Sometimes molten lava remains for a long time in its magma chamber, allowing crystals of a specific mineral to form first, so that when the whole rock is finally solid it contains these large crystals, called phenocrysts, set in a ground mass of small crystals. A surface lava sometimes cools very quickly, allowing no time for crystals to grow; this results in the formation of a glass.
Acidic, intermediate or basic, depending on the amount of the elements of silica, SiO 2 , they contain. Acidic rocks such as granite, microgranite and rhyolite are rich in silica and contain the minerals quartz, feldspar and biotite among others. The intermediate rocks include diorite, microdiorite and andesite.
Montmorillinite can expand by several times its original volume when it comes in contact with water. This makes it useful as a drilling mud to keep drill holes open , and to plug leaks in soil, rocks, and dams. Montmorillinite, however, is a dangerous type of clay to encounter if it is found in tunnels or road cuts.
Because of its expandable nature, it can lead to serious slope or wall failures. Other, less common, members of the smectite group include Beidellite, Hectorite, Nontronite, Sauconite, and Saponite. The Illite clays have a structure similar to that of Muscovite, but is typically deficient in alkalies, with less Al substitution for Si. Thus, the general formula for the illites is:. Thus, the illite clays are non-expanding clays.
Illite type clays are formed from weathering of K and Al-rich rocks under high pH conditions. Thus, they form by alteration of minerals like muscovite and feldspar. Illite clays are the main constituent of ancient mudrocks and shales. Mixed Layer Clay. Mixed layer clays are common, and consist of clays that change from one type to another through a stacking sequence. The sequences can be ordered and regular, or high unordered and irregular. For example montmorillinite layers can alternate with illite layers in an ordered way, or there can be several layers of montmorillinite with random layers of illite.
Distinguishing Clay Minerals Generally, the clay minerals occur as such small mineral grains that they cannot be easily distinguished in either hand specimen or thin section. However, the smectites can be distinguished from the other clays in the field by the "eating test" - place some clay in your mouth. If you can feel it expand as it becomes moistened, then it is one of the smectite clays, and is not a kandite or illite clay. X-ray techniques, are thus usually required to identify the clay minerals.
First, however, the clays have to be separated from other constituents. To do this, we first disaggregate the sample and place it in a settling tube filled with water. Particles will settle in the water according to Stokes Law:. Usually a desegregating agent Calgon is added to the water to keep the individual particles from adhering to one another. The particles are placed in a large glass cylinder filled with water and the desegregating agent, and the mixture is stirred.
One then must use Stokes law to figure out how far particles of clay size will settle in a given time. Calcium is closely linked with soil fertility, and calcium-rich rocks tend to both weather fairly quickly and supply soil with plentiful clays — the particles which facilitate a lot of the essential nutrient uptake by plant roots. Soil weathered from calcium-rich ferromagnesium rocks such as basalt, andesite and diorite thus tends to be more fertile than those developed over acidic igneous rocks such as granite and rhyolite.
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