Chlorite

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.

Physical Properties
Chemical formula Chlorite group minerals vary.
Common chlorite group minerals are close to
(Mg,Fe,Al)6(Si,Al)4O10(OH)8
Class Phyllosilicate
Sheets of linked tetrahedra
Crystal system Monoclinic
Habit Foliated books
Scaly aggregates
Individual flakes
Pseudohexagonal (rare)
Color Green
Variable
Hardness 2 to 2.5
Specific gravity 3.0
Cleavage Perfect basal (001)
Fracture Flexible
Luster Vitreous
Transparency Transparent to translucent
Streak White, green
Optical Properties
PPL Green
Green-brown
Yellow-green
Pleochroic
Moderate to moderately high relief
Often interspersed with other mica minerals
XPL Low birefringence
Yellow or
Purple or
Brown or
Blue or
Sometimes a mixFe-rich chlorite will appear blue,
often called “anomalous blue” or
“Berlin blue.”
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 slickensides in Brallier Formation shale

Scanning electron micrograph of chlorite slickensides in Brallier Formation shale

Chlorite in Balls Bluff siltstone

Chlorite porphyroblasts in either Popes Head Formation schist or Station Hills phyllite (there is some argument about this one)

Chlorite in Castner marble (see snapshots)

Chlorite in Thin Section

Thin Section GigaPans

Chlorite schist, plane polars

Chlorite schist, crossed polars

Chlorite in augen gneiss, plane polars

Chlorite in augen gneiss, crossed polars

Chlorite with muscovite and quartz, plane polars

Chlorite with muscovite and quartz, crossed polars

Clinochlore and laumontite in Castle Danger amygdaloidal basalt, plane polars

Clinochlore and laumontite in Castle Danger amygdaloidal basalt, crossed polars

Clinochlore and laumontite, plane polars

Clinochlore and laumontite, crossed polars

Fine-grained muscovite with chlorite (green), PPL

Fine-grained muscovite with chlorite, XPL

Chlorite peppered with epidote, PPL

Chlorite peppered with epidote, XPL

Chlorite after biotite, PPL

Chlorite after biotite, XPL

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.