URGENT MESSAGE: Time is running out. Click here to find out why.
Log InRegister
Home PageAbout MindatThe Mindat ManualHistory of MindatCopyright StatusWho We AreContact UsAdvertise on Mindat
Donate to MindatCorporate SponsorshipSponsor a PageSponsored PagesMindat AdvertisersAdvertise on Mindat
Learning CenterWhat is a mineral?The most common minerals on earthInformation for Educators
Minerals by PropertiesMinerals by ChemistryAdvanced Locality SearchRandom MineralRandom LocalitySearch by minIDLocalities Near MeSearch ArticlesSearch GlossaryMore Search Options
Search For:
Mineral Name:
Locality Name:
Keyword(s):
 
The Mindat ManualAdd a New PhotoRate PhotosLocality Edit ReportCoordinate Completion ReportAdd Glossary Item
StatisticsThe ElementsUsersBooks & MagazinesMineral MuseumsMineral Shows & EventsThe Mindat DirectoryDevice Settings
Photo SearchPhoto GalleriesNew Photos TodayNew Photos YesterdayMembers' Photo GalleriesPast Photo of the Day Gallery

Determining Color and Streak

Last Updated: 23rd Jun 2018

By Donald B Peck

Realgar
Wulfenite
Uranophane
Malachite
Realgar
Wulfenite
Uranophane
Malachite
Realgar
Wulfenite
Uranophane
Malachite
Actinolite
Turquoise
Azurite
Amethyst
Actinolite
Turquoise
Azurite
Amethyst
Actinolite
Turquoise
Azurite
Amethyst

A Mineralogical "Rainbow" of Color


It has often been said that color is not a good property with which to identify a mineral. It also has been said that one who ignores the color is a fool. Color is a useful property to identify a mineral; but simply put, it should not be the only property.

Streak is often touted as the "true color" of a mineral. It is the color of the powder and is largely devoid of the luster of larger pieces. This article concerns the color of minerals and what they can tell us, as well as how a streak is produced and its interpretation.

Color in Minerals


Color in most minerals results from the absorption of some wavelengths of light and either the reflection or transmission of the wavelengths not absorbed. In general, if a mineral appears to be red, the blue-green wavelengths have been absorbed from the visible spectrum and the red wavelengths have been reflected and/or transmitted. Azurite, a deep blue colored mineral, both reflects and transmits the blue portion of the spectrum while absorbing the wavelengths that are mostly yellow.

Mixing Colors

We are all familiar with the spectrum produced when a glass prism is held in a beam of white light. There is no discernible division between colors. The spectrum, from red to violet, is comprised of progressively shorter wavelengths of light.

Light of different wavelengths (i.e. colors) may be combined. You see this all the time with RGB pixels on your computer and TV screen. From pixels for red, green and blue you see a full color world. Red light added to green and blue light produces white light. So too does red and cyan, green and magenta, and blue and yellow. The reason is that cyan already contains green and blue light, so when added to red in the correct proportion it produces white. It is similar with the green and blue when they are added to magenta and yellow, respectively. The Additive Color Mixing chart (at right) illustrates these combinations. Red Blue and Green are the primary colors of light. Mixing them in equal intensities produces the visual sensation of white Colors which are 180o across the chart are complimentary colors. Adding complimentary colors of light also produces the visual sensation of white.

Colors may be subtracted. It occurs by absorption as light is reflected from an opaque object or filtered as it passes through a colored transparent or translucent body. The chart, Subtractive Color Mixing, shows how the colors are related. It should not be a surprise that subtracting cyan, yellow, and magenta colors from light leaves nothing and the previously illuminated object appears black. Of course, subtracting red, blue, and green light does the same. Subtracting magenta from white light leaves green, and removing yellow leaves blue. Minerals, like pigments, usually absorb colors of light in the first few atomic layers. If they absorb blue light they reflect its compliment, yellow. Transparent crystals act like filters. Their electrons are usually held more rigidly in covalent bonds and light penetrates deeply. Light passing through a transparent body may have a given color absorbed while its compliment is transmitted. For example, an emerald appears to be bright green in white light. The red and blue portions of the spectrum are absorbed.

The science of color in minerals is highly complex, but in general color is produced by the interaction of light with the electrons in the metal ions of the mineral. Most often, but not always, these atoms are titanium, vanadium, chromium, manganese, iron, cobalt, nickel, or copper. The rare earth metals also impart color, as do the actinide metals (principally uranium).

Two Groups of Color in Minerals


With respect to color, minerals fall into two groups. One group has colors that are intrinsic to the species and is said to be idiochromatic. Minerals of the other group can exhibit more than one color and are allochromatic.

Idiochromatic Minerals



Idiochromatic minerals always show the same color (barring tarnish which may in fact be a different mineral). For example: pyrite is always brass-yellow. Native silver is always silver-white, but tarnishes to lead-grey or black, which is acanthite. An essential element in the chemistry of malachite is copper and copper produces the green color. Iron is the essential element in almandine that causes it to be red. Minerals that are native metals, sulfides, sulfosalts, and oxides are typically idiochromatic.

Pyrite
Silver
Gold
Malachite
Pyrite
Silver
Gold
Malachite
Pyrite
Silver
Gold
Malachite


Allochromatic Minerals



Allochromatic minerals display multiple colors that are produced by a chromophore not essential to the mineral's composition. If they are pure, allochromatic minerals are generally colorless. Impurities usually produce color either by occupying voids in the crystal structures or are incorporated as partially replacing another element in the structure. Color producing impurities are usually from the transition metals, which include titanium, vanadium, chromium, manganese, iron, cobalt, nickel, and copper. Transition metals exist in multiple oxidation (valence) states. Thus they can produce more than one color. Chromium, for example, has nine oxidation states (valences). Cr3+ colors corundum red (ruby) and beryl green (emerald), but as Cr6+ in the chromate ion, (CrO4)2-, It colors crocoite bright orange.

Smithsonite
Smithsonite
Smithsonite
Smithsonite
Smithsonite
Smithsonite
Smithsonite
Smithsonite
Smithsonite
Smithsonite
Smithsonite
Smithsonite


Color & Chemistry


Often times, the color of a mineral is a clue to its chemical composition. This is more true with allochromatic minerals than with idiochromatic minerals. There are no hard rules: the causes of color are too complex. But there are some useful trends. Experience shows that while a lot of copper and iron minerals may be green, there are recognizable differences. Green iron minerals most often tend to be darker, trending toward black; whereas many copper minerals are brighter green and some almost emerald green. Iron reds mostly have hues that are earthy, shading toward the browns or brick reds, while red mercury minerals are almost scarlet, and cobalt minerals often pink.

In the Table, below, chemical elements are listed, using their symbols. The numbers following each are the number of minerals that may exhibit that color, taken from a database of 5,060 species. In each case the element is essential to the species, although it may not be the chromophore. The numbers in the bottom row are for all minerals that may show each color. (Data from Dana's New Mineralogy, 1997)

Color as Related to Chemical Ions

Red/PinkOrangeYellowGreenBluePurple/Violet
Fe 193Fe 66 Fe 268Fe 291Cu 154Fe 43
Mn 173V 18 (UO2)2- 123Cu 182Fe 24Mn 18
Co 26U 15U 27V 16Ni 16Li* 13
Hg 17AsS 9Cr 9 U 13V 8Cu 8
Ag 14(UO2)2- 15V 6 Cr 12Co 2Co 2
V 8Cr 4W 3W 8 Mo 1 Ni 2
W 3Mo 2
7972541096865409163

* Lithium is not a chromophore, but several common lithium minerals are lavender/violet in color.

Streak


The streak of a mineral is the color of the powdered mineral. Usually, it is a mark made on a white porcelain plate. Black plates can be used for white or light colored minerals. A point on the mineral is pressed against the plate and drawn 1 or 2 centimeters. Porcelain is hard, about 6½ on the Mohs scale. If the mineral specimen is softer than the streak plate, a pencil-like mark of powder that is rubbed off will be left on the plate. The color of the mark is the "streak". The point drawn across the plate should be clean and not weathered. Occasionally it helps to use a magnifier to read the color.
Comparative Streak of Two Minerals

The streak of minerals that are harder than the streak plate may be obtained by powdering the mineral with a hammer. Place a small piece on a clean (no rust) steel plate and crush it with a clean hammer. Observe the powder against a suitable white or black surface. Ideally, the powder is obtained by grinding a small piece of the mineral in a white porcelain mortar with a porcelain pestle, but they are expensive and difficult to keep clean. For a collector's purpose, the clean hammer and steel plate works well.

Streak tests are especially useful with native metals, sulfides, sulfosalts, and oxides, as the color of their crystals or masses is very often different from the color of their streak. Other chemical classes of minerals normally produce a pale colored streak the same as the color of the mineral.

Porcelain streak plates can be purchased inexpensively. A package of 10 or 12, either white or black, is less than $10.00 US. If you do not have a streak plate, the white back of an unglazed tile often will do. The clay tile is a little softer than porcelain, but in most cases it will work. In an emergency, so will the white, unglazed, bottom rim of most coffee mugs.

Cleaning streak plates is a problem. After use, try using an artist's rubber or gum eraser to remove the mark. It may not fully remove the mark, but it will take off most of it. When the plate becomes unusable, place a piece of 220 grit, or finer, wet emery paper on a flat surface, grit uppermost, and grind the surface of the plate until the marks no longer show. Do not use coarser grits as they will destroy the plate. Then again, those plates do come in packs of ten.

A Little Theory


Light is considered to be either a particle or a wave of energy. Actually it is both, for at the speed of light there is no difference. A photon (particle) of light has a wavelength, a frequency, and energy. They are different ways of describing the same thing.

In any case, color involves the interaction of photons of incident light with the electrons of near surface ions or molecules in the crystalline mineral. Photons penetrate at least a few layers deep into the crystal. They have energy inversely proportional to their wavelength. Violet light has a short wavelength and a high energy. Red has a longer wavelength and a lower energy. The electrons in an atom also have energy, the amount of which is specific to each and determined by where in the structure of the atom the electron is located and how it is influenced by electrons of nearby atoms. There are energy gaps between the permitted positions of electrons. If the electron energy plus the photon energy is a permitted higher energy level for the electron it absorbs the photon and jumps to the higher level, where it is unstable. It soon gives up (radiates) energy and falls back to a stable level. The energy it radiates is equal to the difference between the two energy levels and is a photon of colored light. If the photon is radiated back through the surface, it is reflected light. It may be radiated into the crystal and transmitted until after further collisions it emerges on a far side of a transparent crystal. Or it may be absorbed in the crystal. This all takes place almost instantaneously and in huge numbers, providing a continuous sensation of color. There are several mechanisms by which minerals produce color, but all of them, in some way, involve this process.

Electrons in atoms, ions, and molecules are located in orbitals. An orbital is a space around the nucleus of an atom that has a shape (some are directional, some are spherical) where there is a high probability of finding an electron. Different orbitals have different energies. The highest energy and outermost (valence) electrons of the transition metals are named the "d orbitals".

d to d Transitions


Many metal complexes are colored due to d-d electron transitions. The d orbitals define the transition metals and are on the surface of their atoms or ions. Visible light of the correct wavelength is absorbed, promoting a lower energy d-electron to a higher d energy level, an excited state. The return to a lower level is accompanied by the emission of a photon. Colors of d-d transitions are usually quite faint,

Ligand Field Transitions


Crocoite
Emerald
Ruby
Bariosincosite
Crocoite
Emerald
Ruby
Bariosincosite
Crocoite
Emerald
Ruby
Bariosincosite

Metal ions, usually of transition metals, are bound to ligands that are not metallic, most often in 4 coordination. There is a color producing charge-transfer between the ligand and the metal. High valence metallic ions often are the result (e,g, CrO4 2-, MO4 2-, WO4 2-, VO4 3-). The oxide ion O2- is a common ligand, lending electron pairs from pi orbitals to the metal. A charge transfer can occur from ligand to metal or from metal to ligand. In general, the ligand to metal transfer produces brighter colors. Cr3+ substitutes for Al3+ in both emeralds and ruby, forming ligand fields with oxygen and producing the emerald green or the ruby red.

Charge Transfer


One form of charge tranfer takes place when a photon causes pi orbitals from the metal and ligand to overlap, creating temporary pi bonds that give up photons upon return to the base state.
CdS: The color of artist’s pigment cadmium-yellow is due to a Cd2+ (5s) ← S2−(π) transition.
HgS: is red due to Hg2+ (6s) ← S2−(π) transition.
Fe Oxides: that are red and yellow due to the transition, Fe (3d) ← O2−(π).

Another type of charge transfer occurs when there is a defect in the crystal lattice where a "hole" is created by a "missing" atom. The hole traps an electron which then interacts, usually, with a small monovalent metal ion nearby. Smoky quartz contains some aluminum replacing silicon. Natural irradiation in the rocks causes the aluminum ion to become an aluminate ion and transfer charge, usually to a lithium ion, producing the smoky color center.

Transitions Between Energy Bands


Some atoms have band gaps (or energy gaps), between permitted energy levels, where electrons cannot exist. The band gap is between the valence level and the conductance level of the substance and at a level where a photon can create an electron/hole pair. When this occurs in minerals, it is usually a color center. The rigid covalent bonding of carbon in diamonds produces a relatively high band gap and color centers account for much of the color in diamonds.

The explanations above are greatly simplified. How color is produced is highly complex and for a thorough explanation you are referred to "The Fifteen Causes of Color" (an article) or The Physics and Chemistry of Color: The Fifteen Causes of Color, Edition 2 (a book) by Dr. Kurt Nassau.

The Colors of Diamonds

Diamonds have been classified into 324 different colors (27 hues and 12 levels of saturation). Most of the differences from one to the next are too subtle to be detected by an untrained eye.

Because of the extremely rigid bonding in the diamond crystal, very few impurities can be accommodated in the diamond structure. Very small quantities (ca 1 atom per million) of nitrogen color diamonds yellow and boron makes them blue. Brown diamonds have color centers from lattice defects. Exposure to alpha radiation yields color centers and green diamonds. Plastic deformation produces reds, pinks and some browns.

Nearly 26,000 Kg (12,000 lbs) of diamonds are mined annually. (100,000 Kg are synthesized). Almost half of them are from Central or Southern Africa. Canada, Russia, India, Brazil, and Australia are significant sources, also. Most gem grade colored diamonds are yellow and are mined in Africa. The red, pink and fancy browns come from Australia. The Hope Diamond (blue) in the Smithsoinian Institute (US) is thought to be from India.


Diamond: Australia
Diamond: South Africa
Diamond: Brazil
Diamond: Alluvial Wisconsin, USA (Canada?)
Diamond: Australia
Diamond: South Africa
Diamond: Brazil
Diamond: Alluvial Wisconsin, USA (Canada?)
Diamond: Australia
Diamond: South Africa
Diamond: Brazil
Diamond: Alluvial Wisconsin, USA (Canada?)





Links to tht "Determinig . . ." Series: How To
    What Is a Mineral? The Definition of a Mineral
  1. Determining Color and Streak
  2. Determining Lustre: For Beginning Collectors
  3. Determining the Hardness of a Mineral
  4. Determining the Specific Gravity of a Mineral
  5. Determining Symmetry of Crystals: An Introduction


References


Nassau, Kurt (2001) The Physics and Chemistry of Color: The Fifteen Causes of Color / Edition 2, John Wiley & Sons, Hoboken NJ
Nassau, Kurt (1980) The Causes of Color; Scientific American, October 1980
http://onlinelibrary.wiley.com/doi/10.1002/col.5080120105/abstract ; Nassau, Kurt; The Fifteen Causes of Color
https://en.wikipedia.org/wiki/Color_theory
https://en.wikipedia.org/wiki/Charge-transfer_complex Color and ligand to metal or metal to ligand charge transfer.
https://en.wikipedia.org/wiki/Band_gap ; Theory, examples

Acknowledgements


I am indebted to Erin Delventhal for her constructive comments, and particularly on illustration, of this article. Thank you, also to Steve Hardinger for his review and suggestions on this article, which proved to be more difficult than I expected. DBP

Additive Color and Subtractive Color charts by Mike Horvath, modified by Jacobolus are from Wikipedia. Public Domain.
Streak Plate by: Ra'ike de:Benutzer; Creative Commons Attribute License, Wikipedia.




Article has been viewed at least 1560 times.

Comments

Donald, another great article!! Thanks.

Scott Rider
27th Mar 2018 12:00am
Thank you, Scptt. Fun to do.

Donald B Peck
27th Mar 2018 1:19am
Fascinating read Don, even for someone who teaches this to college-aged students. I will be able to take some of this material and introduce it to my students next year when we do minerals.
Thanks!!!

Paul Brandes
2nd Apr 2018 1:12pm
Thank you, Paul. When I started, I expected that this would be easy to write. I discovered it was anything but easy. I used to live about 4 miles from Bell Labs in Murray Hill NJ. And I met Kurt Nassau a couple of times at mineral club meetings. I didn't really know him, but writing this article, I wished that I had.

Donald B Peck
3rd Apr 2018 1:44am
You write great articles Don!

A great eye opener.

Thank you.



John Attard
10th Apr 2018 6:59am
John,
Thank you! I find writing to be enjoyable. I am a life-long teacher. And, at 88, my field collecting days are done. This fits my life-style nicely.

Don

Donald B Peck
12th Apr 2018 4:46pm

In order to leave comments to this article, you must be registered
Mineral and/or Locality  
Mindat.org is an outreach project of the Hudson Institute of Mineralogy, a 501(c)(3) not-for-profit organization.
Copyright © mindat.org and the Hudson Institute of Mineralogy 1993-2018, except where stated. Mindat.org relies on the contributions of thousands of members and supporters.
Privacy Policy - Terms & Conditions - Contact Us Current server date and time: July 21, 2018 22:29:49
View slideshow - Go to top of page