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 mix

Fe-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 often retains 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

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, Plane Polars

Clinochlore and the zeolite mineral laumontite in plane polarized light. Magnification: 40x Rock: Precambrian-aged basalt collected near Castle Danger, MN Cl…

Clinochlore and laumontite, crossed polars

Clinochlore and Laumontite, Crossed Polars

Clinochlore and the zeolite mineral laumontite in cross polarized light. Magnification: 40x Rock: Precambrian-aged basalt collected near Castle Danger, MN Cl…

Fine-grained muscovite with chlorite (green), PPL

Muscovite and Chlorite, Plane Polars

Muscovite interspersed with chlorite (the brownish green pleochroic mineral) in plane polarized light. Magnification: 100x Rock: garnet mica schist To view t…

Fine-grained muscovite with chlorite, XPL

Muscovite and Chlorite, Crossed Polars

Muscovite displaying its characteristic moderate birefringence and birdseye extinction in cross-polarized light. Magnification: 100x Rock: garnet mica schist…

Chlorite peppered with epidote, PPL

Chlorite with Epidote Grains, Plane Polars

Two large chlorite grains peppered with epidote grains in plane polarized light. Magnification: 40x Rock: Metagranodiorite Click here to watch these grains r…

Chlorite peppered with epidote, XPL

Chlorite with Epidote Grains, Crossed Polars

Two large chlorite grains peppered with epidote grains in cross polarized light. Magnification: 40x Rock: Metagranodiorite Click here to watch these grains r…

Chlorite after biotite, PPL

Chlorite After Biotite, Plane Polars

Chlorite after biotite in plane polarized light. Surrounded by sericitized feldspars. Magnification: 40x Rock: metagranodiorite Click here to watch this grai…

Chlorite after biotite, XPL

Chlorite After Biotite, Crossed Polars

Chlorite after biotite in cross polarized light. Surrounded by sericitized feldspars. Magnification: 40x Rock: metagranodiorite Click here to watch this grai…

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.