Physical Properties Of Minerals II: Polymineralic Analysis
Source: Laboratory of Alan Lester - University of Colorado Boulder
The physical properties of minerals include various measurable and discernible attr…
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
- Examine a selection of mineral samples and observe apparent color.
- Note whether there is color variation within the sample itself.
- Note whether there is color variation within different samples of the same mineral.
2. Observe and Analyze Streak
- Take a mineral sample and drag it across the streak plate.
- Compare the bulk color of the mineral sample with the streak color that is left on the plate.
- 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. - Repeat 2.1-2.3 with other mineral samples
3. Observe and Analyze Hardness
- As an initial step, take each mineral specimen and attempt to scratch the glass plate with it.
- Separate these specimens into those that scratch glass and those that do not.
- 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.
- 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
- 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).
- The minerals with which we can assess magnetism are magnetite, and to some extent hematite and bornite.
- Using a masonry nail, flake a few grains of magnetite from the sample.
- See whether the bar magnet (strong ferromagnet) will pick up the grains of the minerals mentioned above, in 4.2.
- See whether any of the minerals above mentioned (4.2) will affect a compass needle.
- Place the mineral sample and the compass side-by-side with about 6 inches of space between them.
- Slowly decrease the space separating the sample and compass by moving each toward the other.
- 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
- 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.
- 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. - Note how calcite effervesces vigorously with dilute HCl.
- Take the dolomite sample, and by either dragging it along the porcelain plate or scratching it with the masonry nail, create some powder/flakes.
- Repeat step 5.2 but this time place the dolomite sample (not the powder/flakes) into the HCl.
- Note how dolomite barely reacts with dilute HCl.
- Now place some of the dolomite powder/flakes in the HCl, and note the increased reactivity when powdered.
The physical properties of minerals include various measurable and discernable attributes that are unique and mineral-specific.
Rocks are aggregates of mineral grains. Most rocks are polymineralic, meaning that they are composed of multiple types of mineral grains. Some rocks are monomineralic, and are effectively composed of a single mineral. Analysis of crystal form and crystal cleavage is typically used to classify monomineralic compounds. However, geologists often classify polymineralic rocks according to other physical properties such as color, hardness, magnetism, and reaction with acid. This video will introduce the physical properties of minerals, and demonstrate mineral classification using simple standard tests.
A single mineral specimen exhibits a number of unique physical properties that are used in identification and classification. First, minerals exhibit a wide range of colors, often resulting from trace transition metals. Mineral color simply refers to the apparent color of the mineral resulting from the wavelengths of light that are preferentially reflected from a mineral surface.
Streak refers to the color of the powdered sample of the mineral. Streak is observed by dragging a mineral sample across a rough porcelain plate in order to create a line of powdered material. The apparent color of a mineral can vary, due to impurities that absorb or reflect light. However, the streak color is more reproducible, as the fine grains are randomly oriented and less affected by crystal structure and impurities.
Next, mineral luster can be studied. Luster is a subjective measure of how a mineral reflects light. It is divided into two general categories; metallic materials that are shiny and reflective, and non-metallic minerals that appear dull.
Hardness, or a mineral's resistance to disaggregation, is another property used for classification. Hardness is measured according to the Mohs hardness scale, which is a set of ten reference minerals ranked based on their hardness. Minerals are ranked on this scale by their ability to scratch another material or be scratched by another material. A minerals ability to scratch a reference material implies that it is harder than the reference, and vice versa.
Some minerals exhibit magnetism, enabling it to influence a magnet or compass. In general, this property is exclusive to the mineral magnetite, however some other minerals can exhibit weak magnetism after heating. Finally, a mineral's reactivity with dilute acid is measured to test for the presence of carbonate compounds. There are numerous carbonate minerals: the most common being calcite.
Now that you've seen the principles behind these properties, let's look at how some of them are tested in the lab.
To analyze mineral color, first place all mineral samples on a clean tabletop covered with white paper. Examine each mineral and observe its apparent color. Note whether there are color variations within the sample itself. Observe different samples of the same mineral, and note whether there is color variation between samples. Variations can indicate impurities in the mineral. Next, observe mineral streak by dragging a mineral sample across a porcelain streak plate. Compare the streak color to the mineral color. In most cases, the streak color is similar to the mineral color. However, some minerals do exhibit differences between streak color and overall color. Repeat these steps with the other mineral samples.
To analyze mineral hardness, first attempt to scratch a glass plate with the mineral samples. Glass is near the middle of the Mohs Hardness scale. Minerals that are able to scratch glass are generally classified as hard materials. Separate the samples by ability to scratch glass. Test materials within the hard and soft groups by scraping the minerals against each other. Those that are able to scratch a mineral are harder than those that are scratched. Rank the minerals according to their hardness.
Next, ferromagnetism can be measured by first flaking a few grains of the mineral, magnetite in this example, using a masonry nail. Using a bar magnet, observe the behavior of the mineral flakes with the magnet. If the magnet picks up the flakes, the mineral exhibits ferromagnetism. Next, check for interaction with a compass needle. Place the mineral sample side-by-side with about six inches of space between them. Slowly decrease the space between the mineral and compass. If the sample is magnetic, the needle of the compass will point toward the sample, increasing as the space is decreased. Repeat these steps for the other mineral samples.
The identification of the physical properties of rocks and minerals is a key first step in mineral identification. While these physical property tests are valuable tools for identifying minerals in the field, laboratory techniques are now available that enable detailed characterization of materials. For example, the detailed characterization of materials for use in applications such as lithium ion batteries can be conducted using x-ray diffraction, or XRD. XRD utilizes the regular diffraction pattern of x-ray beams to determine a materials crystal structure, and enable detailed structural characterization.
Diamond anvil cells are devices able to reach extremely high pressure, due to the extreme hardness of diamonds. In this example, a diamond anvil cell was used to synthesize and analyze new phases of matter at extremely high pressure. The sample was loaded into a diamond anvil cell, and mounted inside of a water cooled copper chamber. The device was then mounted on a stage in line with a synchrotron X-ray source.
Material synthesis at 15 GPa and 1,700 Kelvin was measured using X-ray diffraction.
You've just watched JoVE's second video on the physical properties of minerals. You should now understand the basic field tests using color, streak, hardness, magnetism, and reactivity with acid to identify and characterize a mineral sample.
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Q1: What is the difference between polymineralic and monomineralic rocks?
Polymineralic rocks contain multiple types of mineral grains, while monomineralic rocks are composed of a single mineral. Most rocks are polymineralic. Geologists classify polymineralic rocks using physical properties like color, hardness, magnetism, and acid reactivity, whereas monomineralic rocks are typically classified by crystal form and cleavage.
Q2: Why is mineral streak a more reliable indicator than color?
Streak represents the color of a powdered mineral sample created by dragging it across a porcelain plate. Unlike apparent color, which varies due to impurities and crystal structure, streak is more reproducible because fine grains are randomly oriented and less affected by these factors. This makes streak a more consistent identification tool in the field.
Q3: How is mineral hardness measured using the Mohs hardness scale?
The Mohs hardness scale ranks ten reference minerals by their ability to scratch or be scratched by other materials. A mineral that scratches a reference material is harder than that reference; conversely, a mineral that is scratched is softer. Geologists test hardness by attempting to scratch glass, which sits near the middle of the scale, then rank samples accordingly.
Q4: What does mineral luster describe?
Luster is a subjective measure of how a mineral reflects light, divided into two categories: metallic minerals that are shiny and reflective, and non-metallic minerals that appear dull. Luster depends on a mineral's atomic structure and chemical composition, making it a useful field identification property alongside other physical characteristics.
Q5: Which mineral commonly exhibits ferromagnetism, and how is it tested?
Magnetite is the primary mineral that exhibits ferromagnetism. To test for magnetism, flake mineral grains and observe whether a bar magnet picks them up. Additionally, place the mineral sample near a compass needle with about six inches of space, then slowly decrease the distance. If magnetic, the compass needle will point toward the sample as proximity increases.
Q6: What does the acid reactivity test reveal about mineral composition?
The acid reactivity test identifies carbonate minerals by observing their reaction with dilute acid. Calcite is the most common carbonate mineral that reacts with acid. This simple test helps geologists quickly classify polymineralic rocks and mineral samples in the field without requiring advanced laboratory equipment.
Q7: How do modern laboratory techniques improve upon traditional mineral identification methods?
Advanced techniques like X-ray diffraction (XRD) enable detailed characterization of crystal structure and material composition beyond field tests. Diamond anvil cells reach extremely high pressures to synthesize and analyze new phases of matter. These methods provide precise structural information useful for applications like lithium ion battery development, complementing traditional physical property testing.