Metamorphosed pyroxene and plagioclase in Litchfield (meta)norite

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Metamorphosed orthopyroxene (left, bottom, upper right, lower right) and plagioclase (center) in  norite. Pyroxene grains show varying degrees and types of metamorphism. Original linear structure of large pyroxene grain at left is detectable only as a weak lination imparted by dark specks. Plagioclase is more intact, but shows degradation (blurring) of twin lamellae. Mt. Prospect complex, Litchfield County, northwestern Connecticut. XPL. Imaged area 1.3mm x 2mm.

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Higher-magnification image of replacement product (blue) in large orthopyroxene grain (left side of top photo). XPL. Imaged area 0.5mm by 0.8mm.

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Higher-magnification (10x objective) image of heavily-altered orthopyroxene grain (lower right corner of top photo). Green areas are remnants of original mineral. Note linear (chain silicate) structure. Also note that small grains of opaque mineral, presumably an iron-oxide mineral, are preferentially concentrated in pyroxene remnants. XPL. Imaged area 0.5mm by 0.8mm.

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PPL image of above, showing relict lineation and concentration of opaques in pyroxene remnants.

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Higher-magnification (40x objective) image of cluster of opaques in center of photo above. Note that many of the "opaques" appear to be slightly translucent brown ovals, aligned to the lineation in the pyroxene. PPL. Imaged area 0.13mm by 0.2mm.

Hydrous minerals replacing plagioclase and pyroxene in metabasalt

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Above: Ophitic-texture pyroxene and plagioclase in a metabasalt are being replaced by hydrous minerals of the greenschist facies. The large orange and brown cluster was a pyroxene grain enclosing the now-corroded plagioclase laths. The pyroxene has been replaced by uralite, a massive amphibole usually composed mainly of actinolite (less commonly of hornblende). The scattered plagioclase laths show varying degrees of replacement by epidote, and probably by other minerals too small to identify. Extensive replament has occurred in the two adjacent, parallel plagioclase laths at right of center (See second image below for greater detail). Greenstone cobble in glacial gravel, southeastern Michigan, Specimen No. 49. Provenance likely northern Ontario. XPL. Imaged area 1.3mm by 2mm.

Below: PPL image of above. Green color of uralite is characteristic of actinolite (as well as hornblende).

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Below: Higher-magnification image of area right of center in top photo. Note the cluster of small epidote grains that have replaced much of the two adjacent plagioclase laths. The replacement of plagioclase by epidote in this location was likely speeded by a channel between the two grains (visible as a narrow strip of uralite) that provided access by percolating fluids. XPL. Imaged area 0.5mm by 0.8mm.

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Epidote-rimmed albite and sieve-textured hornblende in Buck Creek (NC) amphibolite.

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Epidote (cyan and magenta) rimming albite grains (white or gray) and sieve-textured hornblende (stippled orange and brown grains: left center, bottom center, lower left, bottom right) in Buck Creek  amphibolite. Blue Ridge Mountains, Clay County, western North Carolina. XPL. Imaged area 1.3mm by 2 mm.

Below: Higher magnification PPL image of sieve texture in hornblende grain at bottom center of image above. Imaged area 0.27mm by 0.4mm.

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Below: Epidote-rimmed albite in another area of the same thin section. XPL.  Imaged area 0.76mm by 1.14mm. Ab=albite, Hbl=hornblende, Mt=magnetite. Epidote, not labeled, is the high-birefringence mineral rimming the albite grains.

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Below:  PPL image of above. Note green color of hornblende (stage is rotated to give a pale green color) and sieve texture in hornblende grain at left of image.

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Pseudomorphs after pyroxene in greenstone

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Above: Serpentine pseudomorphs after pyroxene. Greenstone, specimen No. 5. Pebble in glacial gravel, southeastern Michigan. Provenance likely northern Ontario. XPL. Imaged area 2.7mm by 4mm.
Below, as above. Imaged area 3.2mm by 3.5mm.

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Below, as above. Imaged area 1.3mm by 2mm.

Epidote, pyrite, and titanite in greenschist.

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Epidote (high-birefringence, high-relief mineral, at right), pyrite (opaque mineral grains at left), and titanite (brown mineral surrounding pyrite). Greenschist,  specimen No. 1. Pebble in glacial gravel, southeastern Michigan. Provenance likely northern Ontario. XPL above, PPL below. Imaged area 1.3mm by 2mm.

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Subhedral pyroxene in greenstone, No. 48 - full thin section

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Subhedral pyroxene grains in greenstone. Specimen No. 48, cobble in glacial gravel, southeastern Michigan. Provenance likely northern Ontario. Macrophotograph in cross-polarized light. Imaged area 22mm by 38 mm.

Below: polished cross-section of cobble from which sample was cut. Optical scanner image. Long dimension 6.3cm.

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Euhedral pyroxene in greenstone (full thin section)

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Euhedral pyroxene crystals in greenstone. Specimen No. 54, cobble in glacial gravel, southeastern Michigan. Provenance likely northern Ontario. Macrophotograph in cross-polarized light. Imaged area 21mm by 39mm.

Actinolite in greenschist

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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.

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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

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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.

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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.

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Below: As above, PPL.

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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).

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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.

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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

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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.

Greenstone pebble, No. 5, composite image

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Composite image of section of greenstone pebble. Optical scan of polished section in on left, macrophotograph (in cross-polarized light) of part of thin section on right. Long dimension of composite photo 38mm.

Metagabbro with sericite and titanite

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Diorite-gabbro altered to metagabbro. A grain of feldspar has been largely altered to sericite (large mass of fine-grained gray-brown material in center). Two green hornblende grains are at top center and right center. A brown grain of titanite is at bottom right, Mt. Prospect complex, Bantam, Connecticut. XPL. Imaged area 1.3mm x 2 mm.

Triassic Orenuag basalt with evidence of low-grade metamorphism

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Orenaug basalt (from the Pomperaug basin in western Connecticut) that has undergone low-grade metamorphism. XPL. Imaged area 2mm x 3mm.