The terms used to describe luster are: Metallic also known as splendent Submetallic Vitreous also known as glassy Adamantine also known as brilliant or diamondlike Resinous also known as resinlike Silky Pearly also known as mother-of-pearl Greasy also known as oily Pitchy also known as pitchlike Waxy also known as waxlike Dull also known as earthy Metallic - Minerals with a metallic luster are opaque and reflective, like metal. The metal lic elements , most sulfides , and some oxides belong in this category.
Submetallic - Describes a mineral that is opaque to nearly opaque and reflects well. Thin splinters or sections of submetallic minerals are translucent. Vitreous - This luster accounts for roughly 70 percent of all minerals. Minerals with a vitreous luster have reflective properties similar to glass.
Most of the silicates , carbonates , phosphates , sulfates , halides , and hydroxides have a vitreous luster. Adamantine - Transparent to translucent minerals with a high refractive index yield an adamantine luster, meaning they display extraordinary brilliance and shine.
Resinous - This is the luster of many yellow, dark orange, or brown minerals with moderately high refractive indices - honey like, but not necessarily the same color.
Silky - A silky luster is the result of a mineral having a fine fibrous structure. Minerals with a silky luster have optical properties similar to silk cloth. Pearly - Describes a luster similar to the inside of a mollusk shell or shirt button. The luster of a material can also determine how it will be used in industry. For example, jewelry manufacturers would not be the top consumer of gold if the metal had an unattractive luster.
The pearly luster of muscovite makes ground muscovite a common ingredient in cosmetics. The luster of a mineral is best observed on a surface that is free of moisture, dirt, tarnish, and abrasion. Geologists in the field usually carry a rock hammer to break rocks so that their true luster and color can be observed.
Breakage is usually not necessary when observing the luster of cleaned and cared-for specimens in a laboratory or classroom.
Luster is best observed under direct illumination. That allows the light that strikes the specimen to reflect to the eye of the observer.
Proper examination includes moving the specimen or the light source, or the head of the observer through a range of angles to observe the full character of the luster. The photographs and descriptions on this page illustrate some of the most common lusters observed in minerals.
Silver Metallic Luster in Galena: This photograph shows the silver metallic luster of a nice cubic crystal of galena. The galena crystal is about two inches on a side, and the adjacent white crystals are calcite.
Specimens with a metallic luster exhibit the reflectivity and brightness of a metal and are always opaque. The smoother the surface, the brighter their luster, and the higher their reflectivity. When a beam of incident light is reflected from a perfectly smooth reflective surface, the angle of reflection is equal to the angle of incidence.
Smooth surfaces have higher lusters because all of the light that strikes them has an opportunity to be reflected. However, when light strikes a rough surface, much of the light is hitting irregularities in the surface.
This light is scattered in many directions. These specimens with an irregular surface will have a lower luster than specimens with a smooth surface. Most metallic minerals have a color similar to native metals such as gold , silver , or copper. Just because a specimen is highly reflective does not give it a metallic luster. It must also be opaque and exhibit the color of a metal. Opacity is an important part of a metallic luster. Light enters specimens that are transparent or translucent.
When a specimen is opaque, then all of the incident light has an opportunity to be reflected. Many sulfide and sulfosalt minerals have a metallic luster, such as pyrite , galena , chalcopyrite , and pyrrhotite.
Some oxide minerals such as hematite , rutile , magnetite , and cassiterite may exhibit a metallic luster. Submetallic Luster in Magnetite: A specimen of magnetite variety: lodestone exhibiting a submetallic luster. The specimen has attracted numerous tiny particles of iron. This specimen is approximately 10 centimeters across. Some specimens exhibit a luster that falls short of being called "metallic" or makes the observer doubtful about using that adjective.
The word submetallic might be used for these specimens. These specimens are usually opaque, and they are often black in color. Others have a small grain size, or an irregular or pitted surface that interferes with the reflection of incident light.
Observers should be careful, because tarnish will sometimes mislead them into deciding that a specimen is submetallic rather than metallic or nonmetallic.
This is when observations of luster on a freshly broken surface become important. Hematite , magnetite , graphite , and chromite are examples of minerals that can exhibit a submetallic luster. Most mineral specimens do not exhibit a metallic or submetallic luster. These specimens are said to have a "nonmetallic" luster. There are many varieties of nonmetallic lusters, and the most common are described below.
Note: The name "nonmetallic" applies to the luster of these minerals and has nothing to do with their elemental composition. Nonmetallic Vitreous or Glassy Luster in Apatite: These small greenish yellow crystals of apatite exhibit a vitreous luster.
Vitreous means "the appearance of glass". Some people would call this a "glassy" luster, and that would be perfectly correct. The apatite crystals are from Cerro del Mercado, Durango, Mexico, and they are mostly about 8 millimeters in length. Image copyright by Geology. Specimens that have a vitreous luster have a reflective appearance that is similar to glass.
This luster is sometimes called "glassy. Some specimens of calcite have a vitreous luster on their cleavage surfaces. Vitreous is the most common type of luster. Dull or Earthy Luster: A specimen of massive hematite that is non-reflective and would be said to have a dull or earthy luster.
It is about four inches across ten centimeters and was collected near Antwerp, New York. Bottom Line : I created a replacement activity for this traditional observation of luster - the flashlight circuit test.
My students find it fast and fun. Results are almost always unequivocal. Check out the section on electrical conductance for details. Luster is a property of the mineral response to light. When light illuminates any matter, you may see see one of several different responses.
Although the appearance of a mineral may vary with conditions of illumination, we will simplify our interpretation into two broad categories - metallic and non-metallic luster. Metallic luster means reflected light resembles a polished metal surface.
Otherwise, the appearance of reflected light is termed non-metallic and this appearance may vary. Terms used for non-metallic luster include glassy or "vitreous" , pearly like mother-of-pearl , etc. For more details about the different terms given for luster, consult this link - luster in mineral identification.
Metallic luster. Cubic single crystal of galena. The crystal is opaque. Image by R. Non-metallic luster. Cubic single crystal of halite. The crystal is transparent. Octohedral crystals of magnetite. The mineral is opaque. Octohedral crystals of fluorite. These crystals are transparent. Metallic luster - brownish cubic crystals of galena. Non-metallic luster - purple cubic crystals of fluorite. Non-metallic luster - single crystal of quartz smoky. Note that you can see the inside of the crystal.
Non-metallic luster - single crystal of calcite dogtooth spar. Note that you can see that light penetrates inside of the crystal.
The crystal is translucent. A mineral, by definition, is a solid chemical compound with ions and molecules arranged in long-distance, repetitive, geometric order. The above examples show single crystals of significant size for handling and observation. The determination of luster becomes more difficult and uncertain as crystal size becomes smaller, eventually microscopic. That's why you'll need to examine a sample to determine multiple properties that help you to narrow down a list of possible names for identification.
Don't spend too much time on evaluating luster of rock samples.
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