(RM) Minerals Lesson
Minerals
Rocks and minerals are not the same thing; rocks are composed of minerals which are naturally existing chemical compounds. Minerals are extremely important for daily life. Almost every product consumers use contains minerals that have to be mined.
Rocks are composed of minerals. Minerals are the building blocks of rocks.
What is a mineral? These are the characteristics of minerals followed by a brief explanation of each characteristic.
A mineral:
- Is naturally occurring
- Is a solid
- Is inorganic (mostly)
- Has a fixed chemical formula
- Has an orderly crystalline structure
Naturally Occurring
To be considered a mineral it must have been formed by natural geologic processes. Laboratory created gems (synthetic diamonds, rubies, etc.) don't count.
A Solid
By definition, minerals are solid within the normal temperature ranges of the earth's surface.
Inorganic
Generally, a mineral is a naturally occurring solid with a crystalline structure.
This is where it gets a little tricky.
It has a fixed chemical formula
Each mineral has a particular chemical make up.
It Has an orderly crystalline structure
Minerals have an orderly crystalline structure. This means that the atoms or ions that make up a mineral are arranged in an orderly and repetitive manner.
Crystals are solid material in which the atoms are arranged in regular geometrical patterns. The crystal shape is the external expression of the mineral's regular internal atomic structure. Temperature, pressure, chemical conditions and the amount of space available are some of the things that affect their growth.
Each mineral has its own specific properties that can be used to identify it. Some properties can be identified simply by looking at it, while other identifications require a test.
There are eight groups of minerals that are common.
- Native Elements
- Sulfides
- Oxides
- Carbonates
- phosphates
- Sulfates
- Halides
- Silicates
Mohs Hardness Scale
One indicator that can be used to identify a mineral is the mineral's hardness. Friedrich Mohs, an Austrian mineral expert, published a scale to describe and compare the hardness of minerals. The published scale is named after him and it ranks ten minerals from softest to hardest.The higher the number on the scale, the harder the mineral is. Each mineral can scratch only those minerals below it on the scale.
For example: Your fingernail has a hardness of 2.5 on the Moh's scale. If you can scratch the mineral with your fingernail then you know the mineral must be on the scale lower than 2.5. If you can't scratch it with your fingernail it must be harder than 2.5.
Identifying Minerals by Eight Properties
Color
Color is a physical property used to identify a limited number of minerals that have their own characteristic color. For example, the mineral malachite is always green and the mineral azurite is always blue. Many minerals occur in a variety of colors though so it is necessary to use more than one test.
Streak
A streak test is made by rubbing a piece of the mineral on a tile and looking at the color of the streak it leaves.
Even though the color of a mineral may appear different, its streak will always be the same. Sometimes the streak color and the mineral are different.
Luster
Luster is a scientific description of the way a mineral surface looks when light reflects off of the surface.
There are two basic categories of luster: metallic and non-metallic
Metallic
Metallic Luster refers to minerals that look like a shiny metal.
Examples include galena, pyrite, magnetite, and some varieties of hematite.
Non-Metallic
Nonmetallic Luster. There are a number of different nonmetallic lusters.
- Vitreous. Also called glassy. Examples include quartz and tourmaline.
- Resinous. Minerals that look like resin (a little bit like the look of plastic). Examples include sulfur and sphalerite.
- Pearly. Minerals with a pearl-like, iridescent luster. Talc is a good example.
- Greasy. Some minerals look like they have a thin layer of oil on them. Graphite is the best example.
- Silky. Minerals that look like fibers of silk. Fibrous gypsum and malachite have this luster.
- Adamantine. This describes minerals with a brilliant luster, sometimes with colors flashing in the mineral.
Diamond is the best example. Some clear lead minerals, like cerussite and anglesite, also have adamantine luster.
Hardness
Hardness is a measure of the resistance to scratching. We have already discussed Mohs Hardness Scale.
Density
A mineral's mass divided by it's volume (D=M/V). The more "dense" it is the "heavier" it seems to be.
Density is the mass in a given space, or mass per unit volume. You can determine the density of a mineral by first finding the mass of the mineral with a balance. Second, find the volume by determining how much water is displaced when you put the mineral in the water. Finally, divide the mineral's mass by its volume. Density of a mineral remains the same regardless of how much of the mineral you have.
Crystal Structure
Atoms are arranged so that they form a particular geometric shape or crystal.
Cleavage and Fracture
How a mineral breaks apart can be observed and used to help identify the mineral. A mineral that splits easily along flat surfaces has the property called cleavage. Cleavage is determined by how the atoms in its crystals are arranged. Most minerals do not break apart evenly but fracture instead.
Fracture describes how a mineral looks when it breaks apart in an irregular way.
Special Properties
Special properties are unique characteristics a mineral may have.
Fluorescence - it glows
May be magnetic
Taste - Is it salty?
May be radioactive
Chemical reactivity - bubbles in acid
Optical properties - causes double images
Identifying Minerals
Watch the following video presentation to recap how scientists and collectors identify minerals.
[CC BY 4.0] UNLESS OTHERWISE NOTED | IMAGES: LICENSED AND USED ACCORDING TO TERMS OF SUBSCRIPTION