What causes zoning in plagioclase?
What causes zoning in plagioclase?
For example, a plagioclase can be zoned from a Ca-rich core to an Na-rich rim. Zoning results from the mineral’s inability to maintain chemical equilibrium with a magma during rapid cooling; the zonation represents a frozen picture of the continuous reaction series for that mineral.
What does it mean if a mineral is zoned?
Crystal zoning is a texture developed in solid-solution minerals and characterized optically by changes in the color or extinction angle of the mineral from the core to the rim. Usually reflects progressive change of melt composition during growth of the crystal.
Is plagioclase a mineral?
Plagioclase is a member of the feldspar group (like orthoclase) and is a framework silicate. Plagioclase consists of a solid solution between the albite and anorthite end-members, and together with quartz is the most common of the rock forming minerals.
What is normal zoning in plagioclase?
Growth of plagioclase crystals accompanied by rapid cooling and contemporaneous crystallisation with falling temperature is considered to cause disequilibrium due to incomplete reaction between the growing crystal and the residual sodic melt. This causes the formation of normal zoning.
Where is plagioclase found?
Oligoclase, the most common plagioclase, occurs in granite, diorite, and other felsic igneous rocks and in some metamorphic rocks; notable occurrences are at Aust-Agder, Norway, and Fine, New York, United States.
What causes plagioclase?
Each plagioclase grain is made up of stacks of thin crystal with their molecules arranged in opposite directions. This causes the cleavage planes to have fine parallel line across them called striations.
What are the different types of zoning in plagioclase?
Zoning can be of three types, the first two applying mostly to plagioclase feldspars. ( a) Normal zoning is where the mineral is zoned from a high-temperature core composition to a low-temperature rim composition. ( b) Reverse zoning is where a mineral is zoned from a low-temperature core composition to a high-temperature rim composition.
Why are plagioclases zoned from calcic cores?
Plagioclases are normally zoned from calcic cores to more sodic rims. Reverse zoning: zoning in which the outer portions of the crystal have a higher temperature composition than the core. Usually reflects mixing between host magma and more primitive magma during crystal growth.
Why are plagioclases zoned to sodic rims?
Usually reflects progressive change of melt composition during growth of the crystal. Plagioclases are normally zoned from calcic cores to more sodic rims. Reverse zoning: zoning in which the outer portions of the crystal have a higher temperature composition than the core.
How is plagioclase feldspar chemically zoned as a mineral?
“Zoning in Minerals”. Feldspar can also be chemically zoned. Plagioclase feldspar grows in igneous environments and crystals which grow early in a cooling magma contain more calcium than those formed at the end of crystallization, which are enriched in sodium. This is predicted by the Bowen’s Reaction Series.