We’re back this week to a slightly unusual mineral to examine in thin section: halite. In my copy of Tröger (there is an English translation, but its also out of print), halite (NaCl) is listed as mineral number 4. Right up there with ice, lime (CaO), villiaumite (NaF), and sylvite (KCl).
Minerals 1 – 64 in Tröger are isotropic, which means that we’re only going to have to worry about properties in PPL this week.
(Finding photomicrographs of halite is not exactly easy. Quite a few of the images I found were out of sedimentology papers and in black & white. I’ve pulled what I can for color, but if someone knows of a good source, please add a comment at the end!)
In PPL, halite can vary from colorless, to a light blue-violet or yellow color. Inclusions of fluid or air within halite are quite common, which can make the crystals appear “dirty” in thin section. The refractive index of halite is 1.544, which is very, very close to the mounting epoxy, so halite will have a low relief and will make cleavage very difficult to see (even though halite has three perfect planes at 90).
(Black & white image, unfortunately.) Laminae of well-rounded, medium-sized quartz sand grains, inclined at approximately 30°. Interstices are partly filled with fine sand grains. Many grains are floating in halite cement. Cedar Hills Sandstone, 2016.6 ft. (Plane Polarized light, x 32). http://www.kgs.ku.edu/Publications/Bulletins/215/
- Photomicrograph of an unirradiated sample (salt layer D, 423.3 m) showing primary fluid-inclusion-rich bands (c) and zones of clear halite (d). Clear halite commonly truncates fluid-inclusion-rich domains across sharp, curved, intracrystalline boundaries. Grain boundaries show up as dark curves in thin section. Secondary phases occurring at grain boundaries are anhydrite and polyhalite. Transmitted light image, image width is 2 cm.http://www.ged.rwth-aachen.de/Ww/projects/salt/IJES/SchlederUraiIJES2005.html
To examine the internal structure of halite, geologists may choose to irradiate the halite. Images look ridiculously cool 🙂
Overview image of a thin section of sample gamma-irradiation decorated at 100 ºC from salt layer A (457.5 m), photographed in transmitted light. Grain boundaries occur as dark curves; white patches are fluid-inclusion-outlined chevrons and hoppers; and white polygons are subgrains. A few new, recrystallized, strain-free grains or grain regions are visible (see arrows). Image width is 4 cm.http://www.ged.rwth-aachen.de/Ww/projects/salt/IJES/SchlederUraiIJES2005.html
Photomicrographs show the microstructures of sample from salt layer A (457.5 m). Plane polarized transmitted light images of sample gamma-irradiated at 100 ºC. a) Migration of high-angle grain boundaries as recorded by elongated subgrains, and strain-free new material, which grows at the expense of old, heavily substructured grains. Image width is 11 mm. b) Strain-free grains grew at the expense of deformed ones. The size of some of the new grains is comparable to that of subgrains. Image width is 7 mm.http://www.ged.rwth-aachen.de/Ww/projects/salt/IJES/SchlederUraiIJES2005.html
Microstructures of sample from salt layer A (457.5 m, gamma-irradiated at 100 ºC), photographed in plane polarized transmitted light. In both images the grain boundaries occur as dark curves, while the white, milky areas are remnants of chevron grains. a) The clear, fluid-inclusion-free regions could be explained by a recrystallization mechanism of grain boundary migration. However, the presence of subgrains both in the chevrons and in the fluid-inclusion-free region implies that the structure is syndepositional and is a product of dissolution and precipitation processes. Image width is 16 mm. b) Similar structure as shown in image “a”. Milky, fluid-inclusion-rich (upper left corner) and clear, fluid-inclusion-free halite are separated by a sharp, curved line. The presence of subgrains in both the milky and clear parts is strong evidence that the structure was not developed by grain boundary migration. Note the serrated grain boundary at the right side. Image width is 1 mm.http://www.ged.rwth-aachen.de/Ww/projects/salt/IJES/SchlederUraiIJES2005.html
Ok, so who is looking at halite in thin section? Halite can form as a cement in sedimentary rocks and also as the primary component in some evaporite deposits. In the case of the group in Aachen (where most of the images above came from), the geologists are trying to discover how the halite has deformed over time to because it has implications for oil-bearing rock evolution. In the image below, droplets of oil has been incorporated as fluid inclusions in the halite.
Before we come to an end, we should quickly discuss what halite might be mistaken for.
- halite vs. quartz / feldspars / calcite -> halite is isotropic, all the others are anisotropic
- halite vs. garnet (though why they would be in the same rock…) -> garnet has high relief vs. halite’s very low relief
- halite vs. fluorite -> fluorite has a lower refractive index (1.43) than epoxy, so it will have a negative relief / Becke Line; also, fluorite is present mainly in igneous rocks where halite is unlikely to be found
- halite vs. sylvite -> sylvite also has a lower refractive index (1.49) and that’s the best way to distinguish between them
- halite vs. holes in the thin section -> this actually comes up as a problem, since halite may pluck out in the polishing process; glass / epoxy will tend not to have the bubbles that are frequently found in halite, but the refractive index is about the same; irradiation may really help this situation
Next week we’re going to talk about plagioclase, which means that the Michel-Levy technique will come up.
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