Source: Laboratory of Alan Lester - University of Colorado Boulder
The physical properties of minerals include various measurable and discernible attributes, including color, streak, magnetic properties, hardness, crystal growth form, and crystal cleavage. These properties are mineral-specific, and they are fundamentally related to a particular mineral’s chemical make-up and atomic structure.
This video examines several physical properties that are useful in field and hand sample mineral identification— color, luster, streak, hardness, magnetism, and reaction with acid. Unlike crystal form and crystal cleavage, these properties are somewhat more closely linked to mineral chemical composition than to atomic structure, but both do play a role.
It is important to recognize that rocks are aggregates of mineral grains. Most rocks are polymineralic (multiple kinds of mineral grains) but some are effectively monomineralic (composed of a single mineral). Unlike crystal form and cleavage, which are terms reserved for mineral specimens, geologists might on occasion refer to a rock as having a general sort of color, hardness, magnetism, or reaction with acid. In other words, the physical properties looked at here are potentially appropriate for use with rocks as well as with specific minerals.
A single mineral specimen may exhibit very few, if any, key physical properties. For any demonstrations or experiments that address physical properties, it is first necessary to select a suitable group of mineral samples that actually display the key features or properties under investigation. Below, we address the fundamental definitions, in geological context, for the physical properties— color, luster, streak, hardness, magnetism, and reaction with acid.
Color— Color simply refers to the apparent color seen with the naked eye when looking at a mineral. Ultimately, this is a result of wavelengths of light that are preferentially reflected from a mineral surface. (Figure 1)
Streak— Streak is the color of a powdered, i.e. very fine grained, sample of the mineral. This is observed by taking a mineral sample and dragging it across a porcelain plate in order to create a line of powdered material. (Figure 1)
Hardness— Hardness is effectively a mineral’s surface strength, or resistance to disaggregation, i.e. whether or not it can be scratched. A mineral is said to be harder than another mineral if it is capable of scratching the surface of the other mineral. The mineral hardness scale, ranging from 1-10, was developed in the early 19th century by the mineralogist Friedrich Mohs, but based on modern materials science, the scale is not linear. (Figure 2)
Magnetism— Magnetism refers to a mineral’s ability to influence a magnet or compass. In general, this property is exclusive to the mineral magnetite (Figure 3), but other minerals can show weak magnetism (especially after heating), such as hematite and bornite. Ultimately, magnetism is a result of spatial organization of electron spin directions, or moments.
Reaction with Acid— Geologists often test rocks and minerals with dilute acid (almost invariably 2-3% HCl) in order to assess the presence of carbonate compounds. There are numerous carbonate minerals, but the most common are calcite (a key component of the rock limestone), which effervesces vigorously with diluted HCl, and dolomite (a key component of the rock dolomite), which effervesces weakly.
Luster— Luster is a subjective measure of how a mineral surface tends to reflect light. It is divided into two general categories:
- metallic (highly reflective and shiny), as seen in minerals such as pyrite (Figure 4) and galena (Figure 5)
- non-metallic (more dull in appearance), as seen in minerals such as feldspar (Figure 6), quartz (Figure 7), and muscovite (Figure 8).
As luster is a subjective property (perhaps better termed a “quality”), and typically more the concern of gemologists opposed to geologists, the rest of the lesson will focus instead on the properties color, streak, hardness, magnetism, and reaction with acid.
Figure 1. Color, streak, and luster. The mineral hematite is a good example of how bulk color (in this case, silvery-dark) and the color of a powder, which is called “streak” (in this case reddish-orange), can be quite different. Hematite can express different kinds of luster, but here it shows a metallic luster.
Figure 2. Hardness scale. The hardness scale is a way of comparing minerals on the basis of how easily a mineral surface can be disaggregated, i.e. scratched. A mineral that is “harder” will scratch a “softer” mineral.
Figure 3. Magnetite mineral sample. Magnetite is an iron oxide. Although iron is a primary constituent of planet earth, it’s only present in pure elemental form in the remote region of earth’s core (some 2,900 km beneath the surface). In earth’s crust and at the surface, iron is bound with oxygen and hydroxyl groups to form the common minerals magnetite, hematite, and limonite. Magnetite is the most magnetic of all the naturally occurring minerals on earth.
Figure 4. Pyrite. Pyrite is also known as fool’s gold due to its metallic luster and pale brass-yellow hue.
Figure 5. Galena. Galena (sometimes called lead glance) is another example of a mineral with metallic luster. It is the main ore of lead, a source of silver (sometimes containing up to 1-2% silver), and has a low melting point.
Figure 6. Feldspar. Feldspars are a group of rock-forming minerals which comprise up to 60% of the Earth’s crust. They are a good example of a mineral that displays a non-metallic luster.
Figure 7. Quartz. Quartz is another good example of a mineral with non-metallic luster. It is the second most abundant mineral in the Earth’s crust after feldspar.
Figure 8. Muscovite. Commonly known as mica, muscovite is another mineral that displays a non-metallic luster.
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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Applications and Summary
Historically, the evaluation of the physical properties of minerals has been a key first step in mineral identification. Because microscopic and modern analytical instrumentation (e.g. petrographic microscopy, x-ray diffraction, x-ray fluorescence, and electron microprobe techniques) are not available in the field, recognition and use of observed physical properties can be important diagnostic tools.
Evaluating and observing the physical properties of minerals is an excellent means to demonstrate how the macroscopic features of minerals are in fact the external manifestation of either atomic-level structure or chemical composition. This process provides insight into:
1) How chemical composition influences the interplay of light with reflecting surfaces.
2) How chemical composition and atomic bond strengths influence a mineral’s resistance to disaggregation (scratching).
3) How chemical composition and atomic scale ordering influence properties such as magnetics (e.g. presence of Fe-bearing substances) and reaction with dilute acid (e.g. presence of the CO32- anion group).
There are also industrial and engineering applications that require some knowledge of the physical properties discussed in this video. For example, machines that need to cut or grind may use mineral substances to aid in the process. Additionally, gemologists (who typically identify and prepare gem-quality minerals for sale) may be concerned with properties like color and luster.