Chemistry of Color

1

The Chemistry of Color

Why do minerals

have different colors?

Characteristics of Minerals

NATURALLY OCCURRING

SOLID

INORGANIC

SPECIFIC CHEMICAL COMPOSITION

SPECIFIC ATOMIC ARRANGEMENT

Mineral Properties

Predictable physical properties

Controlled by chemical composition

and/or structure

Mineral Properties

Color

Streak: Color of powdered mineral

Luster: Character of reflected light

Cleavage/Fracture: Mineral breakage form

Habit: Mineral growth form

Hardness: Resistance to scratching

Specific Gravity (Density)

Miscellaneous Properties: e.g., smell, magnetism

Chemistry of Color

2

Aspects of Light

Amplitude (height of wave) corresponds to the intensity (brightness) of light

Wavelength corresponds to the energy of light

In the visible spectrum wavelength corresponds to color

Wavelength of Light

Visible light is only a

tiny, moderate energy

fraction of the

electromagnetic

spectrum

Longer wavelengths

(e.g., radio waves)

have less energy

Shorter wavelengths

have more energy

Absorption Results in Color(A Familiar Example from Biology)

What Color?

What is the color of a mineral that absorbs

green and yellow light?

What spectral colors are absorbed by a

yellow mineral?

Chemistry of Color

3

Absorption and Emission of Light Color and Compositon

Certain elements are strong pigmenting

agents

Produce strong color when they are

present in either crystal lattice or impurity.

Called chromophores.

Color and Compositon

Chromium (Cr): orange, green

Cobalt (Co): pink, purple, blue

Copper (Cu): blue, green

Iron (Fe): red, green, yellow

Manganese (Mn): pink

Transition Metals

Chemistry of Color

4

Color vs Streak

If mineral color due to major constituent…

color and streak similar

If mineral color due to trace constituent (ppm)

streak will be pale or white