The diagram on the following page shows the phase relations of plagioclse feldsp
ID: 158976 • Letter: T
Question
The diagram on the following page shows the phase relations of plagioclse feldspars in 2. The diagram on the following page shows the phase relations of plagioclase feldspars in anhydrous melts at low pressure and in hydrous melts at high pressure. Note the crystallization of melts with the same composition begins and ends at higher temperatures under low pressure than under high pressure hydrous conditions. Consider a melt having the composition of labradorite (60% anorthite), and answer the following questions: a) Under high pressure (5000 bars H,0) what would be the composition of the first b) Assuming equilibrium conditions, what would be the compositions of the liquid c) Suppose under these conditions (1000 C, 5000 bars H,0 pressure), the pressure crystals to form, and at what temperature would they begin to crystallize? and solid phases at 1000 C? was reduced with the escape of water from the melt of this composition (An), as might happen during volcanism where magma rises toward the surface, with temperature remaining constant at 1000 C. What might happen to the phases described in (b), and what would be the character of the resulting product? Write a well thought out answer to this question for each of the following situations: 1) An abrupt drop in pressure to near surface conditions, and 2), a very gradual drop in pressure.Explanation / Answer
Sometimes, a system can vary in composition, then the temperature at which the first liquid appears upon heating depends on the composition of the solid. Plagioclase feldspar exhibits complete solid solution from albite (Ab) NaAlSi3O8 to anorthite (An) CaAl2Si2O8. The temperature for which the first liquid appears for each plagioclase composition is given by the plagioclase solidus curve. A solidus is the locus of compositions of solids that are in equilibrium with liquids and can be a curve, surface, or hypersurface depending on the number of components in the solid solution. Just as the composition of a liquid follows the liquidus with changing temperature, so also the composition of a solid follows the solidus with changing temperature. However, because it is kinetically mode difficult to change the chemical composition of a solid (diffusion can be vanishingly slow in a solid), solid compositions may not change with temperature as predicted by the phase diagram. During crystallization this resistance to change may lead to chemically zoned crystals -- a feature commonly observed for plagioclase crystals with the petrographic microscope.
Crystallization and Temperature at which different systems begin to crystallize depends on the following:
• Magma composition
• Pressure
• Water pressure
• Cooling rate, and
• many other factors
a)
Under high pressure (5000 bars H2O), formation of the first crystal composition is Ab is 30% and An is 70% at temperature between 800 and 1000 degree Celsius
b)
Under equilibrium conditions, the phase is usually in solid phase, but in a solid systems, 1000 degree Celsius is less temperature to melt all from solid to liquid. So under equilibrium conditions, approximately we can say 90% solid phase and 10% liquid phase.
Note: A pure substance, under equilibrium conditions, may exist as either of a phase namely vapor, liquid or solid, depending upon the conditions of temperature and pressure.