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Primary Characteristics of Minerals
1) Crystal Shape
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if grown unimpeded, minerals will have a consistent shape specific to that mineral
often confused with cleavage
crystal shape is the original shape of the mineral
very useful if present
2) Lustre
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the reflective nature of the mineral's surface
split into two broad categories, metallic or non-metallic
non-metallic can be further broken down to glassy, pearly, silky, resinous, earthy or dull
these descriptions are somewhat subjective
often a starting point for classification
3) Colour
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a relatively useless characteristic
some minerals do have distinct colours, but there are two problems
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many minerals have similar colours
some minerals have a variety of colours
Quartz
Given the samples of quartz shown, why might the following properties not be useful?
Crystal shape
Lustre
Colour
4) Streak
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colour in a powdered form
the "true colour" of the mineral
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obtained by rubbing a mineral off of a streak (porcelain) plate
while quartz may be many different colours, the streak is always white
5) Hardness
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the ability to resist scratching
has to do with the strength of bonds within the minerals
a harder mineral is capable of scratching a softer mineral
Moh's Hardness Scale
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a series of benchmark minerals ranked from 1 - 10
1.
2.
3.
4.
5.
6.
7.
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10.
Ex) An unknown mineral can scratch apatite and can be scratched by a steel screwdriver. What might its
hardness be?
Ex 2) There are 3 minerals (A, B, and C)
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"A" can cut glass
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"B" can cut "A"
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"C" can be scratched by fingernails
Rank them from softest to hardest
Ex 3) Sort the given list into the following catergories.
Can Cut Glass
Can Be Scratched by Finger Nails
6) Cleavage / Fracture
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the way that minerals tend to break
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cleavage - breaks along planes
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fracture - breaks unevenly
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cleavage can be described by:
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the number of planes
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angle that the planes meet
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for example:
How would we describe the cleavage of calcite?
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not all cleavage is perfect, but the patterns should be regular enough to spot
if the mineral breaks in an irregular manner, it fractures
fracture can be broken up in several categories, but we limit it to either
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conchoidal:
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nonconchoidal:
7) Specific Gravity
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similar to density
a number that compares the mass of a mineral to the amount of water it displaces
if a mineral displaces 3 times more water than its mass, then it has a specific gravity of 3
if the mineral has a low specific gravity it will feel "too light"
a high specific gravity feels "too heavy"
Describe the specific gravity of hematite versus gypsum.
Calculating Specific Gravity
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a cubic centimetre (one millilitre) of water has a mass of approximately one gram
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by comparing the mass of the rock to the amount of water displaced, we can calculate the specific
gravity
Ex) How much water is displaced by the by the rock?
Ex 2) If the rock has a mass of 150 g, what is the specific gravity of the rock?
Secondary Properties
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extremely useful when present, but not common enough to start with
1) Taste
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some minerals have a distinct taste
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for example halite tastes salty
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can be unsafe with some minerals
2) Smell
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a few minerals have a distinct smell
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for example, sulfur smells like rotten eggs
3) Magnetism
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some minerals are ferromagnetic meaning that they are attracted to magnets
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for example, magnetite
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some minerals are only slightly magnetic and difficult to notice
4) Fluorescence
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some minerals glow under UV light (blacklight)
for example, fluorite has a greenish glow
5) Acid reaction
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carbonate minerals bubble in the present of acid
for example, calcite
6) Optical Properties
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some minerals cause light to bend in strange ways that can be observed
for example, calcite displays double refraction
Commonly Confused Minerals
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often minerals that look the same can be confused for each other
this is why pyrite is known as fool's gold (it is similar in appearance to gold)
it is important to know some quick ways to tell these common minerals apart
Ex 1) Pyrite and Gold
Pyrite
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lustre: metallic
color: brass yellow
streak: greenish-black
hardness: 6
specific gravity: 5.2
Gold:
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lustre: metallic
color: rich yellow
streak: rich yellow
hardness: 2.5 - 3
specific gravity: 19
Ex 2) Barite and Quartz
Barite
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lustre:
color:
streak:
hardness:
specific gravity:
cleavage:
Quartz
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lustre:
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color:
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streak:
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hardness:
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specific gravity:
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fracture:
Ex 3) Halite and Calcite
Ex 4) Calcite and Quartz
Ex 5) Talc and Gypsum