minerals
STREAK. The color of the mineral in powdered form, called streak, is
often useful in identification. A mineral’ s streak is obtained by rubbing it
across a streak plate (a piece of unglazed porcelain) and observing the color
of the mark it leaves (FIGURE 2.11). Although the color of a mineral may
vary from sample to sample, its streak is usually consistent in color.
Streak can also help distinguish between minerals with metallic luster
and those with nonmetallic luster. Metallic minerals generally have a dense,
dark streak, whereas minerals with nonmetallic luster typically have a light
colored streak.
It should be noted that not all minerals produce a streak when
rubbed across a streak plate. For example, the mineral quartz is harder
than a porcelain streak plate. Therefore, no streak is observed using this
method.
Crystal Shape or Habit
Mineralogists use the term crystal shape or habit to refer to the
common or characteristic shape of a crystal or aggregate of crystals. A few minerals exhibit somewhat
regular polygons that are helpful in their identification. For example, magnetite crystals sometimes occur as octahedrons, garnets often form
dodecahedrons, and halite and fluorite crystals
tend to grow as cubes or near cubes. While
most minerals have only one common habit,
a few have two or more characteristic crystal
shapes such as the pyrite sample shown in
FIGURE 2.12.
Aragonite crystals.
(Photo by Ross Frid)
D I D Y O U K N O W ?
The name crystal is derived
from the Greek (krystallos,
meaning “ice”) and was applied
to quartz crystals. The ancient
Greeks thought quartz was
water that had crystallized at
high pressures deep inside
Earth.
45
Physical Properties of Minerals
FIGURE 2.10 Quartz. Some minerals, such as quartz, occur in a variety of
colors. These samples include crystal quartz (colorless), amethyst (purple
quartz), citrine (yellow quartz), and smoky quartz (gray to black). (Photo
courtesy of E. J. Tarbuck)
FIGURE 2.11 Although the color of a mineral is not always helpful in
identification, the streak, which is the color of the powdered mineral,
can be very useful. (Photo by Dennis Tasa)
STREAK. The color of the mineral in powdered form, called streak, is
often useful in identification. A mineral’ s streak is obtained by rubbing it
across a streak plate (a piece of unglazed porcelain) and observing the color
of the mark it leaves (FIGURE 2.11). Although the color of a mineral may
vary from sample to sample, its streak is usually consistent in color.
Streak can also help distinguish between minerals with metallic luster
and those with nonmetallic luster. Metallic minerals generally have a dense,
dark streak, whereas minerals with nonmetallic luster typically have a light
colored streak.
It should be noted that not all minerals produce a streak when
rubbed across a streak plate. For example, the mineral quartz is harder
than a porcelain streak plate. Therefore, no streak is observed using this
method.
Crystal Shape or Habit
Mineralogists use the term crystal shape or habit to refer to the
common or characteristic shape of a crystal or aggregate of crystals. A few minerals exhibit somewhat
regular polygons that are helpful in their identification. For example, magnetite crystals sometimes occur as octahedrons, garnets often form
dodecahedrons, and halite and fluorite crystals
tend to grow as cubes or near cubes. While
most minerals have only one common habit,
a few have two or more characteristic crystal
shapes such as the pyrite sample shown in
FIGURE 2.12.
Aragonite crystals.
(Photo by Ross Frid)
D I D Y O U K N O W ?
The name crystal is derived
from the Greek (krystallos,
meaning “ice”) and was applied
to quartz crystals. The ancient
Greeks thought quartz was
water that had crystallized at
high pressures deep inside
Earth.
45
Physical Properties of Minerals
FIGURE 2.10 Quartz. Some minerals, such as quartz, occur in a variety of
colors. These samples include crystal quartz (colorless), amethyst (purple
quartz), citrine (yellow quartz), and smoky quartz (gray to black). (Photo
courtesy of E. J. Tarbuck)
FIGURE 2.11 Although the color of a mineral is not always helpful in
identification, the streak, which is the color of the powdered mineral,
can be very useful. (Photo by Dennis Tasa)