Mineral Lab A

GLS100 Lab 5: Mineral Properties

Physical Geology– Dr. Lindley S. Hanson

PART I

What is a mineral?

A mineral is naturally occurring, inorganic, homogeneous solid with a characteristic chemical composition, and ordered atomic arrangement. In order for a substance to be labeled a mineral all of these criteria have to be true.

Explanation:

1. Naturally occurring: Formed by natural processes (not created in a laboratory).
2. Homogeneous solid: A mineral consists of a single solid compound or element, which cannot be physically subdivided into other chemical components.
3. Inorganic: Not created by organisms or from their remains. This qualification excludes those substances, which are solely a product of organic activity (e.g. sugar) and not substances, which are commonly inorganic but may be precipitated by organisms (i.e. calcite).
4. Characteristic chemical composition: This signifies that a mineral can be represented by a specific chemical formula.
5. Ordered Atomic Arrangement: The atoms forming a mineral are arranged in a geometric pattern, which persists throughout the entire mineral. All minerals by definition are crystalline. Substances in which atoms are randomly arrange are called amorphous. All glasses by definition are amorphous.

Note: A mineraloid is a solid, which has all the properties of a mineral except that it has no ordered atomic arrangement and has a slightly variable chemical composition. Two common mineraloids are limonite (rust) and bauxite (major ore of aluminum).

EXERCISE 1: Is it a mineral?

In table 1 are eight common substances. Review each property discussed above and determine whether or not each solid would be considered a mineral. If you think a substance would not be considered a mineral then state why.

Table 1. Which are minerals and which are not?
Material / yes/no / Explanation
volcanic glass / . / .
sugar / . / .
rust (limonite) / . / .
ice / . / .
steel / . / .
coal / . / .
salt (halite) / . / .
cubic zirconium / . / .

Part II

Mineral Properties and Identification

Each mineral has a unique set of physical properties from which it can be readily identified. The physical properties that you will need to know are:

1.  Luster / 5.  Color
2.  Cleavage, Fracture and Parting / 6.  Streak
3.  Crystal form / 7.  Tenacity
4.  Hardness / 8.  Specific gravity

There are many other physical properties, such as magnetism, crystal form, habit (mode of occurrence), and diaphaneity (transparency), that may be useful in mineral identification.

EXERCISE 1

Luster is the measure of the quality or intensity of reflected light. When identifying luster it is important to look at unweathered, freshly broken surfaces; in particular, cleavage surfaces. The materials listed below (table 2) are used to define the different types of luster.

Table 2. Lusters used to describe minerals.
Type of luster / Appearance / Examples
metallic / Like that of metal / gold, silver, copper
adamantine / Like that of diamond / diamond
vitreous / Like that of glass / fluorite, quartz, mica
greasy / Like that of grease / quartz
pearly / like that of pearl / mica, talc
waxy / like that of wax / serpentine, talc
dull / like that of chalk / clay

State the luster(s)* exhibited by each specimen listed in table 3. These minerals are labeled L1-L6 in the mineral drawers found in the gray cabinet in the back of the lab.

Table 3. Luster exercise.
# / Mineral / Luster(s)
L1 / galena / .
L2 / muscovite / .
L3 / kaolinite / .
L4 / pyrite / .
L5 / talc / .
L6 / quartz / .

*Note: Some minerals have only one characteristic luster while others may have two or more. For example, the mineral hematite may be metallic or dull and the mineral gypsum may be vitreous to pearly.

EXERCISE 2: Cleavage, Fracture, Parting, and Habit

Cleavage is the splitting of a mineral along one or more inherent planes of weakness. The inner atomic structure of a mineral governs whether or not the mineral will cleave, and the type of cleavage it displays. Cleavage is easily demonstrated with the structure of graphite (fig. 1.1). The lines between the carbon atoms represent the bonds holding the atoms together. The longer the line the weaker the bond.
Can you determine how this mineral will break? /

Figure 1.1. Structure of graphite.

Cleavage: The flat shiny surfaces that are produced when a mineral cleaves are called cleavage planes. A mineral may exhibit 1,2,3,4, or 6 (rare) different directions of cleavage. (Note: Planes that are parallel are not considered different cleavage directions.) The different types of cleavage are described in the Table 4.

Table 4. Types of cleavage.
Type of Cleavage / Description / Examples
Pinacoidal / 1 plane of cleavage. Forms flat fragments or flakes with ragged edges. / muscovite, biotite, talc, chlorite, graphite
Prismatic / 2 planes of cleavage.(Often the most difficult cleavage to identify.) / feldspars, amphiboles, pyroxenes
Cubic / 3 planes of cleavage intersecting at right angles. / halite, galena
Rhombic / 3 planes of cleavage not intersecting at right angles. / gypsum, calcite
Octahedral / 4 planes of cleavage. (Forms triangular cleavage faces) / fluorite

Fracture is any break other than cleavage. The break does not yield a smooth and flat surface as with cleavage. Fracture occurs when bonds between atoms are more equal in strength and breakage is random. In the absence of cleavage, the fracture (table.5) is identified.

Exercise 2a: Cleavage

Five mineral fragments produced by breakage are illustrated in Figure 1.2. Identify the cleavage that is exhibited by each and fill in the blanks below.
1. ______
2. ______
3. ______
4. ______
5. ______/

Figure 1.2. Fragments formed by the cleavage of five different minerals
Table 5. Types of fracture
Type of fracture / Description
conchoidal / Fracture yields a smooth, often shiny, curved surface
subconchoidal / Moderately smooth, curved surface
uneven / Fracture yields a rough, irregular surface

Parting and habit

Crystalline aggregates: Some minerals have a particular mode of occurrence or habit; they may typically occur as granular, fibrous, or platy crystalline aggregates. The term granular refers to aggregates of small equant crystals. For example the mineral olivine occurs in granular aggregates. As shown in figure 3, crystalline aggregates generally display a characteristic parting or fracture.

Crystalline aggregates exhibit parting, similar in appearance to cleavage, but unlike cleavage, occurs between individual crystals, rather than through them. Examples (Figure 3) are shown below.

Examples of partings and habits
1.  fibrous parting: Asbestos occurs in fibrous aggregates of crystals.
2.  basal parting: Corundum parts between crystals that are in a column.
3.  flaky parting: Graphite, chlorite, and talc typically occur in aggregates of small platy crystals that flake off from the mineral aggregate. /
Figure 3. Partings and modes of occurrence (habit).

The term massive is commonly used to describe aggregates of microscopic or cryptocrystalline crystals forming a compact featureless mass with no preferred parting. Kaolinite, quartz (var. chert), pyrite, and hematite often occur in massive aggregates. The luster and breakage of a mineral aggregate may be different than that of the individual mineral crystal.

NOTES:

Exercise 2b: Cleavage and Fracture

Fill out table 6. For specimens labeled C1-C8, identify the cleavage and then briefly describe the mineral (e.g. color, luster, etc.). For specimen F9 and F10, identify the fracture and then describe the sample. (F10 is a volcanic glass and is therefore not a mineral.)

Table 6 Cleavage and fracture exercise.
# / Mineral / cleavage/fracture / Description
Cl / gypsum / . / .
C2 / calcite / . / .
C3 / muscovite / . / .
C4 / galena / . / .
C5 / fluorite / . / .
C6 / orthoclase (feldspar) / . / .
C7 / halite / . / ..
C8 / hornblende (amphibole) / .. / ..
F9 / chalcopyrite / .. / .
F10 / obsidian / . / .

Crystal Form

When minerals grow in an unrestricted environment (i.e. rock cavity) they obtain geometric shapes which reflect their inner atomic arrangement. Such perfectly shaped forms are euhedral crystals (fig. 4) and each mineral displays a unique crystal form which can aid in its identification. This euhedral crystal is what most people think of as a crystal. However, geologist use the term crystal in a more general sense; a crystal is any single mineral grain that makes up a crystalline rock. Typically, growing crystals interfere with one another and become distorted and malformed. When crystals are anhedral or subhedral the mineral's crystal form is generally not identifiable.

Minerals that commonly occur as euhedral (well shaped) crystals are:
1. quartz (trigonal)
2. fluorite and pyrite (cubic)
3. garnet (dodecahedral)
4. gypsum (rhombic)
5. fluorite (octahedral)
Note: the terms cubic, rhombic and octahedral are also used to describe cleavage. To avoid confusion state what you are referring to when using these terms. /

Figure 1.4. Examples of crystal forms.

EXERCISE 3: HARDNESS

Hardness is a mineral’s resistance to abrasion and is a measure of the bond strength between atoms. The stronger the interatomic bonds the harder the mineral.
Determination of hardness: Hardness is determined by comparing the hardness of the mineral in question with that of a mineral from Moh’s Hardness Scale (Table 1.7), or with another object of known hardness (relative to Moh’s Scale).

For this exercise use the plastic boxes labeled “hardness exercise” that contain minerals 1-9 on Moh’s Hardness Scale. The number on each mineral specimen does not correspond to its hardness. However, by determining the relative hardness of the nine minerals you can identify them.

Procedure:

1. Use a glass plate (H=5.5) to separate the minerals into two groups (<5.5 and >5.5).

2. Determine the relative hardness of the minerals by comparing them with each other. Your fingernail (H-2.5) will help with those minerals < 5.5.

3. In Table 1.6, place the number of each specimen where it belongs relative to the hardness scale. Once you have done that you have identify the minerals.

Table 1.7. Moh's hardness scale and hardness exercise
Moh's Scale of Hardness
Hardness (H) / Mineral / Sample # / Object
1 / Talc / ..
2 / gypsum / .. / Fingernail: H = 2.5
3 / calcite / .. / Copper penny: H = 3
4 / fluorite / ..
5 / apatite / ..
6 / feldspar (orthoclase) / .. / Glass plate: H = 5.5
7 / quartz / ..
8 / topaz / ..
9 / corundum / ..
10 / diamond / .


EXERCISE 4; Color and Streak

Color is a physical property that is a function of the wavelengths or frequencies of light reflected and absorbed form a material. Minerals that are colorless reflect all wavelengths. Some minerals, such as the metallic minerals exhibit only one characteristic color. However, most minerals occur in more than one color. A mineral that is normally colorless may become slightly colored if it contains minute impurities. For example, quartz may be white, rose, gray, brown, black, yellow, or amethyst (purple) if trace amounts of iron, chromium, vanadium or some other chromophore are present. (A chromophore is an element that strongly absorbs light; it need only be present in trace amounts to color a mineral.) Be familiar with all the colors a particular mineral may exhibit.

The streak is the color of a mineral’s powder. Unlike color there is generally only one streak for each mineral. For example, fluorite may be green, colorless, yellow, or blue, but its streak is always white. Hematite may be metallic gray or dull reddish brown, but its streak is always a brick red.

Determination of streak: The streak is determined observing the powder produced when firmly grinding a corner of the mineral on an unglazed piece of porcelain (streak plate). Metallic minerals yield dark streaks. Most vitreous minerals that streak produce white streaks.
Caution: The hardness of the streak plate is around 6. Therefore any mineral harder than 7 will not streak, and any powder produced will be from the glass plate, not the mineral.

For this exercise there are 6 minerals. Record the color and streak of each in Table 1.8.

Table 8. Color and streak exercise.
# / Mineral / Mineral Color / Streak Color
CS1 / chalcopyrite / . / .
CS2 / fluorite / . / .
CS3 / talc / . / .
CS4 / galena / . / .
CS5 / hematite / . / .
CS6 / malachite / . / .

EXERCISE 5: Tenacity

Tenacity (table 9) is a measure of the toughness of a mineral, or the manner in which it will deform (crush, bend, tear, change shape, etc.) under stress. DO not to confuse tenacity with hardness.

Table 9. Examples of different types of tenacity.
Tenacity / Definition
Brittle / Crushes easily into fragments. Examples: most minerals, especially those that cleave easily.
Malleable / Can be pounded into flat sheets. Example: gold
Elastic / Will bend, but regains original shape when stress is released. Examples: biotite and muscovite
Flexible / Will bend and stay bent once the stress is released. Talc, graphite and chlorite

Find the boxes labeled T1 – T4 and identify the tenacity for each sample. For T4 you need to analyze the individual flakes. Try to bend one by pressing your fingernail into it. After you’ve identified the tenacity, identify the mineral.

Table 10. Tenacity exercise
Sample / Tenacity / Mineral
T1
T2
T3
T4

EXERCISE 6: Specific Gravity

The ratio of a mass of a body to the mass of an equal volume of water is known as the specific gravity. A mineral having a specific gravity of 2.5 weighs two and a half times more than water or 2.5 grams/cm3.

(Specific Gravity): Three tin canisters contain the minerals in the table 11. Write the number of the canister in which each mineral is located.

Table 11. Specific gravity exercise.
Mineral / SG / #
Quartz / 2.6 / ..
Pyrite / 6 / ..
Galena / 7.6 / ..

EXERCISE 7: Economic uses for common minerals