Transcript Mineral Identification

Mineral Identification
Minerals combine with each other to form rocks. For
example, granite consists of the minerals feldspar,
quartz, mica, and amphibole in varying ratios.
Rocks are thus distinguished from minerals by their
HETEROGENEOUS composition. A mere 100 of
the several thousand known types of minerals
constitute the main components of rocks.
PHYSICAL PROPERTIES
Approximately 3,000 minerals exist in nature.
How do we identify them? Remember minerals differ from one another
because each has a specific chemical composition and a unique threedimensional arrangement of atoms within its structure. These differences
result in a variety of physical properties, including the minerals'
appearance, how they break, how well they resist being scratched, even
how they smell, taste, and feel. But not all of these properties are equally
useful.
The physical properties we use to identify minerals
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Color
Streak
Luster
Cleavage/Fracture
Hardness
Crystal Shape
Specific Gravity
Other Properties
Mineral Identification
Color
The color of a mineral is one of its most obvious attributes, and is
one of the properties that is always given in any description. Color
results from a mineral’s chemical composition, impurities that may
be present, and flaws or damage in the internal structure.
The Many Colors of Fluorite
Unfortunately, even though color is
the easiest physical property to
determine, it is not the most useful in
helping to characterize a particular
mineral. The problem is shown to the
right, in which the mineral fluorite
(CaF2) displays a rainbow of colors.
Color
*
Some minerals do have only a single color that can be diagnostic,
as for instance the yellow of sulfur, the green of malachite or the blue of
azurite. Also, although many minerals vary in color few span the spectrum
of colors as fluorite does.
Streak
* The color of a mineral when it is
powdered is called the streak of the
mineral. Crushing and powdering a
mineral eliminates some of the effects of
impurities and structural flaws, and is
therefore more diagnostic for some
minerals than their color. Streak can be
determined for any mineral by crushing
it with a hammer, but it is more
commonly (and less destructively)
obtained by rubbing the mineral across
the surface of a hard, unglazed
porcelain material called a streak plate.
Streak
• The color of the powder left behind on the streak plate is
the mineral's streak. The streak and color of some
minerals are the same. For others, the streak may be
quite different from the color, as for example the redbrown streak of hematite, from a mineral that is often a
gray to silver-gray mineral. The combination of luster,
color, and streak may be enough to permit identification
of the mineral.
Examples of Streak
Luster
*
The luster of a mineral is the way its surface reflects
light. Most terms used to describe luster are selfexplanatory: metallic, earthy, waxy, greasy, vitreous
(glassy), adamantine (or brilliant, as in a faceted
diamond).
*
A metallic luster is a shiny, opaque appearance
similar to a bright chrome bumper on an automobile
*
Other shiny, but somewhat translucent or
transparent lusters (glassy, adamantine), along with
dull, earthy, waxy, and resinous lusters, are grouped
as non-metallic.
Cleavage
In some minerals, bonds between layers of atoms aligned
in certain directions are weaker than bonds between
different layers. In these cases, breakage occurs along
smooth, flat surfaces parallel to those zones of weakness.
In some minerals, a single direction of weakness exists,
but in others, two, three, four, or as many as six may be
present. These breaking patterns are known as the
mineral’s cleavage.
Examples of Cleavage
Cleavage
Fracture
When bonds between atoms are approximately the same
in all directions within a mineral, breakage occurs either
on irregular surfaces or along smooth, curved surfaces.
This is described as the minerals fracture. Some
examples of fracture are splintery or irregular or
conchoidal (similar to those formed when thick pieces of
glass are broken.
Hardness
Hardness Testing
* The Mohs Hardness Scale is a
relative scale. This means that a
mineral will scratch any substance
lower on the scale and will be
scratched by any substance with a
higher number. Diamond is not 10
times harder than talc or 1.1 times
harder than corundum, as would be
the case with an absolute hardness
scale. Most often we are able only to
narrow down hardness to within a
certain range; for example, if an
unknown mineral scratches a copper
penny but does not scratch a glass
plate, its hardness must be greater
than 3.0 and less than 5.5.