Grain Shape and Size

Grain Shape

Particle size is an important textural parameter in the classical rock since it provides information on the transport, sorting and deposition conditions of the sediment and gives some information on the history of events that happened at the site of the deposition before the final induration. It may be difficult to determine the sizes of discrete particles that make up sedimentary rock, especially when the rock is firmly indurated. Different methods have been designed to measure grain-size distribution, as well as several grade-size systems.

The size of sediment and sedimentary rock materials can be determined by measuring the ratios that accumulate in a series of wire mesh displays, by visually counting grains with a petrographic microscope, or by measuring the rate at which particles of various diameters settle in a water-filled glass tubes (known as settling tube).
The grade scales and terms of the millimetre and phi unit:

The standard table of rocks and sediments. The table is the standard table. Each size category varies by the constant ratio of 1:2 on the millimetre scale; each size class has a certain class name used for the particles it contains. The scale is a dimensional grain dimension scale, because the category boundaries are continuously proportioned. The scale is Udden-Wentworth. This form of plan is well suited to sediment definitions, as size proportions, be they in relation to gravel, sand, silt or clay, are of equal importance.

The phi scale is a positive, logarithmic shift in the Udden-Wentworth scale. Grain size diameters in mm will be translated to Phi units by means of a conversion equation (phi (ć)=-log2S), in which ć is the phi unit size and S is the millimeter grain length. Phi values are negative for grains of less than one mm, while phi values for grains of greater than one mm are neutral.

After the grain-size distribution for a given sediment or sedimentary rock has been determined by sieving, microscopic analysis, or use of a settling tube, it can be characterized using standard statistical measures in either of two ways: (1) visual inspection of various types of graphs that plot overall percent abundance versus grain-size diameter (e.g., histograms or bar diagrams, size frequency and cumulative size frequency curves, and probability curves that compare the actual grain-size distribution to a normal straight-line Gaussian distribution) or (2) arithmetic calculations made using diameter values in either millimetres or phi units that are read off the graphic plots and inserted into standard formulas. For siliciclastic sedimentary rocks, 

The following standard statistical measures are conventionally described for grain-size distributions: (1) mode, the most frequently occurring particle size or size class, (2) median, the midpoint size of any grain-size distribution, (3) mean, an estimate of the arithmetic average particle size, (4) sorting or standard deviation, a measure of the range, scatter, or variation in grain size, (5) skewness, the degree of symmetry or asymmetry of the grain-size distribution, which is in turn a function of the coincidence or noncoincidence of mean, median, and mode, and (6) kurtosis (peakedness) of a grain-size distribution, which compares sorting in the central portion of the population with that in the tails.

Particle shape

The particle shape is defined by three separate, but related characteristics: size, round and surface structure. The partial form is the overall particle shape, typically defined for the longest, shortest, and intermediate axes in terms of relative lengths. Sphere, prismatic or bladelike can be part of the partitions. The calculation of the smoothness of the particles is roundness and angular. Surface texture means the presence or absence of small, varying marks on grain surfaces (pits, paint, scratches).

Traditionally, each of these attributes of particle type is standardly calculated to define the transportation agent and the depositional environment. The shape is either determined by carefully measuring individual three-dimensional particles or by an analysis of the structure from Fourier using harmonic analysis and computer scanning to accurately describe two dimensional component.

Shape alone is limited in terms of deposition, but it represents more specifically the mineralogy of the grains concerned. Roundness is defined by a contrast between grains and regular profiles of the silhouette. Much of it is the product of the past of abrasion, which is influenced by deposition and the climate. Wind and surfing zones, for example, are well-rounded with glacial sands and existing deposits of turbidity.

Particle roundness or angularity is also expressed in mineralogy (soft minerals are easier to abrad than hard minerals), clast size (coarse particles get rounded faster than fine), and transport length (sand is abraded and rounded as the distance is widening). The pitting, labeling and polishing of particle surfaces can be studied visually with a microscope or hand lens or, in some cases, a microscope scan electron (SEM). Several surface textures have been genetically linked to specific deposition agents; classic V-shaped beach and nearshore areas, for example, classify quartz grains.

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