Terminology and the Classification of Fine Grained
SEDIMENTARY ROCKS – is there a difference between a claystone, a mudstone and a shale? Summary Fine grained sedimentary rocks, both clastic and carbonate, are believed to be the most abundant rock type on the Earth " s surface 1924) of all the sediment ever produced. In sedimentology the size grade scale most commonly used is that which was introduced by Udden in 1898, modified by Wentworth in 1922 and then converted into a logarithmic form by Krumbien in 1934. For a single particle, the sand-silt boundary, which is the distinction between coarser grained materials and those which are fine grained, is placed at 1/16 mm (≈63 microns or +4 Φ). For a given rock the proportion of material containing particles less than 63 microns in size is normally greater than 50% to satisfy the classification criteria of being fine grained. Despite their abundance, fine-grained sedimentary rocks, to date, have not been satisfactorily classified. Perhaps this is because they are not well exposed in the field compared to their coarser grained, more weather resistant counterparts, the sandstones (Pettijohn, 1975; Spears, 1980); perhaps it is due to their fine grained nature that makes their study more noted by Rodgers (1950) the problem of classifying fine grained sedimentary rocks, simply, but inevitably entails the problem of terminology. It is the usage of imprecisely defined terms that has often led to confusion, and frustration, when trying to describe and discuss fine grained sediments. This report will identify where confusion has arisen, and determines the type of terminology and classification that will be used in future works. In the literature there is a plethora of names for fine grained sedimentary materials – Definition and usage of the word 'shale' The term shale has been around for a few centuries. The first recorded use of the written word shale appears in Hooson " s " The Miners Dictionary " , published in 1747; within which the term shale was meant as an indurated, laminated, clayey rock. Tourtelot (1960) gives a good summary on the origin and use of the world shale, and by outlining its historical use as i) a general term for describing " laminated clayey rock " ; ii) as a formation name, e.g. the " Pierre Shale " ; and iii) it " s widespread use as a bucket term for the entire class of fine grained rocks (whether they are laminated …
Fine-grained rocks: Shales or physilites
Abstract Fine-grained sediments and rocks containing a high percentage of phyllosilicate minerals have never been satisfactorily classified. Rocks names are poorly defined. The definition of many terms such as clay, clay mineral and shale are logically inconsistent. The major problems are caused by the assumption that there is a fixed relation between grain size and mineralogy and that structure is a basic parameter. Shallow-buried recent and ancient marine sediments with a high content of phyllosilicates commonly contain 60–80% clay and 20–40% silt. Phyllosilicates are present in approximately the same amounts as the clay-size material. Many fine-grained sediments that have been deeply buried commonly contain 30–40% clay; the phyllosilicate mineral content is in the range of 60–80%. In most fine-grained sediments, as deposited, the majority of the material is in the clay size. With burial and diagenesis, much of the clay-sized material grows to silt size. Thus, claystones are converted to siltstones. The term physil, an abbreviated form of phyllosilicate, is proposed to describe all sheet silicate minerals regardless of grain size. Physilites are rocks with a high content of physils. A simple classification is proposed in which size, mineralogy and structure are treated as independent variables.
The fragmentation of small asteroids in the atmosphere
The fragmentation of a small asteroid in the atmosphere greatly increases its cross-sections for aerodynamic braking and energy dissipation. The differential pressure across a meteoroid disperses its fragments at a velocity that increases with atmospheric density and impact velocity and decreases with meteoroid density. At a typical impact velocity of 22 km/s, the atmosphere absorbs more than half the kinetic energy of stony meteoroids with diameters, D M < 220 M AND IRON METEOROIDS WITH D M < 80 m. The corresponding diameter for comets with impact velocity 50 km/s is D M < 1600 m. Most of this energy dissipation occurs in a fraction of a scale height, which causes large meteoroids to appear to »explode« or »flare« at the end of their visible paths