Regents Prep: Earth Science: Rocks & Minerals:
Minerals

Minerals are also known as “rock ingredients”. In order to properly identify a mineral one needs to be aware of the characteristics a mineral may posses and its physical properties.

Characteristics of Minerals
Minerals may be composed of single or multiple elements. The minerals that are found in the Earth's crust are basically composed of two elements: they are oxygen and silicon. According to page 11 of the Earth Science Reference Tables, oxygen and silicon make up about 75% of the total mass of elements in the Earth's crust. This leads to the basic chemical structure of 90% of all minerals, which is the silicon-oxygen tetrahedron (SiO4).

Before we discuss the physical properties of a mineral take a glance at the chart below which lists the major minerals that are to be identified. The chart is from page 16 of the Earth Science Reference Tables.

Physical Properties of Minerals
Minerals have distinct physical properties that can be used to correctly identify the name of a mineral. The properties that are commonly used are hardness, streak, luster, cleavage or fracture, color, and a few more specific properties such as magnetism and odor. I will discuss each property in order to identify a mineral sample.

Hardness is the resistance of a mineral to being scratched. This test is also the most useful in narrowing down the type of mineral to be identified. The Moh's Scale of hardness lists ten minerals in order of hardness from 1-10 with 1 being the softest and 10 being the hardest. Mineral hardness may be compared to other minerals or to the hardness of common materials such as a fingernail (2.5), copper penny (3.5), iron nail (4.5), glass (5.5), steel file (6.5), and a streak plate (7.0). See Moh's Scale of Hardness below.

Moh's Scale of Hardness

The number next to the mineral is also the mineral's hardness. For example, diamond has a hardness of 10 where as calcite has a hardness of 3.
 

10

Diamond

9

Corundum

8

Topaz

7

Quartz

6

Orthoclase (K-feldspar)

5

Apatite

4

Fluorite

3

Calcite

2

Gypsum

1

Talc

Some common items used for comparison:

5.5

Glass

5.5

Steel Nail

3.5

Penny

2.5

Fingernail

 

Streak is the powder or residue left after a mineral has been scratched on a streak plate. Note: the color of the powder does not always resemble the color of the mineral. For example, Galena is a shiny sliver but will leave a dark gray streak.

Luster is the shine a minerals might posses. Usually if a mineral shines it will have a metallic luster. If not it is classified as a non-metallic mineral. For example, Galena has a shiny appearance like tin foil thus it is a metallic mineral.

Cleavage or fracture is how the mineral will break apart. If the mineral break apart in similar pieces it is said to have good cleavage. And if the mineral breaks like a piece of glass with uneven, jagged edges it would be classified as fracture. An example of a mineral with good cleavage would be Galena. If Galena breaks it breaks into cubes called “cubic cleavage”. There are other forms of cleavage, refer to your text to research them. An example of fracture would be Quartz. If Quartz were to split it would break in all different shapes and sizes. Due to this Quartz is said to have good fracture.

Color may be used but is not always a good identifier due to many minerals having the same color or the same minerals having multiple colors. This is true with quartz. Due to the amount of trace elements in a quartz sample the colors that can be found range from red to purple and even clear. So as one can see color would not be useful to identify a quartz sample.

Magnetism may be used to identify if a sample has iron (Fe) properties. The strongest attraction to a magnet will be found in the mineral magnetite.

Odor and taste may also be used, but these would most likely identify only two minerals they are: sulfur and halite.

After looking at a mineral and testing for the above physical properties along with using the Earth Science Reference Tables mineral chart on page 16, one should have a less difficult time properly identifying the mineral sample.

For additional information on minerals and how to identify them visit TheImage.com's Mineral Index.  

 

Created by Luke Carnicelli
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