Polished Rock Research Articles

Mudrocks examined by backscattered electron microscopy

The small size of many of the particles in mudrocks makes it almost impossible to image and identify them individually and in situ using conventional light microscopy. Partly for this reason, the petrography and genesis of mudrocks is poorly understood compared with sandstones and limestones. This deficiency presents a serious problem hi sedimentary geology, as mudrocks comprise almost two-thirds of the stratigrapnical column1,2. We introduce here a new method of studying mudrocks, using backscattered electrons (BSE) in scanning electron microscopy (SEM). When thin sections or polished rock chip surfaces are examined with BSE, both atomic number (Z) contrast and topographic contrast can be obtained, providing much more detailed compositional and textural information than was previously possible. Used in conjunction with X-ray microanalysis, the new technique promises to revitalize mudrock petrology.

Rock-fluid interactions in a temperature gradient: biotite granodiorite + H 2O

A biotite granodiorite was reacted in a controlled temperature gradient with distilled water for 60 days at 1/3 kbar P Tot = P H 2 o . Polished rock prisms were placed in the gradient at 72, 119, 161, 209, 270, and 310°C. Scanning electron microscope and microprobe analyses show the appearance of several secondary phases: Ca montmorillonite at 72°C and 119°C; zeolite, either stillbite or heulandite, at 161°C; and another zeolite, thomsonite, at higher temperatures. Solution analyses show a constant composition after about 2 weeks of reaction. The phase assemblages observed in this experiment are connected by reaction relationships. The reactions show the transition from clay to zeolite I (stilbite or heulandite) to zeolite II (thomsonite) with increasing temperature and decreasing chemical potential of silica. Additional relations can be used to predict mineral assemblages not directly observed in this experiment but which may occur at different pressures and temperatures as well as at different chemical potentials of water and silica.

68.Improved convenient preparative method of polished rock thin section. : the Fall Meeting of the Society

68.Improved convenient preparative method of polished rock thin section. : the Fall Meeting of the Society

Proof for the synsedimentary biota in Pre-Cambrian shales

Three optical techniques are described to differentiate synsedimentary biota from contaminants and artifacts in Pre-Cambrian shales. These techniques are (a) study of polished rock surface in a dark field in incident light illumination and with the heating on stage microscope in fluorescence light, (b) study of thin sections in interference contrast, and (c) examination of macerated and isolated specimens in Zernike phase-contrast and in interference contrast. Two young Pre-Cambrian shales, one from the Vindhyan Supergroup of India and the other from the Bushimay Group of Zäire have been examined by these techniques. The results demonstrate that both the shale samples contain biota of the synsedimentary deposits.

Evidence for a Former Large Ice Sheet in the Orville Coast–Ronne Ice Shelf Area, Antarctica

AbstractThe Orville Coast area of the Antarctic Peninsula was extensively glacierized in the past. Striations, polished rock surfaces, and erratics on nunatak summits indicate that this area was covered by a broad regional ice sheet whose grounded ice margin was on the continental shelf, in the present-day Ronne Ice Shelf area. If the glacial history of Antarctica has been controlled by eustatic sea-level changes, the destruction of this ice sheet would have been contemporaneous with that of the Ross Sea ice sheet due to the world-wide rise of eustatic sea-level at the end of the Wisconsin glaciation.

Evidence for a Former Large Ice Sheet in the Orville Coast–Ronne Ice Shelf Area, Antarctica

AbstractThe Orville Coast area of the Antarctic Peninsula was extensively glacierized in the past. Striations, polished rock surfaces, and erratics on nunatak summits indicate that this area was covered by a broad regional ice sheet whose grounded ice margin was on the continental shelf, in the present-day Ronne Ice Shelf area. If the glacial history of Antarctica has been controlled by eustatic sea-level changes, the destruction of this ice sheet would have been contemporaneous with that of the Ross Sea ice sheet due to the world-wide rise of eustatic sea-level at the end of the Wisconsin glaciation.

Principles of sampling, preparation and constituent selection for microphotometry in measurement of maturation of sedimentary organic matter

SUMMARYSamples may be obtained from outcrops, cuttings, cores, modern sediments and old collections. Coals, brown or grey siltstones and fine‐grained sandstones generally yield the most satisfactory organic matter for maturation analysis, while red beds and evaporites are often barren. Shales with a high organic content are not especially desirable for optical studies of maturation because the organic matter is likely to be very fine‐grained and high in pyrite. The whole rock or a concentrate of organic particles may be prepared for microscopical study, the former being preferred for analysis of solid bitumens and fluorescence study, the latter for measurement of vitrinite reflectance, microspore translucency and refractive index. Crushing, ultrasonic disintegration, handpicking, heavy liquid separation, flotation, settling, sieving and demineralization with HC1 and HF are used to concentrate organic particles. Preparations include polished rock slabs, concentrate briquettes and strew mounts.The constituents with the lowest maturation in a rock must be selected if maturation indigenous to the rock is to be measured. Organic matter that has been previously altered at its site of origin, reworked and altered during diagenesis, or redeposited from older rocks, must be excluded from consideration. The organic constituents are recognized most easily by reflected light microscopy. A combination of fluorescence and reflectance techniques is useful for low rank samples.

Use of Polished Rock Slabs to Facilitate Hand Specimen Studies

In a time when rocks are being examined in ever more minute detail, it is important that gross mineralogical and textural relationships also be more carefully studied. To this end a technique is described by which hand specimens may be studied through the use of polished rock slabs. A slab one-half to one inch thick is cut from a hand specimen and then polished with a 600 grit powder. This produces a specimen that can be easily examined and photographed. Polished rock slabs are particularly useful in studying larger scale features in all types of rocks, although special features of fine grained rocks also can be studied with considerable success. Several examples are described that demonstrate the usefulness of the technique, as applied to the study of mineral deposits. The technique is inexpensive, easy to apply, and can be used by geology teachers and researchers alike, at all educational levels.

Glaciation and Periglacial Morphology in the Northwestern Venezuelan Andes

Abstract. Evidences of previous glaciation in the northwestern Venezuelan Andes, mainly in Páramo de Piedras Biancas, include a main morainic level at 3400 to 3700 m, and minor moraines at higher elevations up to about 4300. Glaciated valleys, hanging valleys, rock, steps, glacial channels, cirques, arêtes, horns, striae and grooves on polished rock, roche mountonnés, and whaleback forms, are the main evidence of glacial sculpture and erosion. The main morainic level probably represents a Late Wisconsin glacial advance, and the higher moraines are either neo-glacial or due to minor readvances during the main postglacial retreat. The main morainic level, 400 m higher than that in the southeastern Venezuelan Andes, indicates that the glacial climate was drier in the northwestern part than in the southeastern part of this mountain range. Present-day periglacial features include micro-terrassettes, turf exfoliation, talus, and striated ground, above 3500 m; and nonsorted stripes, sorted polygons or sorted circles, and sorted nets, above 4000 m. The present-day periglacial zone is located between approximately 3600 and 4700 m. During glacial times it probably readied down to 2400 m in the southeastern, and 2900 m in the northwestern parts of the Venezuelan Andes.

U, Th and extinct 244Pu in the phosphates of the St. Severin meteorite

Previous work ( G.J. Wasserburg, J.C. Huneke and D.S. Burnett, 1969) on isolated crystals of whitlockite in the meteorite St. Severin has shown that this mineral concentrates U but not to the extent necessary to account for the total uranium. In the present work, the microdistributions of the total Th and U were measured in polished rock sections by the fission track method. Approximately 95% of the Th and U was found to be distributed in the matrix while the remaining 5% was found to be concentrated in two species of phosphate crystals, mostly whitlockite but also in much rarer chlorapatite. The Th/U ratio was measured in seven crystals of whitlockite and one crystal of chlorapatite. The values are respectively (11.8 ± 1.4) and (1.5 ± 0.5). Comparison with rare-gas studies indicates that 244Pu probably followed Th geochemically in the whitlockite.

Autoradiography of neutron-activated rock slabs

Autoradiographs in macro-scale of rocks are produced after subjecting polished rock slabs to thermal neutron activation. The obtained picture shows the distribution of induced activity in the rock slab. 1–5 days after irradiation most of the exposure of the film is due to 24Na formed by ( n, γ) reaction from natural sodium ( 23Na). After about one week the picture is dominated by elements with long-lived radio-nuclides. Iron and cobalt and certain trace elements will produce staining depending upon the composition of the rock.

Ett försök till en geologisk datering av en hällristning

Abstract An attempt to date a rock carving is made by comparing the degree of weathering of the carving with that of glacially polished rock surfaces in the vicinity. The weathering of these rocks began when they rose above sea level, and from their present height above sea level it is easy to calculate how long they have been subject to weathering. The comparison of the degree of weathering was made from close-up stereo-pairs of the rock surfaces under a stereoscope.

Insight into the Magnetic Mineralogy of Antarctic Rocks

A magnetic powder technique is presented, which is considered to be phase I of a methodology being developed at the University of Pittsburgh, to ultimately relate the magnetic properties of rocks to the causitive mineralogy. The methodology, termed magnetic petrology, was suggested by an initial study of plutonic rocks from Ellsworth Land, Antarctica.Phase I, the magnetic powder technique utilizes a strained mechanically polished rock surface upon which a magnetite colloid is deposited. After washing the surface with alcohol or acetone, the magnetic minerals or magnetic phases are patterned with magnetite powder; the non magnetic components remain free of powder.

Analysis of rocks and minerals by reflected infrared radiation

The earliest studies of minerals by reflected and emitted infrared radiation were those of Coblentz (1, 2). Since then mineralogical uses of reflected infrared radiation have been restricted primarily to studies of glass, and to minerals like quartz and cristobalite which play a prominent part in the understanding of glass structures. In a similar manner infrared emission studies have been restricted almost exclusively to ceramics and refractories. This paper deals with the reflected infrared analysis of several common minerals, rocks, and meteoritic materials. The optical principles involved, and the typical spectra to be obtained from quartz plates, fused silica, and glasses are briefly reviewed and recent spectral data from minerals and rocks are examined. Similarities and differences between absorption and reflection spectra for the same materials are discussed, and some of the problems encountered in the calculations of composition (modal analysis) are indicated.Applications to nondestructive testing of polished rock surfaces (or gems) and to the calculation of ambient-temperature emissivity curves for rocks are shown. Such spectral emissivity curves are an important prerequisite for the remote mapping of the lunar and other planetary surfaces.

Quick preparation of peel-prints for sedimentary petrography

ABSTRACT Acetate peels made from sheets of commercial acetate film are quicker, easier, and less expensive to prepare than peels made by the old acetate solution method. Textural details of polished rock sections are best recorded on acetate peels if the film is not firmly pressed against the rock.

Mineralogical analysis of carbonate rocks by X-ray diffraction

Many minerals can be identified by X-ray diffraction from polished rock slices cut to fit in the sample holder of a diffractometer. Scanning from 20 degrees to 35 degrees 2theta gives the strong reflections of common minerals. Typical diffractometer patterns of limestone and dolomites are given.

Erosive Forces in the Physiography of Western Arctic Canada

IN PAST years there has been a tendency to explain all landforms on the Arctic islands as resulting from extensive glaciation. It is true that in western Arctic Canada, in the southern half of the archipelago, there is abundant evidence of glaciation in the form of glacial grooves, striae, roches moutonnees, polished rock surfaces, and rock basin lakes, all of which seem to owe their origin to an ice sheet or sheets that moved into the Arctic islands during the Pleistocene. The northern half of the archipelago, however, probably lay beyond the limits of any continental ice sheet, and it seems possible to explain most of the major topographic forms both there and in the south without recourse to glacial erosion. At present the only island that contains a permanent capping of ice or snow is Meighen, and there the cap appears to be dormant or receding, so that it is the meltwaters emerging from it that are eroding the surface rather than the cap itself The question of the former extent of glaciation in the Arctic Archipelago and its influence on landforms will be discussed in another paper. Here we shall deal only with the processes of erosion that are active today, and their individual and collective role in bringing about the denudation of the Arctic landscape. The principal erosive forces at work on the western Arctic islands are water, both fresh and salt, ice (except glacial ice, which is no longer active), and wind. EROSION BY WATER

The "Polished Rocks" of Cornudas Mountain, New Mexico

The "Polished Rocks" of Cornudas Mountain, New Mexico

The "Polished Rocks" of Cornudas Mountain, New Mexico

The "Polished Rocks" of Cornudas Mountain, New Mexico

The Polished Rocks of Cornudas Mountain, New Mexico

The Polished Rocks of Cornudas Mountain, New Mexico