Determining Lustre: For Beginning CollectorsLast Updated: 26th Mar 2018
By Donald B Peck
Lustre is the character, or quality, of reflected light. It is one of the properties by which we recognize minerals and differentiate one from another. Mineralogists and collectors generally divide lustre into two categories, metallic and nonmetallic. That is not very many and would not be too helpful with more than 5000 known minerals; but the nonmetallic division is further subdivided. Its divisions include: adamantine, vitreous, pearly, resinous, waxy, greasy, oily silky, dull, and earthy. This article is intended to help the neophyte collector distinguish between them. Lustre is largley independent of color.
There is no clear point of separation between, say, adamantine and vitreous. Lustre is a continuum from metallic to earthy. Further, it is not uncommon to see the prefix "sub" attached to any of them or to see two together as "vitreous to adamantine". The best you can do is to keep in mind the general description of the lustre and mentally compare it to some object that you know. For example, if the lustre is said to be metallic and it is yellow, does it look like polished brass. If the lustre is waxy, does it look like the wax in a candle?
As you look through the Mindat Mineral Pages, or a handbook, you will notice that many minerals can exhibit more than one lustre. Luster is affected by the quality of the crystal face. Usually, the smoother the face, the higher the lustre. Further differences in the mineral itself result in different lustres. For example, hematite can be bright slivery-gray metallic crystals, dull gray submetallic and massive, or reddish earthy or brick-like masses.
You cannot learn to recognize different mineral lustres by studying photographs. In general, it does not work because the camera does not capture the quality of the light. For example, the photo of the diamond at the top of this page does not begin to show the adamantine lustre. You need to look at real minerals and their crystals in natural light. Compare what you see to what mindat, or your mineral handbook, tells you the lustre to be.
Does your specimen look like, or reflect light like a piece of polished brass, or a silver spoon, or a piece of cast iron, or . . . You get the idea. Compare to a real metal object. The door of your automobile does not count; it is paint. If the lustre isn't quite as bright as the actual polished metal, then call it "submetallic". Approximately 15% of minerals can have a metallic lustre. Native elements, sulfides,sulfosalts, and oxides are well represented. Among the more common minerals with a metallic lustre are: Arsenopyrite, Bornite, Cassiterite, Chalcopyrite, Chalcostibite, Chromite, Galena, Goethite, Hematite, Magnetite Marcasite, Pyrite. Pyrrhotite, and Rutile.
Adamantine is bright, brilliant, almost like a highly polished metal, but it isn't metallic. Polished diamonds exhibit it. They are almost the definition of adamantine. Minerals containing very heavy ions, like silver, mercury, and uranium, are likely to have an adamentine lustre. A little less than 10% of minerals exhibit adamantine lustre. Some minerals with adamantine lustre are: Anatase, Anglesite, Brookite, Cerussite, Cinnabar, Jahnsite, Miargyrite, Proustite, Pyrargyrite and Sphalerite.
A vireous lustre is the lustre of freshly broken glass. It is bright with a high shine, but not as brilliant as the reflections from minerals with an adamantine lustre. About 70% of all minerals show a vitreous lustre. A large proportion are silicates, carbonates, phosphates, and other oxy-salts. Quartz, one of the most common minerals, has a vitreous lustre. Others include: Azurite, Beryl, Crandallite, Fluorapatite, Lazulite Smithsonite, Uranophane, wavelite, and Wolfenite.
A pearly lustre has the quality of light like that reflected from (you got it) pearls or mother-of-pearl. It is slightly hazy, diffuse, out of focus. A pearly lustre is most often assigned to a mineral which has parallel planes of cleavage, like calcite or muscovite, where there is reflection from closely spaced stacked planes. The pearly lustre shows only on the smooth crystal face parallel to a cleavage plane. Very often, pearly is paired with another luster, usually vitreous, in the mineral descriptions. That is due to the direction of the cleavage planes. A crystal face parallel to the cleavage direction will show the pearly luster, but a face not parallel may be vitreous. Among the minerals that commonly show a pearly luatre are: Calcite, Brucite, the feldspars, the micas, and Hemimorphite.
A mineral with a silky lustre reflects light like a piece of fine silk cloth. Earlier it was stated that one cannot learn to recognize kinds of lustre from a photograph. Perhaps the photo of satin spar at the top of this article is an exception. It definitely displays a silky lustre. The silky appearance is due to long fine crystals laid down in parallel. Minerals that can exhibit a silky lustre include: Anthophyllite, Aurichalcite, Chrysotile, Epsomite, Gypdum (var. Satin Spar), Malachite, Pectolite, Mesolite, Tremolite, and Ulexite.
A resinous lustre reflects light like a piece of amber, rosin, or a piece of plastic. Minerals with a high bright luster and a brown, yellow or red color often appear to be resinous. Minerals that can display a resinous lustre include: Almandine, Epidote, Grossular, Pyromorphite, Realgar, Sphalerite, Sulfur, and Tryphlite.
Greasy, Oily, & Waxy Lustres
Greasy, oily and waxy are self explanatory. Minerals with a greasy luster, in addition to looking like a lump of grease, often have a greasy feel to them as well.
Minerals that can have a greasy or oily lustre include: Amblyonite, Chanosite, Cookite, Datolite, Dianond, Dioptae, Foitite, Lizardite, Mottramite, Natrolite, Polucite, Wulfenite, and Zircon.
Minerals that may have a waxy lustre include: Brucite, Chrysocolla, Glauberite, Opal, Talc, and Turquoise.
Dull & Earthy Lustres
A dull or earthy lustre needs no explanation. It depends in large measure on the particle size or the friability of the mineral. However, a surprisingly large number of minerals that generally have higher lustres may also exhibit a dull or earthy lustre. For example azurite most often occurs as adamantine crystals, but it is also found as an earthy blue powder. Minerals that are typically dull or earthy are the clay minerals, such as Kaolinite and Montmorillonite.
A Little Theory
When light strikes the surface of an object, it may be absorbed, transmitted, and/or reflected. Usually all three occur, with absorption predominating in opaque minerals, and transmission accounting for most of the light in clear, colorless minerals. It is the reflected light that accounts for lustre. Even in opaque minerals, the light penetrates a few atomic layers deep. It interacts with the electrons in the near-surface atoms and some of it is radiated back through the surface. The nature of the material and the angle at which the incident light penetrates affect the reflectivity.
Metallic minerals, where ions are immersed in a sea of essentially free electrons, strongly absorb the visible light. The refractive index is generally above 3. If the lustre is submetallic, the minerals are usually semi-opaque to opaque and the refractive index is about 2.6 to 3. An adamantine lustre usually is associated with a refractive index between 1.9 and 2.6 (if the color is yellow or brown, the lustre is resinous). And minerals with a refractive index between approximately 1.3 and 1.9 display a vitreous lustre. Greasy, oily, waxy, dull,and earthy are variations depending mostly on surface conditions and particle size. There are no absolute division points in refractive index between minerals with different lustres. Rather they are fuzzy, overlapping bands in a continuum.
In minerals, as with other objects, the proportion of light that is reflected and its character is greatly affected by the nature, or condition , of the surface. The more irregular the surface, the more diffuse is the reflected light. Thus a mineral that may have a vitreous luster when a crystal face is flat and very smooth, may display a waxy luster if the surface is mildly irregular.
Links to tht "Determinig . . ." Series: How To
- What Is a Mineral? The Definition of a Mineral
- Determining Color and Streak
- Determining Lustre: For Beginning Collectors
- Determining the Hardness of a Mineral
- Determining the Specific Gravity of a Mineral
- Determining Symmetry of Crystals: An Introduction
Mason, Brian and Berry, L.G. (1968) Elements of Mineralogy. W. H. Freeman and Company, San Francisco. pp 110-111, p 172.
Dana, Edward Salisbury, Ford, William E. - Editor (1991) A Textbook of Mineralogy. John Wiley & Sons, Inc., New York. pp 273-274.
Peck, Donald B. (2007) Mineral Identification: A Practical Guide for the Amateur Mineralogist. Mineralogical Record, Tucson, Arizona. pp 9-11.
Pough, Frederick H. (1976) A Field Guide to Rocks and Minerals Houghton Mifflin Company, Boston. p 28
I am indebted to the following who shared photographs that were included in this article: Lucian Aborosa, John Betts, Rock Currier, Paul Hewitt, Photo Guide to Mineral Species, Quebul Fine Minerals, Viteslav Snasel, The Ruff Project.
Article has been viewed at least 1473 times.
Another great article
Thank you Don
24th Feb 2018 4:09am
Thank you Don
24th Feb 2018 4:09am
Thanks Don. Pretty good insight on an important property of minerals and explanation of reflection so that the layman can understand. Good job!!!
9th Mar 2018 1:38pm
9th Mar 2018 1:38pm
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