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Physical Properties Of Minerals II: Polymineralic Analysis



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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