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.

Crystal zoning A texture developed in solid-solution minerals and characterized optically by changes in the colour or extinction angle of the mineral from the core to the rim. This optical zoning is a reflection of chemical zoning in the mineral.

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.



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. 

(c) Oscillatory zoning is where the mineral chemistry continuously oscillates between high- and low-temperature compositions going from the core to the rim. 

The salt deposits of a salt pan are zoned like bathtub rings, with less-soluble sulfates and carbonates at the outer margin and highly soluble sodium chloride (table salt) at the centre. The crystallization of these salts can be compared with the evaporation of brine in a dish.

Various examples of growth zoning features typically found in hydrothermal fluorites

 

No apparent zoning (A) suggests crystallization under constant hydrothermal conditions, whereas concentric zoning (B) indicates discontinuous changes in the fluid chemistry over time; (C) is a typical representation of short-lived oscillating changes in the fluid chemistry. (D,E) Typical sectoral zoning in fluorite crystals with complex crystal habits ((D): cuboctahedral, (E): cubic + rhombic dodecahedral faces) that is only visible in specific plane sections. (F) Combination of sectoral and oscillatory zoning in a cubic fluorite crystal. (G) 'Fir-tree' zoning occurs when growth conditions (T/Eh) alternate over time and thereby favour different crystal morphologies (cuboctahedron ↔ cube), which proves pulsed crystallization (e.g., [106]). (H) 'Petal-type' zoning is the result of a continuous change from octahedral growth to cubic growth; the red line marks the beginning of shape transition (modified from [104]).

 
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