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Phase Equilibria in Igneous Systems - Simple Binary Systems wth no Solid Solutions

4 important questions on Phase Equilibria in Igneous Systems - Simple Binary Systems wth no Solid Solutions

What is the difference between equilibrium and fractional crystallisation?

Equilibrium crystallisation: equilibrium is maintained between liquid and crystals at all times. (Closed system)
Fractional crystallisation: crystals are removed or prevented from being in equilibrium with the melt from which they form. (Open system)
-this effectively changes the bulk composition, depleting it of the phasethat is separating.
-step-wise or continuous, but effect is always to produce liquids with composition closer to the eutectic.

What gives a porphyritic texture in magma?

Early formed crystals of a certain mineral are larger than later-formed crystals, due to the fact that (at a given composition far from the eutectic) the mineral will crystallise over a considerable temperature before the other mineral forms.

What gives a Vitrophyric texture in magma?

When the phenocrysts in a magma sample are surrounded by glass.

e.g. If a volcanic eruption of a melt on the anorthite liquidus resulted in rapid quenching, the liquid may never crystallised diopside. The the liquid cools to contain only anorthite phenocrysts surrounded by glass.

*Regardless of which phase forms the phenocrysts, the surrounding liquid (or glass in this case) is always enriched in the eutectic composition.

What are eutectic intergrowths?

At the eutectic, two minerals crystallise simultaneously in a [limited] space, and so must accommodate each other's growth habits

Ophitic texture: plagioclase laths embedded in large single pyroxene crystals.
Graphic granite: quartz grains embedded in single alkali feldspar crystals, forming shapes that resemble characters of cuneiform (wedge-shaped) writing.

The question on the page originate from the summary of the following study material:

Principles of Igneous and Metamorphic Petrology | 9780521880060 | Anthony Philpotts, et al

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