Mineral Classification
The
Dana System
Mineral
classification can be an organizational nightmare. With over 3,000
different types of minerals a system is needed to make sense of them all.
Mineralogists group minerals into families based on their chemical composition.
There are different grouping systems in use but the Dana system is the most
commonly used. This system was devised by Professor James Dana of Yale University
way back in 1848. The Dana system divides minerals into eight basic classes.
The classes are:
Mineral Classification
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Native Elements This is the category of the
pure. Most minerals are made up of combinations of chemical elements. In this
group a single element like the copper shown here are found in a naturally
pure form.
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Silicates This is the largest group of
minerals. Silicates are made from metals combined with silicon and oxygen.
There are more silicates than all other minerals put together.The mica on the
left is a member of this group.
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Oxides Oxides form from the combination of
a metal with oxygen. This group ranges from dull ores like bauxite to gems
like rubies and sapphires. The magnetite pictured to the left is a member of
this group.
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Sulfides Sulfides are made of compounds of
sulfur usually with a metal. They tend to be heavy and brittle. Several
important metal ores come from this group like the pyrite pictured here that
is an iron ore.
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Slufates are made of compounds of sulfur
combined with metals and oxygen. It is a large group of minerals that tend to
be soft, and translucent like this barite.
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Halides form from halogen elements like
chlorine, bromine, fluorine, and iodine combined with metallic elements. They
are very soft and easily dissolved in water. Halite is a well known example
of this group. Its chemical formula is NaCl or sodium chloride commonly known
as table salt.
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Carbonates are a group of minerals made of
carbon, oxygen, and a metallic element. This calcite known as calcium
carbonate is the most common of the carbonate group.
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Phosphates are not as common in occurrence
as the other families of minerals. They are often formed when other minerals
are broken down by weathering. They are often brightly colored.
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Mineraloid is the term used for those
substances that do not fit neatly into one of these eight classes. Opal, jet,
amber, and mother of pearl all belong to the mineraloids.
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BASIC DEFINITIONS
A mineral is a
naturally occurring inorganic substance with a characteristic chemical
composition and definite crystal structure. The composition and crystalline
structure determines the properties of a mineral. The main mineral properties
used for identification are color, streak, hardness, specific gravity, cleavage
and fracture. Other properties such as luster and habit are also important.
Rocks
are aggregates of minerals. Rocks exhibit not only different mixture of
minerals, but also certain textures. The texture depends upon the size, shape,
and arrangement of the minerals composing the rock. Mineral assemblage and
texture serve in determining the origin and identification of a rock.
MINERAL IDENTIFICATION PROCESS
MINERAL IDENTIFICATION PROCESS
MINERAL PROPERTIES
COLOR - The color of a
mineral serves to narrow down the number
of possible choices since it is the first and most obvious
property noticed. However, because most minerals may
exhibit a variety of colors, color is not a reliable diagnostic
property.
of possible choices since it is the first and most obvious
property noticed. However, because most minerals may
exhibit a variety of colors, color is not a reliable diagnostic
property.
HARDNESS - Hardness is a measure
of resistance to scratching. The hardness of a mineral is based upon
comparisons of scratching tests. Mohs Scale of Hardness is the basic comparison
test.
Mohs Hardness Scale
Hardness of Test Materials
1 - Talc 2.5 -
Fingernail
2 - Gypsum 3.5 - Copper penny
3 - Calcite 5.5 - Masonry nail
4 - Fluorite 5.5 - Glass
5 - Apatite
6 - Orthoclase feldspar
7 - Quartz
8 - Topaz
9 - Corrundum
10 - Diamond
2 - Gypsum 3.5 - Copper penny
3 - Calcite 5.5 - Masonry nail
4 - Fluorite 5.5 - Glass
5 - Apatite
6 - Orthoclase feldspar
7 - Quartz
8 - Topaz
9 - Corrundum
10 - Diamond
Example of hardness
determination: If an unknown mineral cannot be scratched by a masonry nail but
can be scratched by orthoclase feldspar, then the hardness of the unknown
mineral would be between 5.6 and 5.9.
STREAK - The color of the
powder of a mineral is the streak. Rub the mineral on a piece of unglazed
porcelain (streak plate) to obtain the streak.
CLEAVAGE - Certain minerals
have a tendency to part, producing smooth flat surfaces. An example is mica: it
cleaves along one plane thus it has one direction of cleavage. Galena
FRACTURE - If a crystal
does not break along a cleavage surface, it exhibits fracture. Quartz shows no
cleavage when it is broken, instead it fractures in a shell shape known as
conchoidal fracture. Other minerals with well-developed cleavage may fracture
along other surfaces. Common descriptions: conchoidal, splintery, fibrous and
irregular.
LUSTER - The degree or
manner in which the surface of a mineral reflects light is luster. Terms used
to describe luster include earthy, glossy, metallic, pearly, greasy, waxy, and
vitreous (glassy).
SPECIFIC GRAVITY - The weight of a
mineral compared to the weight of an equal volume of water is the specific
gravity. Gold has a specific gravity of 19. This indicates that a cubic
centimeter of gold weighs 19 times as much as a cubic centimeter of water. (One
cubic centimeter of water weighs 1.0 grams) A relative comparison (of what is
light and what is heavy) is satisfactory for this laboratory.
