Guided Exploration: Properties of Minerals

GM Tour Map: Properties


Overview: Scientists perform a range of measurements to determine the crystal structure of minerals, as well as their physical, chemical, and optical properties. Some minerals are easy to identify, but most require multiple tests. In this section, explore four cases to see how mineral properties can be determined and measured.

1. Atoms, Space Lattices, and Crystals

A mineral’s characteristic crystal structure is determined by how its atoms fit together. Compare the models in the center of the case to the crystal shapes of mineral specimens to the right. Explore how atomic arrangements in different minerals are expressed in actual crystals, and how bonded groups repeat in three dimensions to make crystals.

2. Physical Properties of Minerals

These different crystal structures result in a huge variety of physical properties — even when minerals are made of the same element or elements. Compare two carbon-based minerals, diamond and graphite. How do their crystal structures and physical properties differ? Then, observe the samples to the right to explore a variety of characteristic properties (hardness, cleavage, fracture, tenacity, specific gravity).


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Physical Properties of Minerals

The physical properties of a mineral depend on the kind of atoms it is composed of and, more critically, the way these atoms fit together to form the mineral's crystal structure.


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Models of Diamond and Graphite Atoms

Both graphite and diamond are made of carbon. But diamond, with more closely packed atoms, is considerably harder and has a much higher specific gravity than graphite.

3. Chemical Properties of Minerals

These properties reflect the chemical elements that minerals are made of. Scientists use chemical tests to distinguish between minerals with similar physical properties. They determine how minerals react under different conditions, such as exposure to heat, water, or acids. Explore different ways to identify minerals with chemical tests. Note that halite (large specimen on the left) and calcite (#12) share many physical properties, but halite dissolves in water and calcite in hydrochloric acid.


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The ionic bonding in rock salt is readily broken down by water to form a salt solution.


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Calcite (#12)

Calcite is soluble in dilute hydrochloric acid.

4. Optical properties of minerals: Color is an important optical characteristic, but can vary within mineral types and overlap across others. Streak — the color of the residue left when the mineral is scraped against a rough surface — can help distinguish minerals from one another. Look at hematite (#48). Like many minerals, it is black, but its streak is distinctly red. Pyrite (#44) is gold, but its streak is black.


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Streak (#42-51)

Streak is the color of powder made from a mineral.

Luster is another optical property. Minerals can display either a metallic luster (#32-36) or non-metallic luster (remaining specimens in the left-hand case). The way light passes through some minerals can also help with identification.


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Luster (#32-36)

Mineral surfaces may show one of two major types of luster: metallic and nonmetallic.

Look closely at the halite and calcite specimens (#73–75), and their mounting brackets. How do they look different when viewed through the calcite sample? 


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Refraction (#73)

In general, when light passes from one medium such as air to another such as a transparent mineral, it is bent or refracted.