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particle shape

geology
Also known as: grain shape

Learn about this topic in these articles:

sandstones

  • chemistry of sedimentary rocks
    In sedimentary rock: Texture

    …textures, particularly grain-size distribution and grain shape (angularity and sphericity) has been described above. The information that results from textural analyses is especially useful in identifying sandstone depositional environments. Dune sands in all parts of the world, for example, tend to be fine-sand-size (clast diameters from 1/4 to 1/8 millimetre)…

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sedimentary rocks

  • chemistry of sedimentary rocks
    In sedimentary rock: Particle shape

    Three different but related properties determine particle shape: form, roundness, and surface texture. Particle form is the overall shape of particles, typically defined in terms of the relative lengths of the longest, shortest, and intermediate axes. Particles can be spherical, prismatic, or bladelike.…

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cementation, in geology, hardening and welding of clastic sediments (those formed from preexisting rock fragments) by the precipitation of mineral matter in the pore spaces. It is the last stage in the formation of a sedimentary rock. The cement forms an integral and important part of the rock, and its precipitation affects the porosity and permeability of the rock. Many minerals may become cements; the most common is silica (generally quartz), but calcite and other carbonates also undergo the process, as well as iron oxides, barite, anhydrite, zeolites, and clay minerals.

It is unclear just how and when the cement is deposited. Part seems to originate within the formation, and part seems to be brought in from outside by circulating waters.

The reverse process is called dissolution. There is evidence that dissolution has occurred in calcareous sandstones, in which case the calcareous cement or grains are broken down in the same manner as the solution of limestones. The frosted and etched surfaces of quartz grains in some friable and loosely cemented sandstones seem to indicate the former presence of a carbonate cement that has been leached.

This article was most recently revised and updated by John P. Rafferty.