The New Encyclopaedia Britannica in 30 Volumes - Macropaedia 12 [antikvár]

Metamorphic Rocks Metamorphic rocks are rocks that have recrystallized as a result of changes in the physical environment. Such changes may occur by reactions involving only the solid state (i.e., the mineral grains) or, more commonly, by reaction in a fluid médium, which makes up a very small percentage of the volume of the rock at any given instant. Commonly, the fluid médium is an aqueous film present in rock pores and on the boundaries between mineral grains. In a sense, metamorphic rocks are the most common rock types of the solid Earth, primarily because the Earth is a dynamic system whose temperatures and pressures tend to fluctuate in space and time. Because the pressure-temperature (P-T) conditions to which rocks of all igneous and sedimentary types may be subjected are almost infinite, the variety of metamorphic rock types is very large indeed. A very simple mineralogical system and its response to changing pressure and temperature provide a good illustration of what occurs in metamorphism. An uncomplicated sediment at the Earth's surface, a mixture of the clay mineral kaolinite [Al4Si4O10 (OH)8] and the mineral quartz (Si02), provides a good example. Most sediments have small crystals or grain sizes but great porosity and permeability, and the pores are fiiled with water. As time passes, more sediments are piled on top of the surface layer, and it becomes slowly buried. Accordingly, the pressure to which it is subjected increases because of the load on top, or overburden. For rocks with a density of two to three grams per cubic centimetre, the pressure will increase by 200 to 300 bars (one bar equals atmospheric pressure at an altitude of about 100 metres [300 feet] above sea level) for each kilometre of overburden. At the same time, the temperature will increase because of radioactive heating within the sediment and heat flow from deeper levels within the Earth. On the average, the temperature increases by about 30° C for each kilometre (87° F per mile) of burial. In the first stages of incremental burial and heating, few chemical reactions will occur in the sediment layer, but the porosity decreases, and the low-density pore water is squeezed out. This process will be virtually complete by the time the layer is buried by five kilometres of overburden. There will be somé increase in the size of crystals; small crystals with a large surface area are more soluble and less stable than large crystals, and throughout metamorphic processes there is always a tendency for crystals to grow in size with time, particularly if temperature is rising, because it increases the speed of reaction. Eventually, when the rock is buried to a depth at which temperatures of about 300° C (600° F) obtain, a chemical reaction sets in, and the kaolinite and quartz are transformed to pyrophyllite and water: kaolinite -f quartz -> pyrophyllite + water Al4Si4O10(OH)8 + 4Si02 - 2Al2Si4O10(OH)2 + 2H20. The exact temperature at which this occurs depends on the fluid pressure in the system, but in generál the fluid and rock-load pressures tend to be rather similar during such reactions. The water virtually fights its way out by lifting the rocks. Thus, the first chemical reaction is a dehydration reaction leading to the formation of a new hydrate. The water released is itself a solvent for silicates and promotes the crystallization of the product phases. If heating and burial are continued, another dehydration reaction sets in at about 400° C, in which the pyrophyllite is transformed to andalusite and quartz and water: pyrophyllite -> andalusite + quartz + water Al2Si4o10(oh)2 -> ai2sío5 + 3si02 + h2o. After the water has escaped, the rock becomes virtually anhydrous, containing only traces of fluid in minute and small inclusions in the product crystals. Both of these dehydration reactions tend to be fast, because water, a good silicate solvent, is present. Although the mineral andalusite is indicated as the first product of dehydration of pyrophyllite, there are three minerals with the chemical composition Al2SiOs. Each has unique crystal structures, and each is stable under definite P-T conditions (Figure 1). Such differing forms 1 1 1 kyanite1 1 / / // // V / - -yf sillimanite / / /\ \ \ \ \ \ \ - \ i i \ , V andalusite temperature (°C) Figure 1: Pressure-temperature regions where the three polymorphic modifications of AI2Si0s are stable. The dashed and solid lines are boundaries provided by different laboratories (see text). with identical composition are called polymorphs. If pyrophyllite is dehydrated under high-pressure conditions, the polymorph of Al2Si05 förmed would be the mineral kyanite (the most dense polymorph). On the other hand, if the originál temperature gradient persists, then at a depth of burial corresponding to about 700° C (1,300° F) the polymorphic transformation from andalusite to sillimanite will occur: andalusite -> sillimanite ai2sío5 -> ai2sío5 Sillimanite is more stable than andalusite at high temperatures, but, unless a small amount of water is present in the rock, this reaction may not go to completion even in geological time. If sillimanite does form, however, then the temperature rangé within the Earth's crust will preserve the sillimanite-quartz assemblage unchanged. If the forces leading to burial and sinking are reversed when the base of the sedimentary column has reached the sillimanite stage, then the thick column may be pushed up into a mountain rangé, permitting its observation. The reactions that proceeded during burial tend not to be reversed: with the water of the originál sediments gone, the hydrates cannot reform, and chemical reaction rates are always faster in response to rising than to lowering temperatures. The re-exposed column would reveal the metamorphic history of the pile of sediments.