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Identifying Common Minerals (Time required – 80 minutes)

Background Information

Scientists have identified more than 2000 different kinds of minerals. However, most minerals are very rare. Over 95% of the earth’s crust is made of rocks that are composed of about a dozen minerals. These common minerals are called rock forming minerals. While minerals can vary a great deal in their chemical makeup and the forms in which they are found, most common minerals can be identified by observing a few of their basic properties. In this investigation, you will observe the physical properties of some common rock forming minerals and use these properties to identify the minerals. To do so, you will have to test for the physical properties described below.

Some Physical Properties of Minerals

HARDNESS: Hardness is the resistance of a mineral to scratching. A mineral will scratch any other substance less than or equal in hardness to itself.

Table of Hardness of Common Items
Hardness / Mineral / Item / Approximate hardness
1 / Talc
2 / Gypsum / Fingernail / 2.5
3 / Calcite / Copper penny / 3-3.5
4 / Fluorite
5 / Apatite / Glass / 5.5
6 / Orthoclase / Feldspar / Steel nail / 6.0
7 / Quartz / Steel file / 6.5
8 / Topaz
9 / Corundum
10 / Diamond
This means that a fingernail will scratch the mineral

STREAK: Streak is the color of a fine powder of a mineral. Rubbing a mineral sample against a piece of unglazed tile, or a streak plate, will power enough of the mineral to enable you to identify its streak color. If a mineral is harder than the streak plate (H=7), it will not produce a streak.

CLEAVAGE: Cleavage is the quality of a mineral that causes it to split, or cleave, leaving a smooth, flat surface. When a mineral does not split to leave smooth, flat surfaces, the break is called fracture.

CRYSTALLINE SHAPE: A crystal is a solid that has flat surfaces arranged in a defined shape. Most minerals occur naturally as crystals. Every crystal has an orderly, internal pattern of atoms, with a distinctive way of locking new atoms into that pattern to repeat it again and again. The term used to describe general shape of a crystal is habit.

COLOR:Color is sometimes an extremely diagnostic property of a mineral, for example olivine and epidote are almost always green in color. But, for some minerals it is not at all diagnostic because minerals can take on a variety of colors.

DENSITY: Density, or heft, describes how heavy a sample is for size. Samples that seem heavy for their size are described as dense, while samples that seem light for their size are described as not very dense. Density refers to the mass per unit volume. Specific Gravity is the relative density, (weight of substance divided by the weight of an equal volume of water).

MAGNETISM: Some minerals are magnetic and will be attracted to a magnet.

LUSTER: Luster is the way a mineral reflects light. Minerals may be described as having metallic or nonmetallic luster. Metallic luster is the shine associated with a freshly polished metal surface. Nonmetallic luster may be referred to as glassy, brilliant, or greasy. Luster should not be confused with color:

Metallic: very high reflectance, opaque / Submetallic: medium reflectance, opaque
Adamantine: very high reflectance, transparent / Glassy: high reflectance, transparent or translucent
Resinous: medium reflectance, translucent / Waxy: medium reflectance, translucent or opaque
Pearly: low reflectance, translucent or opaque / Dull: no reflectance, opaque

Problem: How can the physical properties of minerals be used to identify them?

Materials: (per group)

Set of common minerals samplesMagnet

Piece of dull tile (streak plate) Hardness kit containing a – copper penny, a steel nail, and a piece of glass

Procedure: Carefully test and observe each mineral sample to determine its physical characteristics. Use the summary of mineral physical properties in the Mineral Identification Key on the next page. Enter your observations in the appropriation space in the Data Table.

Conclusions

Using the Mineral Identification Key and your observations in the Data Table, identify each mineral and enter its name In the Data Table.

Critical Thinking and Application

  1. According to the Mineral Identification Key, what is the only physical property that can be used to distinguish between pyrite and chalcopyrite, assuming that both minerals are the same color?
  1. What are the softest minerals listed in the Mineral Identification Key?
  2. What is the hardest mineral listed in the Mineral Identification Key? What must then be true about the mineral?
  3. Which physical property is the least helpful in identifying minerals? What physical property did you find to be the most useful?
  4. Explain the following statement: You can determine the identity of a mineral by showing what it cannot be?
  1. What is the most important thing you learned about minerals by doing this lab activity?
  2. Complete this activity by identifying the chemical formula for each mineral you have identified. Use your text book.