Why is plagioclase zoning?
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.
What is compositional zoning?
Compositional zoning is a common feature of pyroclastic deposits erupted from calderas worldwide (Smith 1979; Hildreth 1981; Bacon & Druitt 1988; De Silva 1991; Feeley & Davidson 1994).
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 sector zoning?
Sector zoning refers to a compositional difference between coeval growth sectors in a crystal and results from differences in fluid-crystal element partitioning between nonequivalent faces of the crystal.
What is fluorite zoning?
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.
Is fluorite an FCC?
Fluorite structure in general terms is a common motif for compounds with the formula MX2 wherein the X ions tend to occupy the eight tetrahedral interstitial sites. On the other hand, the M ions occupy the regular sites of a face-centred cubic (FCC) structure. The most common mineral fluorite (CaF2) has this structure.
What is Hypidiomorphic texture?
Hypidiomorphic refers to a texture, in which the grains of some mineral species are anhedral, those of. others subhedral, and those of some may even be anhedral. The texture is typical of granitic rocks in. many of which quartz and orthoclase tend to be anhedral, and plagioclase and biotite are subhedral to. euhedral. …
What type of zoning does plagioclase show?
In rare cases, plagioclase shows reverse zoning, with a more calcium-rich rim on a more sodium-rich core. Plagioclase also sometimes shows oscillatory zoning, with the zones fluctuating between sodium-rich and calcium-rich compositions, though this is usually superimposed on an overall normal zoning trend.
What is plagioclase?
“Plagioclase” is the name of a group of feldspar minerals that form a solid solution series ranging from pure albite, Na(AlSi3O8), to pure anorthite, Ca(Al2Si2O8).
What is the temperature at which plagioclase crystallizes?
The liquidus of plagioclase (the temperature at which the plagioclase first begins to crystallize) is about 1,215 °C (2,219 °F) for olivine basalt, with a composition of 50.5 wt% silica; 1,255 °C (2,291 °F) in andesite with a silica content of 60.7 wt%; and 1,275 °C (2,327 °F) in dacite with a silica content of 69.9 wt%.
What is the extinction angle of plagioclase?
The extinction angle is an optical characteristic and varies with the albite fraction (%Ab). There are several named plagioclase feldspars that fall between albite and anorthite in the series. The following table shows their compositions in terms of constituent anorthite and albite percentages.