Actinolite in greenschist

Click on image to enlarge.          Photo © Daniel R. Snyder

Tabular and acicular actinolite crystals in greenschist. XPL above, PPL below. Actinolite crystals are light green or dark green, with some zoned crystals and some crystals at extinction (black). Small, irregularly-shaped, multicolored grains at right are epidote. Black and gray mesostasis is mainly quartz. Greenschist pebble, specimen No. 52, from glacial gravel, southeastern Michigan. Provenance likely northwestern Ontario. Imaged area 0.57mm by 0.8mm.

Click on image to enlarge.          Photo © Daniel R. Snyder

PPL image of same area as shown in XPL image. Actinolite is green to blue-green. Small, irregularly-shaped, high-relief grains at right (pale yellow) are epidote, colorless mesostasis is mainly quartz.

Epidote pleochroism in greenschist

Click on image to enlarge.          Photo © Daniel R. Snyder

Epidote in greenschist. In XPL image above, epidote grains are yellow to orange to magenta; some of them, at or near extinction, are dark brown or black. Many of the larger grains show epidote's characteristic oval shape. Small, irregularly-shaped epidote grains in the upper half of the image are partially masked by their high-relief outlines, and also by magnetite grains and finely disseminated magnetite. Imaged area 0.57 mm x 0.8 mm.

The two PPL images below cover the same area as the XPL image above. The characteristic yellow-green to green pleochroism of epidote is evident in these images, which were made with the polarizer at two different angles, approximately 90 degrees apart. Many magnetite grains, clustered above the center of the image and in the upper right corner, are more clearly visible in these PPL images than in XPL.

Click on image to enlarge.          Photo © Daniel R. Snyder

Click on image to enlarge.          Photo © Daniel R. Snyder

Greenschist, No. 1, full thin section in cross-polarized light

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Greeschist pebble from glacial gravel, southeastern Michigan. Provenance likely northwestern Ontario. Macrophotograph in cross-polarized light. Long dimension of imaged area 36mm.

Greenschist, No. 52 - full thin section in cross-polarized light

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Full thin section of greenschist, specimen No. 52. Cobble from glacial gravel, southeastern Michigan. Provenance likely northwestern Ontario. Macrophotograph in cross-polarized light. Long dimension of imaged area 40mm.

Archean greenstone, No. 92

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Archean greenstone collected from outcrop,  Haviland Bay, Ontario (East end of Lake Superior). XPL. Imaged area 1.33mm by 2mm.

Sub-greenschist facies metamorphism in Keweenawan (Pre-cambrian Y age) Basalt

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Above: Above: Fine-grained metamorphosed basalt from Pre-cambrian Mid-Continent Rift. Note abundant corroded plagioclase. No relict pyroxene is evident. Most of the black masses are magnetite (sample is strongly magnetic). Round black mass at lower left is a chlorite vesicle filling. The light rim is calcite. Mamainse Point, Ontario (eastern end of Lake Superior). XPL. Imaged area 1.3mm by 2mm.  Photo made with 4x objective.

Below: Higher-magnification image of a portion of image above. Note predominance of small grains with upper first-order and second-order interference colors. Black masses are magnetite (compare with PPL image below). XPL. Imaged area 0.5mm by 0.8mm. Photo made with 10x objective.

Click on image to enlarge.          Photo © Daniel R. Snyder

Below: As above, PPL.

Click on image to enlarge.         Photo © Daniel R. Snyder

Below: Anheadral to subhedral grains of crystalline replacement mineral(s) surrounded by partially-altered plagioclase. Dark areas are magnetite. Left image XPL, taken with 10x objective, digitally enlarged to match right image. Right image PPL, taken with 20x oblective. Imaged area of each panel 0.24mm(W) by 0.32mm(H).

Click on image to enlarge.          Photo © Daniel R. Snyder

Below: Higher-magnification image of a field of mineral grains characteristic of this specimen. PPL. Imaged area 0.13mm by 0.2mm. Thus, the long dimension of the imaged area is 200 microns. Photo made with 40x objective.

Click on image to enlarge.          Photo © Daniel R. Snyder

I will write more about the difficulty of making positive identification of the green mineral. It could be any of several hydrous calcium-aluminum silicates. If YOU know what it is, please comment!

More Text coming

Subhedral epidote grain in greenstone

Click on image to enlarge          Photo © Daniel R. Snyder

In rocks of the greenschist facies, such as common greenstones, epidote is typically associated with chlorite, actinolite, albite, and quartz. This roughly diamond-shaped epidote grain is cut by several fractures that are filled with chlorite, which is also the dominant mineral in the mesostasis. Greenstone pebble found in glacial gravel in southeastern Michigan. Provenance likely northwestern Ontario. PPL on left, XPL on right. Imaged area 0.8 mm in height.