Geology Reference
In-Depth Information
Observing transparency, color, luster, and streak
Some of the most obvious observations you can make about a mineral relate to its ap-
pearance in relation to light:
Transparency: One visible characteristic of a mineral is its ability to transmit light,
or allow light to pass through it. This property is sometimes called its clarity . A
mineral that you can see through is transparent. One that allows light to pass
through but is not clear enough to see through is translucent And a mineral that no
light passes through is opaque.
Color: Color is a result of how light is absorbed or reflected by an object. In the
case of minerals, their color can be changed in many ways that do not affect the
crystal structure or composition. For example, heating a mineral may shift atoms
and result in color change, or very small amounts of other elements, called impur-
ities, may be trapped in the crystal structure and give it a color.
For these reasons, color is the least reliable method of identifying min-
erals. After all, a single mineral may come in many different colors. (This is the
secret to many semi-precious gemstones, which I describe later in this chapter.)
Luster: A mineral's luster is a description of how the surface reflects light. Luster is
either metallic (shiny like a metal) or nonmetallic. Nonmetallic lusters include
pearly, glassy, silky (like silk or satin fabric), earthy (dull), greasy, or adamantine
(extremely shiny, or fiery like a diamond).
A metallic luster is the result of light transmitting energy to electrons of
the surface atoms. This energy causes the electrons to vibrate and emit a shini-
ness you observe as metallic. This type of luster is named after metals, which
have a high number of free electrons that respond the same way to light.
Streak: Streak is how the mineral appears in powdered form. To test the streak of a
mineral, you rub the mineral on a streak plate, which is a piece of rough porcelain.
(Streak plates are often included in rock and mineral kits for this particular pur-
pose.) Rubbing the mineral against a streak plate grinds off some of the mineral in-
to a powdered line, or streak, across the plate. Many minerals can be identified by
the color of their streak, which may be very different from the color of the mineral
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