Light in Minerals
Image credit: Scott Brande
The Bottom Line on Non-Metallic Minerals - Here On Top!
Bottom Line: If a mineral transmits any light, it is non-metallic. But if fragment of a sample remains dark in front of a bright light source, we may not conclude the mineral is metallic. Details and explanation below.
Errors/Suggestions: Contact Scott Brande (see footer).
Degrees of Transparency
When lights falls on a mineral, it may interact with the chemical compound in different ways. What's important to us for mineral identification are three terms: opaque (opacity), translucent (translucency) and transparent (transparency).
opaque minerals block light from penetrating through the surface (and therefore through the mineral); one can only see the surface of the crystal
translucent minerals permit some light to enter the interior of the mineral, but the light does not pass through unimpeded; writing or an object cannot be distinguished through a crystal
transparent minerals pass light largely unimpeded; writing or an object can be seen through the crystal in bright enough light
The term "diaphanous" (diaphaneity) refers to the degree of transparency according to these different interactions of light with a mineral.
Opacity
Opaque samples block the transmission of all light through the mineral. A single mineral crystal may appear opaque for various reasons, including
a weathered surface that scatters and blocks light
a thicker sample that cumulatively absorbs light as it passes through the mineral
a darker color due to elements in the compound (such as metals) that selectively absorb light
a metallic element that reflects light
Weathered surface of garnet crystal blocks the entry of light that would otherwise pass into the mineral.
Image by R.Weller/Cochise College http://skywalker.cochise.edu/wellerr/mineral/garnet/dodecahedron2.htm
Really thick red garnet too thick to pass light).
Image by R.Weller/Cochise College http://skywalker.cochise.edu/wellerr/mineral/garnet/darkred1.htm
Darker color (even in thin slab) of biotite (mica), caused by element iron (Fe) selectively absorbing light as it penetrates the mineral.
Image by James St. John (https://commons.wikimedia.org/wiki/File:Biotite_mica_2_(31739438210).jpg), „Biotite mica 2 (31739438210)“, https://creativecommons.org/licenses/by/2.0/legalcode
Metallic crystal of galena (PbS - lead sulfide). Metals reflect all light at the surface.
Image by R.Weller/Cochise College http://skywalker.cochise.edu/wellerr/mineral/galena/octahedron2.htm
Translucency
Translucent samples transmit only some of the light through the mineral. A single mineral crystal may appear cloudy for various reasons, including
inclusions of clay or other external material incorporated into the crystal as it grows
flaws in the crystal lattice that scatter light
the sample is composed of many crystals, oriented in different directions
Translucent calcite.
Image by R.Weller/Cochise College http://skywalker.cochise.edu/wellerr/mineral/calcite/twin1a.htm
Translucent fluorite
Image by R.Weller/Cochise College http://skywalker.cochise.edu/wellerr/mineral/fluorite/fluoriteL.htm
Translucent apatite
Image by R.Weller/Cochise College http://skywalker.cochise.edu/wellerr/mineral/apatite/massive2.htm
Translucent plagioclase feldspar (variety schiller)
Image by R.Weller/Cochise College http://skywalker.cochise.edu/wellerr/mineral/labradorite/schiller1.htm
Transparency
Transparent samples transmit light through the mineral as in a piece of clear glass. However, we recognize that minerals may be colored as well as transparent, as long as the mineral appears clear enough for writing or an object to be seen through the crystal.
Transparent garnet (variety tsavorite)
Image by R.Weller/Cochise College http://skywalker.cochise.edu/wellerr/mineral/garnet/tsavorite1.htm
Transparent to garnet (var. spessartite)
Image by R.Weller/Cochise College http://skywalker.cochise.edu/wellerr/mineral/garnet/garnet12.htm
Translucent garnet (variety grossularite/hessonite)
Image by R.Weller/Cochise College http://skywalker.cochise.edu/wellerr/mineral/garnet/hessonite1.htm
Not quite Transparent garnet (rhodolite).
Image by R.Weller/Cochise College http://skywalker.cochise.edu/wellerr/mineral/garnet/rhodolite2.htm
Transparency in Calcite Crystals
Transparent calcite crystal (variety "Iceland spar"). The lines on paper can be distinguished through the crystal. Calcite (and other minerals) can split light into two different paths, thus doubling the image.
Image by R.Weller/Cochise College http://skywalker.cochise.edu/wellerr/mineral/calcite/calcite1.htm
Translucent calcite due to internal reflections and scattering of light off planes of cleavage.
Image by R.Weller/Cochise College http://skywalker.cochise.edu/wellerr/mineral/calcite/cleavage2.htm
Translucent calcite (variety dogtooth spar) . Light is scattered both on the surface and in the interior.
Image by R.Weller/Cochise College http://skywalker.cochise.edu/wellerr/mineral/calcite/dogtoothspar10.htm
Opaque calcite. No object or writing could be seen through this sample. The two separate shapes belong to crystal "twins".
Image by R.Weller/Cochise College http://skywalker.cochise.edu/wellerr/mineral/calcite/rhomb6.htm
Transparency in Gypsum Crystals
Transparent gypsum crystal, variety selenite. An optically clear crystal through which writing may be read.
Image by R.Weller/Cochise College http://skywalker.cochise.edu/wellerr/mineral/gypsum/transparent2.htm
Translucent gypsum due to inclusions.
Image by R.Weller/Cochise College http://skywalker.cochise.edu/wellerr/mineral/gypsum/utah12.htm
Translucent gypsum (variety selenite) due to inclusions of clay incorporated into the crystal during growth.
Image by R.Weller/Cochise College http://skywalker.cochise.edu/wellerr/mineral/gypsum/clay1.htm
Opaque gypsum (variety selenite) crystals due to growth in sandy matrix.
Image by R.Weller/Cochise College http://skywalker.cochise.edu/wellerr/mineral/gypsum/sand3.htm
Bright Light Through Thin Fragments
Recognizing Non-metallic Minerals by Light Transparency
One fact that distinguishes metallic minerals from non-metallic minerals is that metallic minerals reflect all light at the surface. However, because light may be partially or totally blocked from passing through a mineral for some of the other reasons (above), we can't be sure that a mineral is metallic when a sample is opaque. However, if a sample does pass any light, we can be sure that is non-metallic.
Thin Fragments Test for the Passage of Light
Even the thinnest of fragments of metallic minerals reflect all light at the surface. Consider that even a strong back light will not shine through a thin piece of aluminum foil.
If a small, thin fragment of a mineral DOES transmit ANY light from behind it, we must conclude the sample is non-metallic.
In the images below, small fragments of crushed minerals (a few millimeters in size) are placed on a white translucent plastic bottle cap. The bottle cap is placed over the lens of a bright flashlight. Paired images of reflected and transmitted light demonstrate whether or not light passes through the fragments. If light is transmitted through the fragment, the sample is non-metallic.
Note: If the sample remains dark in transmitted light (opaque), we CAN'T conclude that the mineral is metallic because of the many other factors that might block the passage of light - see the section on Opacity above.
Non-metallic mineral. Even in room light, we can see through the fragments to the white bottle cap below. Note the shadows seen through the crystals.
Image credit: Scott Brande
Non-metallic mineral. The strong back light passes through the fragments. You can see the variation of light intensity caused by flaws and cleavage planes inside the crystals.
Image credit: Scott Brande
Translucent or Opaque mineral. The fragments are either translucent or opaque in room light. You can't any detail below any of the fragments. Also, it appears that these samples are composed of many small crystals (a crystalline aggregate).
Image credit: Scott Brande
Translucent Non-metallic mineral. The fragments transmit light from behind, but are not transparently clear. The nature of the fragment interior is more clearly seen - note many small irregular crystal boundaries. You can see variations in the intensity of light, with some thinner edges appearing brighter, and thicker (center) regions darker. Thicker regions cumulatively scatter and/or absorb more light, leaving less light to emerge towards our eyes. The mineral is translucent because it is not clear (transparent) The mineral is non-metallic because some light passes through the mineral.
Image credit: Scott Brande
Opaque mineral. The fragments appear opaque in room light. The samples appear very fine-grained, with variations of color separated into different regions. We can't see into the material, we can only see the surface. The fragments appear opaque.
Image credit: Scott Brande
Opaque and Translucent (at thin edges) Non-metallic mineral. Under a strong back light, we can see a lighter "glow" coming through the thinnest edges of fragments. The "glow" is seen as a different, lighter color. Notice that as small as these fragments are, only a couple of millimeters of thickness is sufficient to block almost all of the strong back light. However, no matter how little light is in the glow, as long as the thin edge is not completely dark, it is transmitting some light. We must conclude that even though the samples are opaque under normal room light, the mineral is non-metallic.
Image credit: Scott Brande
Opaque mineral.These fragments appear opaque in room (reflected) light. They are also quite dark in color. Would any light be transmitted through the thinnest of edges under a strong back light? We can't know until we test the fragments.
Image credit: Scott Brande
Opaque mineral. Look around the edges of the fragments. Some edges are bound to be thinner than others, The strong back light does NOT pass through any of the edges or other regions of the fragments. They are opaque under a strong backlight. We might be tempted to conclude that the mineral is metallic. However, we learned that there are a number of factors that could block the passage of all light through a sample. So this crude test is NOT definitive of a metallic mineral identification. We'll need to do more testing of other properties.
Image credit: Scott Brande