Chlorite Occurrence Chlorite is common as a secondary mineral, forming after mafic minerals, in rocks of many types. It may also be a primary mineral in low- to medium-grade metamorphic rocks. Composition Although often thought of as being the green-colored equivalent of biotite, it does not have the same atomic arrangement as other micas. Its chemistry is quite variable. Identification In general, keys to identifying chlorite are its lack of pronounced features, habit, (normally) colorless or greenish color, low order or anomalous interference colors. Chlorite's features generally make identification straightforward. Important properties  ·Overlooked - although common, chlorite is, in many cases, a nondescript greenish mineral that is easily overlooked because it is secondary and does not form large well-developed crystals.  ·Habit - when secondary, typically occurs as flake-like crystals (generally anhedral) or replacement patches containing small flakes or scales. It may partially replace grains of biotite or other minerals, appearing to "eat at them" along cleavage traces . When primary , chlorite has habit similar to biotite or muscovite.  ·Color - generally pleochroic, ranging from colorless to pale to medium green. Sometimes darker green or (rare) other colors.  ·Interference colors are distinctive: lower first order, and often anomalous. Anomalous blue, purple or brown are quite common.  ·Cleavage - chlorite may show a single good cleavage.  ·Biaxial (- or +); when +, it is one of the few sheet silicates that is length fast. Interference figures often difficult to obtain. Similar minerals  ·Biotite and other micas are rarely confused with chlorite because of chlorite's greenish color and pleochroism, and low or anomalous interference colors.  ·Serpentinge shows less relief and is less pelochroic than chlorite.  ·Green amphiboles and pyroxenes have two cleavages and significantly greater birefringence.  ·In some samples, glauconite and other clays may be difficult to distinguish from chlorite.

Chlorite in a Metasedimentary Rock The views above show chlorite (light green in PP; anomalous blue in XP) surrounding a flake of graphite (black, opaque). The rock is a metamorphosed shale collected near Poughkeepsie, New York. Most of the other material seen is muscovite, but graphite is scattered throughout in wavy layers, and there is one large quartz crystal on the top center edge. Field of view is 2 mm across.

Chlorite in a Garnet-Paragonite Schist   These photos show a large garnet surrounded by paragonite (a white mica) on the left and below, and by chlorite on the right. Quartz can also be seen, especially along the right edge of the photos. Afew small flakes of brown biotite (PP) are also present. The garnet has high relief and is isotropic (black, XP). The chorite exhibits anomalous metallic gray interference colors (XP). The paragonite shows mottled upper second order interference colors. Paragonite cannot normally be distinguished from muscovite in thin section; chemical analysis is required. This sample comes from southeastern Vermont. The field of view is about 2.5 mm.

Chlorite Replacing Biotite The photos to the left show chlorite as an alteration patch in a grain of biotite (PP on far left, XP on immediate left). Some additional chlorite (light green in PP) is just to the right of the bottom right corner of the biotite. Note the light green color of the chlorite and the anomalous (gray to dark inky blue) inteference colors. The surrounding minerals are mostly quartz and feldspar. The field of view is about 2.5 mm.

Chlorite, Epidote and Magnetite Replacing Hornblende   These photos show a diamond shaped hornblende fragment that has been replaced by chlorite (light green PP, greenish gray interference colors XP), epidote (clear PP, high-order interference colors XP) and magnetite (opaque). The other replacement material is feldspar and quartz.

Chlorite, Glaucophane and Epidote in a Blueschist Green chlorite, blue glaucophane, clear white mica flakes (bottom center and right) and a number of small high-relief epidote grains are visible in PP light. In XP light the chlorite shows anomalous interference colors, the glaucophane shows 2nd order interference colors, not generally typical of this mineral. The epidote grains are hard to pick out in XP light but the mica flakes show obvious mottled interference colors typical of micas. This sample comes from near Panoche Pass, Callifornia. The field of view is about 2.5 mm.

Chlorite Showing Anomalous Brown Interference Colors The PP view shows light green chlorite, blue glaucophane, clear muscovite, and scattered small epidote crystals in the PP view. In the XP view, the chorite can be identified by its anomalous brown interference colors. The muscovite appears as flakes with upper second order interference colors (XP). The glaucophane shows first order interference colors (XP). This sample comes from near Panoche Pass, Callifornia. The field of view is about 2.5 mm.