The name chlorite encompasses a variety of Mg-rich phyllosilicates which are difficult to tell apart without detailed chemical studies. The information below, then, may seem a bit scattered and/or vague. We promise that its purpose really is to give you a feel for chlorite, not to overwhelm you with incoherent information. Trust us: once you learn to recognize it in all its many guises, you’ll never not know it when you see it.
|Chemical formula||Chlorite group minerals vary.
Common chlorite group minerals are close to
Sheets of linked tetrahedra
|Hardness||2 to 2.5|
|Cleavage||Perfect basal (001)|
|Transparency||Transparent to translucent|
Moderate to moderately high relief
Often interspersed with other mica minerals
Sometimes a mixFe-rich chlorite will appear blue,
often called “anomalous blue” or
|δ||0.006 to 0.020|
|Special properties||Chlorite is a common alteration mineral
which will often retain the habit of the original mineral
Commonly occurs with muscovite and biotite
|after Perkins, 324|
Chlorite in Hand Sample
Chlorite in Thin Section
Thin Section GigaPans
What’s that stuff that looks like dirt in PPL and confetti in XPL?
The feldspars alter to sericite, which is a catch-all name for fine-grained white mica whose identity can’t be pinned down just by looking at a thin section. This chlorite grain is surrounded by sericitized feldspars.