The common sedimentary rock minerals include calcite, quartz, dolomite, gypsum, anhydrite, orthoclase, plagioclase, muscovite, glauconite, pyrite, phosphorite, limonite, and all the clays. Less common minerals that may appear in thin section include aragonite, zircon, kyanite, siderite, leucoxene, chamosite, barite, and magnetite. You need to be able it identify these quickly and accurately and you need to be able to deal with the identification of procedures for any other less common mineral or substance that may show up in thin section. The evaluation of a typical thin section generally involves making determinations on several hundred crystals using a systematic procedure called modal analysis. Reflecting back to your mineralogy class, how long did it take you to make a single mineral identification using the petrographic microscope. Making several hundred mineral identifications on a single thin section in a reasonable amount of time will require you to:
You must memorized the optical properties of calcite and quartz and you must learn how to recognize both with out performing any test beyond observation, as one or the other will occur in 95% of the thin sections which you encounter. For starters print out a copy of Mineral Quick-Look and keep it at all times next to your microscope.
The very first thing that you have to do when viewing any thin section is judge the thickness of the section. There is no set standard for thickness, never forget this regardless of what your mineralogy text book said. Most old time mineralogists wanted their thin sections to be 20 microns thick and they took the pains to make them such. Few mineralogists today make their own thin sections but instead have commercial labs produce them. Even though they might advertise that they make a thin section to a specific thickness the end product is frequently not at the specified thickness. Sedimentary petrologists frequently have their thin sections cut thick, 30 to 35 microns, in order to better reveal textural features. Students who make thin sections rarely get them to a uniform thickness hence one side will be 40 microns and the opposite could be 15 micros or worse. Gauging the thickness of a thin section requires that you locate preferably quartz crystals or if no quartz is present then a calcite crystal. View the crystals under cross polarizes or as we used to say under crossed nicols. The colors that you see are the result of :
If you are looking at quartz crystals slowly spin the stage. What is the range of colors that you see? Quartz has a birefringence of 0.009 and thus depending upon the orientation of the crystal lattice an individual quartz crystal will have an apparent birefringence of 0.000 to 0.009. Compare the colors just of the quartz crystals to the colors found on the standard interference color chart. Colors to the left of the chart are the lowest ordered colors while those to the right are the higher ordered colors. As you slowly rotate the stage and are observing several quartz crystals; what the highest color that you see? Next on the chart find the diagonally running line that corresponds to 0.009 birefringence. Follow that line downward until it intercepts the highest color that you saw. At that point track horizontally to the margin of the diagram and note the thickness. This is the approximate thickness of you thin section. (see example of Interference Color Chart) For this to be reliable you need to be sure that it is quartz that you are looking at and you need to be looking at as many crystals of quartz as possible.
If you can not find quartz in the thin section then look for calcite. Find a larger calcite crystal and look very closely at its margin under cross polarizers. Look for a series of bands or rainbow (picture of this). If they are present then what you are looking at is a calcite crystal which has a beveled edge varying in thickness from zero to the thickness of the thin section. Count the number of bands or the number of orders of color that you see on a single beveled edge. Every three bands equals about 10 microns of thin section thickness. It is good to do this with several crystals. If the consensus of a number of crystals is 7 bands then the thickness would be about 22 microns. This is not as reliable as the quartz technique
If you can neither find quartz or calcite than you must apply one or the other techniques to some other mineral that your are reasonably sure of its identification.
Quick and dirty mineral ID
methods:
These are rules-of-thumb which, if you use carefully, will give you a correct identification 95% of the time; 5% of the time you will be wrong, big deal.
Rule #1 Quartz will be the most common mineral that you will encounter in sedimentary rock thin sections. If the thin section is cut to the usual petrographic (20 microns) thickness then quartz under crossed polarized light will appear as shades of gray actually light gray to black or they display 'first order colors'. If the thin section is cut thicker, as many used in sedimentary petrology, then expect other colors; see above. Refer to your Interference Color Chart, which is found in most mineralogy texts. Unfortunately, the Feldspars will also be nearly the same shades of gray as they have nearly the same birefringence. But if
o The grain has any parallel fractures it will not be quartz, (remember quartz has no cleavage), Picture of this.
o Inclusions in quartz, though common, are rarely lineated whereas in feldspars they commonly are lineated, Picture of This. Another Picture
o Inclusions in feldspars are much more dense so if the grain has a lot of garbage in it think feldspar, Picture of this.
o Feldspars, being less stable than quartz, will sometimes exhibit dissolution features like pores within the grain (intragranular porosity), quartz will not,
o Quartz does not have twinning that you can usually see in thin section; feldspars and especially plagioclase do. [Twinning...stripping of shades of gray across the grain].
Rule #2 If it has the tartan pattern it is OK to call it microcline. If you are not using stains and you have no evidence to call it plagioclase or microcline then simply call it "feldspar." Picture of this.
Rule #3 If the grain looks real pretty in thin section (upper first and second order colors) and is elongate and bent; then a good guess would be one of the micas. For the micas assume white mica if clear in plain light and the colors are brilliant under crossed polarized light, chlorite if greenish and pleochroic in plain light and gray in crossed polarized light, biotite if the colors appear to be masked with brown. White Mica, or as sedimentary folks say 'Muscovite' is the most frequently encountered mica in sedimentary rocks.
Rule #4 If there is more than an estimated 10% of combined feldspar and mica present then Rule #1 may not be so good. Frequently orthoclase and plagioclase will appear just like quartz with no linear structures, no cleavage and no twinning. What you have to do is pull an interference figure on each crystal. Quartz is uniaxial and the feldspars are biaxial. The alternative is to stain the thin section for both orthoclase and plagioclase.
Rule #5 If it has brilliant second order colors and occupies the space between grains or what was porosity and does not really look like a mica it is probably anhydrite. This should not be confused with gypsum, which will have gray first order color but otherwise occur in the same position as anhydrite in thin section.
Rule #6 If a grain is opaque in transmitted light and brassy in reflected light call it pyrite. If white in reflected light call it leucoxene, and if you are not sure just call it an 'opaque mineral.' To view in reflected light takes a special lighting system for your microscope. But simply blocking off you substage illumination with your hand and shining a flashlight or similar light down from above onto the thin section will also sort of work; but do not use a LED light. If you really want to get an idea of the whole realm of opaque mineral identification then visit: Virtual Atlas of Opaque and Ore Minerals.
Rule #7 Green grains, which are generally shaped like pellets, are usually glauconite; if brownish possibly chamosite or phosphorite; reddish grains are hematite or a limonite stained grain. Phosphorite will have a low birefringence or appear isotropic.
Rule #8 Zircons are strange looking in thin section. They look like something stuck on the surface of the slide and out of focus. Look for very high relief and lots of color. Do not expect an abundance of these.
Rule #9 Clays are generally difficult to work with in thin section. Usually it is best to just call them clays. Look for tiny yellowish or brown crystals generally located between grains or replacing feldspar grains. Clay could be confused with microcrystalline calcite. Some older adhesive may also look like clay but a good petrologist will know the difference.
Rule #10
Calcite is the most common carbonate mineral in sedimentary rocks. Look for
a salmon color under crossed polarizes (4th 5th 6th order colors). Sometimes you
can see rainbows of color at the edge of the grain. It is very easy to confuse
calcite with dolomite unless you stain the thin section (see below). Aragonite
is also easy to confuse with calcite but rarely occurs in rocks older than the
Quaternary. For some folks dolomite sometimes looks greenish (very faint) in
plain light. It is not greenish to me but some students have said hey doc! What
is this greenish mineral?
Rule #11 Pores are holes in the rock, which in thin section are now filled with the glue that holds the rock to the glass slide. The glue is not crystalline (well in some very old thin sections, which were glued with Lakeside 70 the glue may have partially crystallized). In polarized light a pore will look black, just like an extinct grain and in plain light it will look just like a transparent grain and it is dificult to tell the two apart. Trick one is to memorize the 1st order red color then insert the 1st order red accessory plate into the optical path of the microscope and look for pores. With some skill, this will help. If you plan ahead and have the bucks you can have the sample impregnated with blue epoxy prior to making the thin section. Then the pore will be Carolina blue. Photomicrographs of these.
Rule #12 When you are left clueless as to what mineral you are looking, at try determining its optical properties and then compare your results to those of the common sedimentary rock forming minerals. The table below lists the optical properties of the common minerals and some of the ones that cause students problems. However, remember when all else fails open your optical mineralogy textbook! If that doesn't work then call the strange mineral Unknown Type A and the next one Unknown Type B and get on with it.
Rule #13 Learn what grit looks like!!!! In addition, when you learn how to make a thin section you will know why you should learn what grit looks like. Oh! if it is circular looking best to call it an air bubble.
Staining of Thin Sections to
Reveal Carbonate Mineral Type
Many of the thin sections used in the next several lab exercise have been stained so that one can differentiate between calcite and dolomite; remember the HCl method is a good story but not very reliable. Two stains are used Alizarin red-s and Potassium ferricyanide blue (Table 1). The alizarin red-s will stain any variety of calcite a red or pink color but will not stain any type of dolomite. The potassium ferricyanide blue stain will stain any calcium carbonate or calcium-magnesium carbonate mineral with around 10% or more FeCO3 substitution a blue color. Such minerals are said to be ferroan. Thin sections can be stained with one or both stains. If both stains are sued on the same thin section then the stains can superimpose on one another such that a ferroan calcite will look, once stained, like it is purple [red + blue]. Non ferroan dolomite will not stain at all, non ferroan calcite will be red or pink, and ferroan dolomite will come out Carolina blue. This is the best way to differentiate these minerals. The red and purple colors are distinctive and will not be confused with anything else that you will see. The blue color is almost the same shade of blue as the blue epoxy used to reveal pores.
|
|
Ferroan Calcite |
Non-ferroan Calcite |
Ferroan Dolomite |
Non-ferroan Dolomite |
|
Alizarin Red-s |
Yes Red |
Yes Red |
No |
No |
|
Potassium Ferricyanide Blue |
Yes Blue |
No |
Yes Blue |
No |
|
Combined Stains |
Purple |
Red |
Blue |
Clear |
Table: Colors that result form the use of stains on calcite and dolomite. See (Dickson, J.A.D., (1966), Carbonate identification and genesis as revealed by staining: Journal of Sedimentary Petrology, vol. 36, p. 491-505.) for additional information of the technique.
One problem that you must keep in mind with these stains is that over the years the potassium ferrocyanide blue stain tends to fade; consequently the pretty Carolina blue color of ferroan dolomite may not be overly evident, but then it is not overly common anyway. Also it is really easy to screw up the staining process if one does not follow the correct procedure. Commercially prepared thin sections should always be suspect.
The only reliable and quick way to differentiate these minerals is to stain them with sodium cobaltnitrate and Rhodizonate Solution. However, usually this is not done because the procedure requires the use of hydrofluoric acid which is very dangerous to use and requires special fume hoods which are not readily available. Some commercial thin sectioning firms will do such staining but for a price. If you have a stained thin section then:
Don't worry about confusing the pink or red stained plagioclase with red stained calcite as the hydrofluoric acid treatment will have removed all calcite from your thin section!!!
| Quartz | Orthoclase | Plagioclase>3%An | Plagioclase<3% An | |
| No stains | Clear | Clear | Clear | Clear |
| Sodium Cobaltnitrate | Clear | Yellow | Clear | Clear |
| Rhodizonate Solution | Clear | Clear | Pink to Red | Clear |
| Both Stains | Clear | Yellow | Pink to Red | Clear |
Table: Colors that result from the use of stains on quartz, orthoclase, and plagioclase. See (Bailey and Stevens, 1960. Selective staining of K-feldspar and plagioclase on rock slabs and in thin sections. American Mineralogist, v. 45, p. 1020-1025.
Exercise #1: Examine the collection of reference thin sections for sedimentary rock forming minerals. Spend enough time so that you are VERY familiar with each of these. In the very near future you will have to identify all of these under less than ideal circumstances and very quickly. Print a copy of this KEY to Thin Sections
Modal Analysis
Modal analysis is a survey that you make to determine the percentages of the various minerals and grain types that make up the volume of a rock. We assume that the thin section gives one a representative sampling of the bulk make-up of the rock. There are two steps to a modal analysis (1) survey of what is there and (2) rapid count of what is there.
Step (1) Look at the thin section while it held up to a window or light. Do you see big grains or small ones? Keep in mind the size of the grains. Next put it under the microscope and look at it. Is there any porosity? Make a list from you think is the most abundant to the least abundant of what makes up the sample; this includes pores if present. Set up a form like Point Count Form where you list the perceived most abundant at the top and the least abundant at the bottom. Always leave a few lines for those items you "discover" while doing the modal analysis. You may also do this directly in EXCEL if you have your laptop with you.
For the "unknowns" you need to provide all the information that you can so that later on you might just be able to deduce what it was. Do not spend a whole lot of time on the reconnaissance.
Step (2) Using what we call a random walk (Figure 1) move the thin section and identify what is exactly under the cross hairs.....NO CHEATING, BE HONEST. When you have identified the material type, put a tick mark after the item on your list in the raw data column. Then go on to the next spot on the thin section, but do not be looking down the microscope while you are making the move. The more points you examine the more accurate your analysis becomes. Typically, a petrologist will do 300 identifications per thin section.
Figure 1. Thin section with a map of the track of a random walk for a
modal analysis.
If you have the luxury of a mechanical stage you can adapt a more systematic gird pattern to your search (Figure 2). What happens if when using this grid you discover that the grid lines parallel a fabric feature of the rock? Answer: a little directional bias comes into your results. You also can add bias to your results when using the random walk especially if you watch to where you move the cross hairs. We have a tendency to move toward things we know and avoid that which we don't. This can be overcome by looking away while making the move; also less chance of encountering vertigo and excessive eyestrain when doing this.
Figure 2. Typical search pattern for modal analysis using a mechanical
stage where each dot represents the location of a determination.
Exercise #2: (You must follow these directions to the T. if you don't you will be wasting a whole lot of time because you will have to start over) For any thin section of the special set for this purpose identify the minerals present and record on the form below. Next do a modal analysis of only 25 counts and calculate the percentage of each where percentage equals number of hits on the particular item divided by total number of identifications (25). Record those percentages in the 25-column on Special Form that you should print out before lab time. Then continue on doing an additional 25 more identifications and calculate the percentages. Record this in the 50-column. Continue on doing an additional 50 more identifications, calculate and record in the 100-column. Then do 100 more IDs calculate and record in the 200-column. Think before your leap!
Notice how the calculated percentages differ depending upon how many counts
you made. The more counts you make the more accurate your results are but also
the more time required doing the job. Obviously if you had a dozen thin sections
all from the same sample and did 200 counts per thin section and tallied then
together you would quite accurately "know" the distribution of the
material in the sample..... and you would be all day doing it!!! Always consider:
how much accuracy do I need and how much time do I have to do the job in.
You can quantify your accuracy-using Figure 3, which allows for determining the "ninety-five percent confidence limits" of your modal analysis. That is the results you get will be reproducible 95% of the time within the prescribed limits. The 'n' or the vertical scale is the total number of identifications or hits and the 'p' scale is the percentage of each grain or mineral type. The percentages within the diagram represent the 95% confidence intervals (numbers are posted above their respective lines). For example, if a modal analysis consisted of 300 determinations and a particular grain was hit on 25% of the time then the amount of the volume of the rock occupied by that grain type would be 25%±5% or the 95% confidence limits would be 20% to 30%.
Figure 3. Ninety-five percent
confidence limits for object proportions from modal analysis. The "n"
denotes the number of total identifications and the "p" denotes the
calculated percentage of the total for a particular object type.
Exercise #3: Determine the 95% confidence limits for each stage in your modal analyses or exercise #2. Then completely redo the modal analysis but this time do only 100 hits. Does this analysis agree within the 95% confidence range of the previous 100-count analysis?
From the results of your modal analysis you can do a number of things such as
Determination of the bulk density is simple. For example, from modal analysis you have determined that the rock is made up of 25.0% ±5% quartz, 70.0% ±4%calcite and 5.00% ±4% porosity. Then a one centimeter on a side block of the rock would contain:
quartz
0.250 ± 0.05 X 2.65 grams/ cm3 = 0.66 ± 0.13 grams/ cm3
calcite
0.700 ± 0.04 X 2.72 grams/ cm3 = 1.90 ± 0.11 grams/ cm3
porosity
0.050 ± 0.04 X 0.00 grams/ cm3 = 0.00 ± 0.00 grams/ cm3
Using the error propagation law:
where "t" is main fraction of the sample's mass and 
(sigma) is
the 95% confidence interval of that. Look at the numbers that are plug into the
equation below. The bulk density would be"
grams/cm3
bulk density therefore is: 2.56±
0.17 grams/cm3
EXCEL will do this quite nicely.
Exercise #4: Calculate the bulk density of the sample which you made modal analysis of. You will need to look up the density of the mineral species in Dana's Manual of Mineralogy or your old mineralogy book. (Site your sources.)
Combine exercises 2, 3 and 4 into an EXCEL file and send it to me via email. DO NOT USE SCIENTIFIC OR ENGINEERING NOTATION FOR THIS LAB.