Part 2: Forsterite-enstatite-quartz diagram 1900 1800 Liquid 1700 Qz+ Liq 1600 1
ID: 288017 • Letter: P
Question
Part 2: Forsterite-enstatite-quartz diagram 1900 1800 Liquid 1700 Qz+ Liq 1600 1500 Fo + En Qz + En 20406080100 Mg2SiO4 MgSiO Fo En 2.1. What does the letter "P stand for? What does the letter "E" stand for? 2.2. With a pen or colored pencil, outline (and label) the liquidus and solidus 2.3. Consider a melt of composition A. The following is a point-by-point description of the steps involved in cooling and crystallization of this melt . Melt cools to liquidus at 1700 C Quartz crystals form, shifting melt composition to left Quartz continuously crystallizes as magma cools to eutectic point at 1540 At eutectic, any magma that is left crystallizes to a mix of enstatite + quartz The rock cools with no further reaction . .Explanation / Answer
2.1) The letter P stands for Peritectic point. At this point the earlier mineral reacts with the melt to produce a different mineral of different composition.
E stands for Eutectic point. This point has no reaction relation like P, here the crystal gets fractionated and the whole melt gets crystallized.
2.2) The curve line in the melt phase is liquidus. and the horizontal line (bold line) below point P and E is the solidus line.
2.3) A liquid of compositon X lies towards Fo side from En composition is poorer in silica than enstatite. Such a liquid on cooling down at F begins to produce Fo crystals. On the liquidus phases are Fo+L and the degree of freedom at fixed pressure is 2-2+1 i.e. 1. Crystallization of Fo continues until the liquid reaches the raction point P when the temperature is 1557 degree C. The reaction converts the already formed Fo to En. At this point the forsterite liquidus intersects the En liquidus and the phases are 3 namely Fo, En and melt. Here F=0 to an invariant point. The bulk composition on completion of crystallization at P comprises crystals of Fo+En and no liquid is left over to proceed with further crystallisation on enstatite liquidus.
A liquid of composition Z just to the right of En but to the left of P (Peritectic point) , the liquid produces crystals of Fo on liquidus and these crystals will react with the liquidus at invariant point P. Some liquiud will remain even after the complete conversion of Fo to En. Now one phase disappears the variance is enhanced. The liquid leaves the invariant point and enters the univariant liquidus curve PE forming En throughout the path. At point E cristobalite begins to form along with enstatite until the melt gets exhausted. Final product En+Cristobalite.
2.4)
%Fo %En %SiO2 A 0 50 50 X 50 50 0 Y 0 100 0 Z 0 50 50