Steven Dutch, Professor Emeritus, Natural and Applied Sciences, Universityof Wisconsin - Green Bay
Very common mineral in low-grade metamorphic rocks and as an alteration product offerromagnesian minerals. Low relief, colorless to green. Light green is most common shade.Micaceous cleavage, often matted or feltlike appearance.
Low birefringence results in first-order whites. Anomalous interference colors verycommon (A). Dark blue, brown, purple and green are possible. Fine-grained, matted-lookingvarieties often show undulose extinction.
The light green mass in the center of this plane-polarized view ischlorite. Most of the brownish micaceous mineral around it is biotite.
The crossed-polarizer view below shows dark purple anomalous extinctioncolors. Chlorite is one of the most common minerals to show anomalous extinction.
Another common appearance of chlorite. In this crossed-polarizer view wesee parallel blades of chlorite, very similar in texture to mica, but distinguished byfirst-order white interference colors. Again, we also see dark purple anomalousextinction.
The plane-polarizer view below shows a field mostly of light greenchlorite, with two opaque magnetite crystals at the top.
The same field in crossed polarizers. The chlorite is actually a mass ofcriss-crossing small plates, giving the material a felt-like texture. This is a verycommon texture in chlorite. Note that these extinction colors are normal. Not all chloriteshows anomalous extinction.
The plane-polarizer view below is pretty typical of greenschist faciesmetamorphic rocks. The brown is biotite, the light green is mostly chlorite, and thedarker green includes hornblende (note one grain with 56-124 cleavage just belowleft center).
In the crossed-polarizer view below, the chlorite stands out because ofits anomalous extinction colors.
In the impure quartzite below, the clay filling between the quartz grainshas been metamorphosed to bright green chlorite.
The same field in crossed polarizers. The chlorite is a fine-grainedfelt-like mass showing first-ordred whites and normal extinction colors.
Created 10 Oct 1997, Last Update 31 May 2020