater is about 300 feet in diameter and about 40 feet deep. It is the smallest of three craters where coesite has been found.

Evidence of the impacting body of the Ries crater — the discovery of Fe-Cr-Ni veinlets below the crater bottom

Abstract Fe-Cr-Ni particles and veinlets have been discovered in the top 15 m of the compressed zone with abundant shatter cones below the bottom of the Ries crater. The metallic particles are less than a few microns across. They occur in various minerals along healed intergranular and locally in intragranular microfractures in quartz diorite, amphibolite and chloritized granite of the basement crystalline rocks. The particles consist of major Fe, Cr, and Ni with minor Si and Ca. Origin due to contamination is absolutely ruled out. We believe that these Fe-Cr-Ni particles are probably condensed from the vaporized impacting body which produced the Ries crater. These particles were injected with high velocity into microfractures near the top of the compressed zone, implanted in and across various minerals before these microfractures were resealed. The presence of Si and Ca as well as the fact that the Cr content is nearly twice that of Ni, led us to conclude that the Ries impacting body is very likely not an iron meteorite but a stony meteorite.

Application of automated image analysis to coal petrography

Abstract The coal petrologist seeks to determine the petrographic characteristics of organic and inorganic coal constituents and their lateral and vertical variations within a single coal bed or different coal beds of a particular coal field. Definitive descriptions of coal characteristics and coal facies provide the basis for interpretation of depositional environments, diagenetic changes, and burial history and determination of the degree of coalification or metamorphism. Numerous coal core or columnar samples must be studied in detail in order to adequately describe and define coal microlithotypes, lithotypes, and lithologic facies and their variations. The large amount of petrographic information required can be obtained rapidly and quantitatively by use of an automated image-analysis system (AIAS). An AIAS can be used to generate quantitative megascopic and microscopic modal analyses for the lithologic units of an entire columnar section of a coal bed. In our scheme for megascopic analysis, distinctive bands 2 mm or more thick are first demarcated by visual inspection. These bands consist of either nearly pure microlithotypes or lithotypes such as vitrite/vitrain or fusite/fusain, or assemblages of microlithotypes. Megascopic analysis with the aid of the AIAS is next performed to determine volume percentages of vitrite, inertite, minerals, and microlithotype mixtures in bands 0.5 to 2 mm thick. The microlithotype mixtures are analyzed microscopically by use of the AIAS to determine their modal composition in terms of maceral and optically observable mineral components. Megascopic and microscopic data are combined to describe the coal unit quantitatively in terms of (V) for vitrite, (E) for liptite, (I) for inertite or fusite, (M) for mineral components other than iron sulfide, (S) for iron sulfide, and (VEIM) for the composition of the mixed phases (Xi) i = 1,2, etc. in terms of the maceral groups vitrinite V, exinite E, inertinite I, and optically observable mineral content M. The volume percentage of each component present is indicated by a subscript. For example, a lithologic unit was determined megascopically to have the composition (V)13(I)1(S)1(X1)83(X2)2. After microscopic analysis of the mixed phases, this composition was expressed as (V)13(I)1(S)1(V63E19I14M4)83(V67E11I13M9)2. Finally, these data were combined in a description of the bulk composition as V67E16I13M3S1. An AIAS can also analyze textural characteristics and can be used for quick and reliable determination of rank (reflectance). Our AIAS is completely software based and incorporates a television (TV) camera that has optimum response characteristics in the range of reflectance less than 5%, making it particularly suitable for coal studies. Analysis of the digitized signal from the TV camera is controlled by a microprocessor having a resolution of 64 gray levels between full illumination and dark current. The processed image is reconverted for display on a TV monitor screen, on which selection of phases or features to be analyzed is readily controlled and edited by the operator through use of a lightpen. We expect that automated image analysis, because it can rapidly provide a large amount of pertinent information, will play a major role in the advancement of coal petrography.

Nickel-iron spherules from aouelioul glass.

Nickel-iron spherules, ranging from less than 0.2 to 50 microns in diameter and containing 1.7 to 9.0 percent Ni by weight, occur in glass associated with the Aouelloul crater. They occur in discrete bands of siliceous glass enriched in dissolved iron. Their discovery is significant tangible evidence that both crater and glass originated from terrestrial impact.

Petrology of Unshocked Crystalline Rocks and Shock Effects in Lunar Rocks and Minerals

On the basis of rock modes, textures, and mineralogy, unshocked crystalline rocks are classified into a dominant ilmenite-rich suite (subdivided into intersertal, ophitic, and hornfels types) and a subordinate feldspar-rich suite (subdivided into poikilitic and granular types). Weakly to moderately shocked rocks show high strain-rate deformation and solid-state transformation of minerals to glasses; intensely shocked rocks are converted to rock glasses. Data on an unknown calcium-bearing iron metasilicate are presented.

Mineral-produced high-pressure striae and clay polish: key evidence for nonballistic transport of ejecta from ries crater.

Recently discovered mineral-produced, deeply incised striae and mirror-like polish on broken surfaces of limestone fragments from the sedimentary ejecta of the Ries impact crater of southern Germany are described. The striae and polish were produced under high confining pressures during high-velocity nonballistic transport of the ejecta mass within the time span of the cratering event (measured in terms of seconds). The striae on these fragments were produced by scouring by small mineral grains embedded in the surrounding clay matrix, and the polish was formed under the same condition, by movements of relatively fragment-free clay against the fragment surfaces. The occurrence of these striae and polish is key evidence for estimating the distribution and determining the relative importance of nonballistic and ballistic transport of ejecta from the shallow Ries stony meteorite impact crater.

Preliminary petrographic description and geologic implications of the Apollo 17 Station 7 boulder consortium samples

Abstract Preliminary petrographic description and mineral composition of four hand samples (77135, 77115, 77075 and 77215) are presented. 77135, 77115, and 77075 all crystallized from fragment-laden melts; they are similar in textures but differ in grain size. 77135 and 77115 are pigeonite feldspathic basalts. On the basis of geologic and petrographic evidence, 77115 and 77075 are related; they formed, cooled, and consolidated before being engulfed in the vesicular 77135. The impact or igneous origin of the melts from which these rocks crystallized cannot be determined. 77215 is a shocked, strongly sheared and granulated microbreccia consisting of three major lithologies dominated by mineral clasts of orthopyroxene and calcic plagioclase. The orthopyroxene clasts contain coarse exsolved blebs of augite, suggesting a deep-seated origin. The major, minor, and trace element compositions of 77135, 77115, and 77075 are in general similar. They represent a major highland rock type, perhaps more important than anorthosites.