Why is zoning in plagioclase so common?
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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 is Zone in crystal?
A zone is defined as a group of crystal faces that intersect in parallel edges. Since the edges will all be parallel to a line, we can define that the direction of the line using a notation similar to Miller Indices. This notation is called the zone symbol.
What is meant by mineral zoning?
Quick Reference. The term ‘mineral zoning’ refers to the systematic spatial variation in the composition of a single mineral grain. Concentric zonation reflects compositional variations perpendicular to the crystal surfaces, whereas sector From: mineral zoning in The Oxford Companion to the Earth »
What is zoning in fluorite?
Beautiful Zoning Purple Fluorite Crystals from Minerva Mine, Hardin Co., Illinois, USA. Photo credit: Anton Watzl. 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.
What kind of twinning does plagioclase have?
Plagioclase commonly shows this type of twinning, called the Albite Twin Law, with {010} as the twin plane. Such twinning is one of the most diagnostic features of plagioclase.
What is Perthitic texture?
Strictly speaking a perthite has blebs or irregular lamellae of sodic feldspar within potassic alkali feldspar, however, the term perthite is often used to describe all types of exsolution in the feldspars.
What is zone and zone axis?
Definition. A zone axis is a lattice row parallel to the intersection of two (or more) families of lattices planes. It is denoted by [u v w]. A zone axis [u v w] is parallel to a family of lattice planes of Miller indices (hkl) if. uh + vk + wl = 0.
What would be the normal zoning pattern in a zoned crystal of olivine?
Olivine crystals show variable zoning patterns: (b) – normal, (d) – complex reverse, and (c) – complex repetitive.
What is exsolution lamellae?
Exsolution lamellae are fine crystals that were dissolved entirely in their host mineral structures at high temperature (e.g., clino- and orthopyroxenes) and/or pressure (e.g., majorite-pyroxenes), but are produced when their host minerals lose the solubility as a result of cooling to low temperatures (T-controlled …
What is the streak of plagioclase?
| Plagioclase | |
|---|---|
| Luster | Vitreous |
| Streak | White |
| Diaphaneity | Transparent to translucent |
| Specific gravity | 2.62 (albite) to 2.76 (anorthite) |