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NOTE: Picture 7 is an animated GIF of pictures 1-6, concocted by the MPOD Web Master, aka The Idiot Programmer™, for his own amusement.


      ^{Michel-Lévy Color Chart}

Bill writes:

Thin Sections and Polarized Light Microscopy: A Brief Introduction
Transmitted polarized light microscopy provides scientific information and, many times, a mentally aesthetic sensibility for the observer. Proper orientation of the sample grain with respect to the aligned crossed polars and accessory plates, or compensators, give the information; the "mentally aesthetic sensibility" is ancillary.

Optically transparent crystals have one, two, or three refractive indexes. Both "indexes" and "indices" are acceptable English usage for the plural of "index". Cubic crystals without substantial lattice defects and all other isotropic materials, such as gases, liquids, and unstrained glasses, under ambient conditions have only one refractive index. Between crossed polars these materials appear black regardless of their inherent color or orientation. When any material, such as a grain in a meteorite thin section, has more than one refractive index, colors result between the crossed polars. The observed colors are interference colors and are dependent mainly on the sample's orientation, thickness, and birefringence – the difference between two refractive indexes.

When a birefringent crystal grain is oriented such that it is black or at "extinction" between crossed polars and then rotated to the next adjacent extinction position, note is made of the microscope stage's angular displacement to determine whether the extinction is parallel, symmetrical, oblique, or undulose relative the initial position. This helps in crystal identification. Addition of compensators, or accessory plates, further aids identification.

Figure 4 illustrates the third order blue 1200 nm retardation color of this sample. Adding a first order full wave plate of 530 nm produces a total retardation of 1730 nm: See Figure 5 and compare it to the Michel-Levy diagram.

Figure 6 illustrates the addition of the quarter wave plate of 147.3 nm. Add this retardation to the original 1200 nm blue and compare it to the Michel-Levy diagram.

More on "order" another time.

Two easily readable books on the above subject that I used with my college freshman chemistry scholar (independent study) students were:

Optical Crystallography by F. Donald Bloss, Mineralogical Society of America, 1999, ISBN 0-939950-49-9

Microscopic Identification of Crystals by Stoiber and Morse, Robert E. Krieger Publishing Company, 1986, ISBN 0-88275-975-2

Several basic experiments below that you can do will give you a better understanding of what is described above and a bit more:

1) The transmission vibration direction of a polarized sheet is determined by viewing glare from a shiny surface such as a puddle of water, ice, etc. View this shiny reflection through a sheet polarizer. Rotate the sheet; when the glare is minimum, the vibration direction of the polarized sheet is now vertically oriented.

2) Place two linear polarizers one on top of the other such that they allow the maximum amount light to pass through. The sheet nearest the light source is called the polarizer and the sheet furthest from the light source is called the analyzer. This parallel orientation of the two polars allows for the maximum transmission of light. Note that when the polarizer's transmission direction is oriented east-west and the analyzer is oriented north-south with respect to observer looking straight ahead, the "x" axis is the E-W direction; originally the German convention.

3) Now rotate one sheet by 90^o with respect to the other; light barely, if at all, passes through. The sheets now should be perpendicular to each other. This is what is meant by crossed polars and the observed effect is called extinction.

4) Take the assembly in experiment 2) and insert a third linear polarizing sheet between the two that are crossed. As the third sheet is rotated relative to the crossed polars varying amounts of light pass through the assembly. The maximum occurs at a 45^o orientation of the third sheet. This effect is called compensation and the third sheet is called the compensator. A mineral grain of the sample (read thin section) is essentially a compensator. Polarized light microscopes have compensator slots in their barrel (body tube) that can be oriented at 45^o.

5) Take an unstrained piece of glass (most glass microscope slides), place a piece of non-opaque sticky tape on it about .5 cm from the left end all the way to the right end of the slide. Repeat this taping process several more times by placing each successive piece of tape .5 cm from the previous one until the right end of the slide is met. Going from left to right, the first part of the slide is uncovered by the sticky tape and the last part has the thickest layer of tape. This essentially is a "Fox Wedge" compensator. Place your wedge orientated 45^o to the crossed polars of experiment 2) and note the interference colors – they are the colors of the Michel-Levy diagram posted with every MPOD thin-section. The same material of different thicknesses produces different interference colors. If sufficient layers of tape are present, the observed "color" is high order white.

A sample of unshocked quartz of 30 micrometer thickness viewed perpendicular to its c-axis, when inserted at 45^o between crossed polars, generates a first order white on the Michel-Levy diagram. This is the reference standard that is easily attainable, stable under ambient conditions, and non-toxic unless ingested.

Linear polarizing material may be obtained from inexpensive polarized sunglasses or scientific supply houses. If using bulk material be sure to remove the plastic protective coverings.

This hopefully help explain what you are looking at when viewing thin sections between crossed polars.