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Mineral and Gem Characteristics Help (page 3)

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By — McGraw-Hill Professional
Updated on Apr 25, 2014

Color

Most people are most interested in the color of a mineral. This is especially important for choosing minerals as gemstones for jewelry. After all, jewelry has to match the outfit (maybe that’s why “diamonds are a girl’s best friend,” they go with everything)!

One of the problems geologists find in using color to identify minerals of a certain group is that some minerals can be very different.

Some colors are called idiochromatic . Their chemistry gives them their color. Malachite which has a lot of copper is always green because copper gives it that color.

Minerals that are usually colorless and take on the color of small impurities are called pseudochromatic . Depending on the impurity, they can have a variety of colors. If a mineral contains bits of iron, it will take on a reddish color.

Allochromatic minerals are generally colorless and transparent. They get their color from the small changes in their crystalline make up or from structural flaws. In corundum, for example, the substitution of iron and titanium for aluminum gives a blue sapphire, while iron by itself produces a yellow sapphire.

Minerals like quartz come in lots of colors. Some of the colors that quartz can take are listed below:

  • Colorless quartz,
  • Rose quartz (all shades of pink),
  • Milky quartz (white and whitish gray),
  • Citrine quartz (yellow, yellowish brown, and orange),
  • Smoky quartz (brown, brownish black, and black), and
  • Amethyst (light to deep purple).

Many minerals, depending on their chemical content and formation, are found in different colors. Ruby and sapphire are both varieties of corundum with the same chemical composition (AlO 3 ) and hardness (9), but two very different colors. Most people know that rubies are red and sapphires are blue. However, just to keep you guessing, sapphires can also be colorless, green, yellow, or purple!

Tourmaline is thought to have the greatest number of color variations. A “chameleon of color,” tourmaline is a prismatic crystal and is found to occur in seven different forms. These include, elbaite (multicolored), schorl (black), buergerite and dravite (brown), rubellite (pink), chromdravite (green), and uvite (black, brown, and yellowish green). Long, tourmaline crystals can be pink on one end and green on the other! They look like some rare cosmic gem from a science fiction movie.

A few minerals, like ruby, are fluorescent. They absorb blue and ultraviolet light and then release some of the energy back in the red part of the light spectrum.

Streak

The streak of a mineral is simple to remember. It is just what it says, a powdery streak made when rubbing a sample across an unglazed surface. Streak is a more dependable way to test a mineral than color since it is nearly always the same for different minerals. Sometimes a hard sample must have a small bit crushed with a geological hammer to get a sample to test.

A streak may be colorless, white, golden yellow, yellow, reddish brown, red, gray, brown, or black. The streak of a mineral is often not the same color as the mineral appears to the eye. For example, the mineral crocoite is orange–red and its streak is yellow. Wulfenite can be orange, yellow, brown, gray, or greenish brown, but its streak is white. Table 9-7 shows a variety of minerals with their visible color, streak, and luster.

Table 9-7 A mineral’s streak can be a different color from the mineral itself.

Mineral

Visible color

Streak

Luster

Albite

Colorless, white, bluish, gray, greenish, or reddish

White

Pearly to vitreous

Antimony

Silvery-gray

Gray

Brilliant metallic

Arsenic

Pale gray, dark gray

Pale gray

Metallic

Bismuth

Silvery-white (reddish tarnish)

Silvery-white

Metallic

Cinnabar

Brownish-red or scarlet

Scarlet

Adamantine, submetallic, or dull

Copper

Rose-red, copper-red

Copper-red

Metallic

Corundum

Many colors, red (ruby), blue (sapphire)

White

Vitreous to adamantine

Cuprite

Red

Brownish-red

Adamantine, submetallic, or earthy

Diamond

Colorless, white, gray, orange, yellow, brown, pink, red, blue, green, or black

White

Adamantine or greasy

Dioptase

Deep bluish-green

Pale bluish-green

Vitreous

Erythrite

Deep purple to pale pink

Purple to pale pink

Adamantine, vitreous, or silky

Fluorite

Colorless, purple, green, white, yellow, pink, red, blue, and black

White

Vitreous

Galena

Lead-gray

Lead-gray

Metallic

Gold

Yellow

Golden yellow

Metallic

Graphite

Dark gray to black

Dark gray to black

Dull metallic

Hematite

Bright red, brownish-red, steel-gray, iron-black

Brownish-red

Metallic to dull

Jadeite

Green, white, gray, or mauve

Colorless

Vitreous to greasy

Labradorite

Colorless, blue, gray, or white

White

Vitreous

Lazulite

Blue, bluish-green

White

Vitreous to dull

Malachite

Deep green

Pale green

Vitreous to silky

Microcline

Green, white, gray, yellowish, reddish, or pink

White

Pearly or vitreous

Molybdenite

Gray

Gray

Metallic

Nickel–iron

Steel-gray, dark gray, or blackish

Steel-gray

 

Olivenite

Olive-green, brown, yellowish, gray, or white

Olive-green

Vitreous to silky

Olivine

Green, greenish-yellow, white, or brown

Colorless

Vitreous

Platinum

Silvery-gray to white

White, silvery-gray

 

Pyrite

Pale yellow

Greenish-black

Metallic

Rhodochrosite

Pink to red

White

Vitreous to pearly

Silver

Silvery-white

Silvery-white

Metallic

Sulfur

Lemon-yellow to yellowish-brown

White

Resinous to greasy, adamantine

Wulfenite

Brownish-black

Reddish-brown to black

Submetallic

Luster

Luster is the word geologists use to describe the way light reflects off the surface of a mineral or crystal. The amount of light absorbed and a mineral’s texture affect luster. The different types of luster consist of dull, metallic, vitreous (glassy), adamantine , pearly, greasy, silky, and waxy. These are pretty straightforward and were used by some of the earliest people in describing different minerals. For example, gold and platinum have metallic lusters, but not microcline , which has a vitreous or pearly luster. Most silicates, sulfates, halides, oxides, hydroxides, carbonates, and phosphates have a vitreous luster.

A diamond’s high luster or that of highly reflective, transparent, or translucent mineral is known as an adamantine luster. Zircon , cuprite , and some forms of sulfur and cinnabar have this type of luster.

One thing to remember is that depending on the mineral and the environment in which it was formed, luster can be different in different parts of the same sample, as well as in different samples (from different places) of the same mineral.

Transparency

Depending on the way minerals are bonded, light will pass through a mineral in different amounts. When you can see right through a mineral like glass, it is said to be transparent . If light is slightly blocked, making the mineral look foggy and unclear, it is said to be translucent . If a mineral sample is solid and lets no light pass through at all, it is called opaque .

A transparent mineral can be seen through, while a translucent mineral is hazy, and an opaque mineral lets no light pass through at all.

A sample’s transparency isn’t always the same all the way through. Crystals are often transparent to translucent across a sample. For example, amethyst and olivine crystals are usually transparent to translucent within the same sample. Opal is transparent to opaque across a sample, while copper and jamesonite are opaque.

Specific Gravity

When geologists are trying to figure out the identity of an unknown sample, they use the above-mentioned characteristics as well as specific gravity (SG). The density of a sample is measured in terms of its specific gravity.

Specific gravity is the ratio of the mass of a substance compared to the mass of an equal volume of water at a specific temperature.

To find the specific gravity of a mineral, compare its weight to the weight of an equal volume of water. For example, a specific gravity of 4 tells geologists that an unknown sample is four times heavier than water. Size doesn’t matter. A larger sample can have a lower specific gravity. This is the case with talc and mercury . A large amount of talc would have a lower specific gravity than a small amount of mercury. The specific gravity of talc is 2.8, while mercury ’s specific gravity is 13.6. Table 9-8 gives some common minerals and their specific gravities.

Table 9-8 Minerals of different densities have different specific gravity values.

Mineral

Specific gravity

Albite

2.61

Antimony

6.7

Arsenic

5.7

Cinnabar

8.09

Copper

8.95

Corundum

4.0–4.1

Diamond

3.5–3.53

Fluorite

3.18

Galena

7.58

Gold

15–19

Graphite

2.09–2.23

Hematite

5.26

Jadeite

3.33

Labradorite

2.7

Lazulite

2.4

Magnetite

5.18

Malachite

4.05

Manganite

4.33

Marcasite

4.89

Mercury

13.6

Molybdenite

4.6–4.7

Olivine

3.2–3.4

Opal

2.1

Platinum

14–19

Pyrite

5.02

Scheelite

6.10

Silver

10–11

Spinel

3.55

Sulfur

2.07

Talc

2.8

Topaz

3.5–3.6

Zircon

4.7

 

Practice problems of this concept can be found at: Minerals and Gems Practice Test

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