Why does exsolution occur in alkali feldspars?
The intergrowth forms by exsolution due to cooling of a grain of alkali feldspar with a composition intermediate between K-feldspar and albite.
How do you identify alkali feldspar in a thin section?
Alkali feldspars can often be distinguished from plagioclase feldspars because most grains of the latter exhibit albite twinning (see above Crystal structure), which is manifested by parallel lines on certain cleavage surfaces, whereas grains of alkali feldspars do not.
Which feldspar has exsolution lamellae?
The exsolution lamellae of Na- and K-rich feldspar are created in the bulk composition of the K-feldspar. A Na-rich host with K-rich lamellae is an antiperthite.
What is the microscopic characteristics of feldspars?
Feldspars usually are white or nearly white, though they may be clear or light shades of orange or buff. They usually have a glassy luster. Feldspar is called a rock-forming mineral, very common, and usually makes up a large part of the rock.
How does exsolution occur?
exsolution, in mineralogy, process through which an initially homogeneous solid solution separates into at least two different crystalline minerals without the addition or removal of any materials. In most cases, it occurs upon cooling below the temperature of mutual solubility or stability of the solution.
What causes 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 …
How do you identify minerals in a thin section?
We use cross-polarized light to learn a crystal’s optic class and optic sign, to measure extinction angles and sign of elongation, and to measure 2V. A combination of optical properties allows us to identify minerals in thin section and to interpret geologic histories.
How can you identify plagioclase feldspar in a thin section?
Hardness (6-6.5) and cleavage are good indicators of a plagioclase as opposed to other similar silicates. In thin section or grain mount, the general lack of pleochroism and the twinning are good indicators. The presense of parallel striae on cleavage surfaces distinguishes plagioclase from K-feldspar.
Does plagioclase have exsolution?
Plagioclase feldspars exhibit exsolution microtextures in three compositional ranges, the Peristerite, Bøggild and Huttenlocher ‘miscibility gaps’ shown in orange on Fig. 4 and in white in Fig. 8. These intergrowths are coherent and usually below the resolution of an optical microscope.
What is exsolution lamellae?
[¦ek·sə′lü·shən lə′mel·ē] (geology) Layers of sedimentary rock that solidify from solution by either precipitation or secretion.
In which mineral do you find the exsolution lamellae?
igneous pyroxenes
Exsolution occurs only in minerals whose compositions vary between two or more pure endmember compositions. Many igneous pyroxenes show exsolution textures of thin lamellae of one pyroxene in a host of a different composition. The lamellae occur as oriented intergrowths that display parallel and herringbone textures.
How can you identify Orthopyroxene in a thin section?
The keys to identifying orthopyroxene are its relief, usually pale green (sometimes pleochroic to pink) color, low-order interference colors, and near 90o cleavage seen in some views. Color – Commonly light green, but some orthopyroxene shows marked pleochroism to pink. Relief is moderate to fairly high.
What are plagioclase feldspars?
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 the meaning of exsolution?
exsolution, in mineralogy, process through which an initially homogeneous solid solution separates into at least two different crystalline minerals without the addition or removal of any materials.
How do you identify a mineral in the thin section?
What is mineral exsolution?
How are exsolution lamellae formed?
What is an alkali feldspar?
Alkali Feldspar 1 Basic Mineralogy. The alkali feldspars include monoclinic feldspars (orthoclase and sanidine) and triclinic feldspars (microcline and anorthoclase). 2 Volume 1. 3 Plutonic Geology. 4 Igneous rocks
What is the most common weathering product of alkali feldspar?
The most common weathering product of alkali feldspar is kaolinite, in various climatic conditions ( Melfi et al., 1983 ).
Is strontium compatible with alkali feldspar?
Barium and strontium are well known to be compatible in alkali feldspar. Several experimental studies demonstrate this over a wide range of pressures and temperatures (Long, 1978; Guo and Green, 1989; Icenhower and London, 1996 ).
What is the difference between K-feldspar albite Intergrowths and Perthite?
K-feldspar albite inter-growths is known as perthite and albite intergrowths K-feldspar as antiperthite. Antiperthite is albitic plagioclase with small discrete K-feldspar that has become exsolved from the albite.