For instance, minerals with a cubic atomic structure will have a tendency to grow into cube-shapes. Crystal shapes are usually determined by the arrangement of the atoms within the crystal structure. Besides color, it is often the first thing you might notice about a mineral. It can refer to the expression of a crystal shape or the shape of multiple crystals aggregated or bunched together. Pyrite with metallic luster.Ī mineral’s habit is the crystal shape or texture in a given specimen. Minerals such as quartz have a non-metallic luster, but there are still a variety of ways to describe how the light reflects off the mineral. Minerals that are opaque and shiny, such as pyrite, have a metallic luster. One straightforward way to classify luster is based on whether the mineral is metallic or non-metallic. Mineralogists have specific terms to describe luster. Luster describes the reflection of light off a mineral’s surface. Some minerals leave a different streak color than their bulk mineral would indicate. Yellow-gold pyrite has a blackish streak, another indicator that pyrite is not gold, which always has a golden-yellow streak. To check streak, scrape the mineral across an unglazed porcelain plate. Many minerals, such as the quartz, do not have a streak. Minerals that are the same color may have a different colored streak. Streak is a more reliable property than color because streak does not vary between two samples of a mineral, even when those samples are different colors (see above example). Streak is the color of a mineral’s powder. Both samples of hematite (Fe 2O 3, gray on the left, red on the right) leave a diagnostic red-brown streak on a porcelain streak plate. Quartz can be nearly any color as shown in these pebbles. Quartz is a good example – depending on the trace elements hiding within its structure, it can be clear, white, pink, gray, black, yellow, or brown! Fig. However, there are a lot of minerals that take on a wide variety of colors. Knowing these patterns makes it easier to identify certain minerals. There are certain minerals that are reliably a specific color. These trace elements are often metals such as chromium (Cr), manganese (Mn), or titanium (Ti), and because they are present in such limited amounts, they do not appear in the mineral’s chemical formula. Sometimes, minerals have trace elements within their structure that cause them to take on unique colors. Rock containing green malachite and blue azurite. For example, copper (Cu), will cause azurite to be blue and malachite to be green. These colors are determined by the main elements in a mineral’s chemical formula. Some minerals are bright with reds, greens, blues, and yellows. Which tests will you find to be the most reliable when identifying an unknown mineral? ColorĬolor is one of the first things we notice about minerals, and for good reason. You will find that some tests will be more useful than others. With these physical identification tests, most minerals can be identified in the field by scientists and nonscientists alike! The physical properties of minerals include:īelow, we will explore each of these tests in more detail. These tests are ideal because prior to their use, scientists had to rely on complex (and expensive!) instruments such as X-ray Diffractometers. Mineralogists have identified some simple physical tests (described in the above video) to determine the identity of different minerals. These properties are usually related to the type of elements within a mineral or how the atoms are bonded. Some minerals within the same compositional group will have similar physical characteristics such as color or shape. Minerals are categorized based on their chemical composition (see table below). What might this mineral be? Physical Properties of Minerals A cluster of colorless crystals, each with well-defined faces. If there are so many minerals on Earth, how does a scientist tell one apart from another? If you found the crystal cluster pictured below in a cave, how would you know whether it was gypsum and not quartz? Fig. There are over 5,600 minerals that have been discovered on Earth so far! We also think there are at least another thousand or more that have yet to be discovered by mineral scientists, called mineralogists. However, our planet (and the greater Universe) has found thousands of unique ways to combine elements into crystalline solids. Given the narrow criteria, you might think that Earth’s minerals are pretty rare.
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