New York State High School Regents Exam Prep
Earth Science

What are minerals?

Minerals are naturally occuring, inorganic, uniform materials that have definite chemical and physical properties.

Identifying Minerals
Geologist often identify minerals based on their physical and chemical properties. A description of these properties are listed below. Many of these properties are controlled by the elements present in the mineral and their arrangement.

Color- The most easily observed property, but usually the least useful. A mineral's color can be changed by the impurities that are found in the mineral.

Luster- The way light is reflected from a newly exposed surface. Described as either metallic or nonmetallic. Some examples of minerals with metallic luster are pyrite, galena and magnetite. Some examples of minerals with nonmetallic luster are calcite, quartz and feldspar. Nonmetallic luster can also be described as glassy, pearly, waxy and earthy (dull).

Streak- The color of the mineral in powder form. This test is done by rubbing the mineral across a white streak plate. Several minerals have a streak that is not the same color as the mineral itself. Most metallic luster minerals have a dark colored streak.

Crystal Structure- Some minerals form crystals, if there is time and room for the crystals to form. The crystal pattern of a mineral is controled by the internal arrangement of the atoms that make up the mineral. Some examples of these crystal structures are quartz which has a hexagonal (six-sided) crystal and halite which has a cubic crystal.

Hardness- The ability of one mineral to scratch another. The softer mineral gets scratched. You test a mineral's hardness by scratching the unknown mineral with an object of known hardness. Moh's Scale of Hardness is used to rate the hardness of a mineral. The chart below shows the ten minerals that make up the hardness scale and some common materials with their hardness to test unknown minerals. One on the scale is the softest and ten is the hardest.

To find the hardness of a mineral you first have to find out what it can scratch and what can scratch it. For example, if a mineral can scratch glass but can't scratch a streak plate, the mineral has a hardness of 6 on the Moh's Hardness Scale.

Use Moh's Hardness Scale to answer the following questions.
1. If your fingernail can easily scratch the mineral, what is the hardness of this mineral?

2. A student notices that their unknown mineral will scratch a penny but it will not scratch an iron nail. What mineral might this unknown be?

3. What if the unknown mineral scratches all the common objects listed in the chart?

Cleavage or Fracture- If a mineral breaks along flat, smooth surfaces it shows cleavage. Cleavage can be in one, two or three directions. Some examples are:

Mica- cleavage in one direction, see the diagram below (upper right),
Feldspar- cleavage in two directions, see the diargam below (lower left),
Calcite, Galena and Halite- cleavage in three directions, see the diargam below (lower right).

If a mineral breaks along irregular rough surfaces it shows fracture. Quartz shows a special type of fracture called concoidal (shell-like) fracture.

Acid test- This test is performed by dropping weak hydrochloric acid on the mineral. If it reacts (fizzes) then the mineral is calcite. This test will also help to identify the rocks limestone and marble, because calcite is the principal mineral in both.

Magnet test- If there is a magnetic attraction between your mineral and a metal object, then the mineral has a high iron content. The mineral magnetite will attract metal objects.

Remember on the Regent's Exam most of the the information about the mineral in a question will be given to you or will observed from the diagrams on the Exam. Use them to your advantage. Make sure you know the basics about mineral identification and observe carefully.

New York State High School Regents Exam Prep Center: Earth Science