Physical Properties Of Minerals II: Polymineralic Analysis

Preparation

In order to observe and analyze the physical properties of minerals as is done in this video, there are a few preparatory steps that should be taken. First, collect a group of mineral samples. Suggested samples include hematite, magnetite, calcite, dolomite, and galena. Establish a surface for examining the specimens. A clean table-top is suitable, perhaps with a piece of white paper on the table surface. Obtain a porcelain streak plate, a hardness kit, magnet and compass, and dilute HCl (2-5%).

1. Observe and Analyze Color

1. Examine a selection of mineral samples and observe apparent color.
2. Note whether there is color variation within the sample itself.
3. Note whether there is color variation within different samples of the same mineral.

2. Observe and Analyze Streak

1. Take a mineral sample and drag it across the streak plate.
2. Compare the bulk color of the mineral sample with the streak color that is left on the plate.
3. In most cases there will be little difference between the bulk color and the streak color, however a few minerals are notably different, e.g. galena, hematite.
   Streak, i.e. the color of powdered microscopic grains, is occasionally different from bulk color, because of reflectivity effects or the limited control of impurities over color at the small-grain scale.
4. Repeat 2.1-2.3 with other mineral samples

3. Observe and Analyze Hardness

1. As an initial step, take each mineral specimen and attempt to scratch the glass plate with it.
2. Separate these specimens into those that scratch glass and those that do not.
   1. Glass plate is near the middle of the Mohs Hardness scale (hardness 5.5). This separates the group into what geologists refer to as generally hard, versus generally soft minerals.
3. Within each group (hard minerals, soft minerals) test to see which are harder or softer. This is done by seeing which mineral will scratch another.

4. Observe and Analyze Magnetism

1. Easily measurable and identifiable magnetism is restricted to the magnetic group known as ferromagnetism (as opposed to paramagnetism or diagmagnetism, which are very weak and difficult to measure).
2. The minerals with which we can assess magnetism are magnetite, and to some extent hematite and bornite.
3. Using a masonry nail, flake a few grains of magnetite from the sample.
4. See whether the bar magnet (strong ferromagnet) will pick up the grains of the minerals mentioned above, in 4.2.
5. See whether any of the minerals above mentioned (4.2) will affect a compass needle.
   1. Place the mineral sample and the compass side-by-side with about 6 inches of space between them.
   2. Slowly decrease the space separating the sample and compass by moving each toward the other.
   3. The needle of the compass should start to point toward the sample, increasingly so as the space separating the compass and the sample is decreased.

5. Observe and Analyze the Reaction with Acid

1. Minerals that react with dilute hydrochloric acid are carbonates. Examples being calcite—CaCO₃; dolomite—CaMg(CO₃)₂. These are the primary constituents of the important and common carbonate rocks, limestone and dolomite.
2. Take the dropper bottle of dilute HCl and carefully place one to two drops on the surface of the sample.
   Note: Although dilute HCl is not particularly dangerous, it’s best not to get the acid on one’s skin (possible rash), or on one’s clothing (possible staining), and after testing, it’s a good idea to wash off the sample.
3. Note how calcite effervesces vigorously with dilute HCl.
4. Take the dolomite sample, and by either dragging it along the porcelain plate or scratching it with the masonry nail, create some powder/flakes.
5. Repeat step 5.2 but this time place the dolomite sample (not the powder/flakes) into the HCl.
6. Note how dolomite barely reacts with dilute HCl.
7. Now place some of the dolomite powder/flakes in the HCl, and note the increased reactivity when powdered.