The Earth Lab 4
From GeoClasses
Contents |
An introduction to the study of minerals
Introduction
Almost everyone can appreciate the value and beauty of diamonds, rubies, emeralds, and other gemstones. These gems, and other minerals, are proudly worn by people, displayed in stores, and exhibited in museums (such as the Smithsonian in Washington, D.C.) all over the world. Still, many people don’t realize the important roles that minerals play in their lives each and every day. Bricks, concrete, nails, screws, insulation, plaster, wires, and pipes are all made from minerals. A modern society, such as ours, requires a constant supply of a wide variety of minerals in order to function, particularly in order to maintain the standard of living that we currently enjoy. Because minerals are so important in our lives, it is essential that we know something about them.
Lab objective
In this exercise, you will learn how to make field identifications of minerals. You will also learn about some mineral properties that will enable you to quickly identify some of the more important ones. In the field, using these same simple tools and methods, a geologist will make initial identifications of different rock and mineral samples, which are called specimens. After a geologist makes an educated guess or a field identification, specimens are brought back to the laboratory for further study with sophisticated instruments.
Getting started
Read the following definitions and explanations before beginning your field identification of the specimens provided by your lab instructor. First, you will learn the definition of a mineral. Then, you will learn how to classify minerals. Finally, you will learn the physical properties of minerals, which will assist you with your field identification lab exercise.
What is a Mineral?
A mineral is a naturally occurring, inorganic crystalline substance with a chemical composition that may vary within fixed limits. Thus, any solid may be called a mineral if all of the following apply:
- it occurs in nature
- it is not an organic compound
- it has a definite atomic structure that, under appropriate conditions, exhibits a definite external crystalline form
- it has a constant (or relatively constant) chemical composition
Minerals are made up of specific atomic structures, and each mineral has its own unique pattern. If atoms, of similar size and kind, replace some existing atoms within a particular mineral, the unique pattern of that mineral remains intact: the mineral’s pattern is not destroyed.
For example, common olivine (an olive-green mineral that crystallizes at high temperatures) has a formula (Mg, Fe)2 SiO4 : Mg (magnesium), Fe (iron), Si (silicon), and O (oxygen). For all olivines, the arrangement of atoms is the same; but in some olivines, some magnesium (Mg) atoms are substituted with iron (Fe) atoms. However, the ratio of magnesium and iron (Mg+Fe) to silicon and oxygen (Si+0) always remains the same.
The previous example illustrates how minerals have a fixed atomic structure with a restricted range of chemical composition. This atomic arrangement, specific to each mineral, controls a mineral’s external crystalline shape and physical properties such as cleavage, density, etc. Minerals, unless their growth is impeded, will develop perfect crystal forms with smooth, perfect faces. If crystalline growth is impeded during its formation, some crystalline structures may not exhibit smooth, perfect faces. In contrast to a mineral that manifests its orderly internal atomic arrangement by an externally perfect shape, some solid substances that may be perceived as having crystal form—such as glass, or an exceptional “mineral” like opal—lack an orderly internal atomic arrangement. These types of substances are said to be amorphous (without form); naturally occurring amorphous substances are called mineraloids.
Classification of Minerals
A group of minerals occurring as an aggregate is typically called a rock. Rocks are therefore aggregates (a collection of units or parts that form a group or mass unit) of minerals. Through scientific research, we know that a specific kind of rock is formed only if certain geologic conditions—characterized by a narrow range of temperature, pressure, and various chemicals—are present. Understanding these limited conditions of formation enables us to correlate the occurrence of a particular mineral in a rock (how and why it formed) with the environment in which it formed (when and where it formed).
Two important groups of minerals are rock-forming minerals and ore minerals. Rock-forming minerals are those that usually determine a rock’s classification, depending upon the presence or absence of that mineral within the rock. Rock-forming minerals are composed of the most common elements found in the earth’s crust. These minerals, combined singly or in suites, compose the vast majority of all rocks. Conversely, ore minerals are those mined for the extraction of metallic elements. These minerals, when present in a rock, do not determine its classification.
There are approximately 3,500 types of recognized minerals. Minerals are arranged in large sets, called Groups, which are defined primarily according to their chemical composition. The largest of these Groups is the Silicate Group. This Group contains almost all of the rock-forming minerals found in common igneous and metamorphic rocks. Table 1-1 lists several mineral Groups and some mineral samples found within a specific Group.
Within these groups, minerals are subdivided by their similarity of atomic structure. Sometimes, minerals of the same group exhibit such similar physical and chemical properties that one mineral is indistinguishable from another by field methods.
| Group | Mineral Examples |
|---|---|
| Carbonate Group | calcite and dolomite |
| Halide Group | halite and fluorite |
| Native Element Group | gold and copper |
| Oxide Group | hematite and magnetite |
| Silicate Group | quartz, feldspar, mica and pyroxene |
| Sulfate Group | quartz, feldspar, mica and pyroxene |
| Sulfide Group | pyrite and sphalerite |
Physical Properties of Minerals
Each mineral has its own unique atomic structure and chemical composition (that may vary within certain limits). As a result, each mineral has certain testable properties (diagnostic properties). Certain common physical properties are used to assist in the identification of most minerals, while other physical properties are less common and are therefore used more carefully for identifying some specific minerals. Furthermore, the physical properties of some select minerals are so unique that their use is limited to a single mineral and are not applied as general tests. Table 1-2 lists some physical properties used in the field identification of minerals.
| Most Minerals | Specific minerals | Select minerals |
|---|---|---|
| crystal form | color | magnetism |
| clevage | tenacity | effervescence with dilute HCl |
| hardness | fracture | feel |
| density | smell | |
| luster | ||
| streak |
Lab I—Exercise
Overview
The goal of this assignment is to correctly identify each of the numbered mineral specimens you are given. Below, the physical properties characteristic of certain types of minerals are explained. Using your numbered lab specimens, examine each mineral and make note of each specimen’s properties. A worksheet is provided at the end of this chapter. When you finish your observations and notes, refer to the charts at the end of the exercise and identify each specimen by comparing your notes with the properties on the provided tables. Turn in the worksheet.
Crystal form
If a mineral is allowed to grow in an unrestricted space, it will develop with natural crystal faces that have a perfect geometric form. This perfect geometric form is a reflection of the mineral’s internal atomic arrangement and can be used to help identify many mineral types. Because some minerals may have similar internal structure, they may develop similar-looking crystals. A couple of examples of minerals in which crystal form is especially diagnostic are quartz (hexagonal section) and halite (cubic form).
Step 1: Examine each mineral specimen and note which ones exhibit crystal form. Make a description of the crystal form.
Cleavage and fracture
Cleavage is a property some minerals have of breaking along repeated, parallel planes. Because cleavage is identical for all specimens of a mineral, cleavage is an important diagnostic tool. Cleavage is characterized as perfect, good, or poor in quality—depending upon how pervasively developed it is in a mineral. Cleavage can be observed parallel to the crystal faces of a mineral; however this is not always so (e.g. fluorite). A mineral species may also possess cleavage in one or more directions. Table 1-3 contains some examples of these species and their relative directions of cleavage.
| Cleavage direction(s) | Sample species with a specific type of cleavage |
|---|---|
| Cleavage in one direction | Micas |
| Cleavage in two direction | Feldspars, pyroxenes (at 90° angles), amphiboles (at 60° and 120° angles) |
| Cleavage in three direction | Galena, halite (at 90° angles), calcite (not at 90° angles) |
| Cleavage in four direction | Fluorite (at angles not parallel to crystal faces) |
| Cleavage in six direction | Sphalerite |
Be careful not to confuse cleavage with crystal faces! Not all minerals have cleavage. When a mineral specimen is broken, one that exhibits cleavage will cleave along repeated, parallel directions. A crystal without cleavage, though perhaps perfect in form, will break irregularly. Irregular breaking in a mineral is called fracture. Fracture may be splintery, conchoidal (on curved surfaces, like glass), hackly (notched or jagged), or earthy (like a brick). Fracture is an important diagnostic property.
Step 2: Examine each numbered mineral specimen and note if it exhibits cleavage and its relative direction of cleavage if more than one is present. Make note of any specimens that exhibit fracture, then describe the type of fracture.
Hardness
Hardness is a mineral’s resistance to abrasion. Qualitatively, this important property is used in observing whether a mineral is scratched or will scratch other minerals or materials. For this purpose, a scale of relative hardness was devised in 1824 by Fredrich Mohs, an Austrian geologist. Mohs’ Hardness Scale has an arbitrary relative value of 10 assigned to diamond, the hardest of minerals, and a value of 1 assigned to talc, the softest mineral on the scale. Between these two values, eight relatively common minerals have an assigned number in an increasing order of relative hardness.
To test the hardness of a mineral, scratch it with a firm stroke. If an unknown mineral can be scratched with apatite but not with fluorite, then its hardness is between 4 and 5. If a standard set of minerals used in Mohs’ Hardness Scale is not handy, use the common materials listed in Table 1-4 for a relatively accurate determination of hardness.
| Mineral | Scale | Approximately as hard as a… |
|---|---|---|
| Talc | 1 | |
| Gypsum | 2 | fingernail (2.5) |
| Calcite | 3 | copper penny (3.0) |
| Fluorite | 4 | |
| Apatite | 5 | glass plate (5.0-5.5) or pocket knife blade (5.5-6.0) |
| Feldspar | 6 | steel file (6.5-7.0) |
| Quartz | 7 | |
| Topaz | 8 | |
| Corundum | 9 | |
| Diamond | 10 |
Step 3: Using a firm stroke, try to scratch the glass plate with each mineral specimen. Note which specimens scratch the glass and which do not. Then, try to scratch each of the mineral specimens with the other common materials provided. Using your observations and Table 1-4, approximate each specimen’s hardness using Mohs’ Hardness Scale.
Luster
The luster of a mineral refers to the manner in which light is reflected from the mineral’s surface. The two broad categories of luster commonly recognized are metallic and non-metallic. Minerals with metallic luster look like metals, meaning that they reflect light like gold, silver, iron or copper. Comparatively, minerals with non-metallic luster do not have a metallic appearance and reflect light like glass, porcelain, clay, etc. Table 1-5 provides some examples of minerals with metallic luster, minerals with non-metallic luster and descriptions of non-metallic luster.
| Minerals with metallic luster | Minerals with non-metallic luster | Luster quality |
|---|---|---|
| Gold | quartz | vitreous (luster of glass) |
| Iron | olivine | |
| Silver | garnet | |
| Copper | diamond | adamantine |
| Galena | sphalerite | resinous (luster of resin) |
| Pyrite | muscovite | pearly (luster of pearls) |
| talc | ||
| limonite | earthy (similar to brick or clay) | |
| red hematite |
Step 4: Examine and note the types of luster exhibited by each numbered mineral specimen
Color
Color is an important diagnostic property for metallic minerals—gold is golden in color, silver is silver in color and copper is, of course, copper in color. However, extreme care must be taken when using color to identify non-metallic minerals. Many species of non-metallic minerals show wide variations in color because of impurities or variations in composition. For example, quartz in its pure state is colorless, but it may also be purple (amethyst), gray (smoky quartz), pink (rose quartz), yellow (citrine), and many other colors. Another example is fluorite, which might be purple, yellow, green, blue, or colorless. However, some non-metallic minerals do exhibit diagnostic colors; these minerals are olivine (olive green), malachite (green), and azurite (blue).
Step 5: Observe and note the color for each numbered mineral specimen.
Streak
Streak is the color of a finely powdered mineral, and streak is obtained and observed by rubbing the mineral specimen with a short, heavy stroke on a piece of unglazed tile called a streak plate. The color of the mark made by the powder is the streak. The color of the streak is always diagnostic; however, the streak color may or may not be the same as the color of the mineral in the original specimen. Many silicate minerals, regardless of their color, will have a white or colorless streak. Many carbonates (malachite or azurite), oxides (limonite, magnetite, and hematite), and sulfides (pyrite and galena) have a constant, characteristic streak. For example, pyrite appears in various shades of metallic yellow, but its streak is always greenish black. Crystalline hematite and magnetite are iron oxides that are black to gun-metal gray in color, but hematite has a dark brick-red streak, and magnetite has a black streak.
Step 6: Rub each mineral specimen with a short, heavy stroke on the streak plate provided. Note the streak color for each numbered specimen.
Step 7: Now that you have examined and noted many of the physical properties of minerals, refer to the following three tables (Table 1-6, Table 1-7, and Table 1-8). Using the notes you took for each specimen and these three tables (called determinative tables), you can identify your mineral specimens. Your lab instructor can help you if you need assistance comprehending them.
Note: A few items (specific gravity and special features) in Table 1-6, Table 1-7, and Table 1-8 are discussed at the end of the chapter and will be important to know in future lab exercises—and on tests.
Specific gravity
The specific gravity of a substance is the ratio of the mass of the substance to the mass of an equal volume of water at a specific temperature. It is helpful to remember that metallic minerals will generally have a specific gravity greater than 5 (with the notable exception being graphite, which has a specific gravity of 2.1). Conversely, non-metallic minerals will normally have a specific gravity of less than 3.5 (with the exception of barite, which has a specific gravity of 4.6). Field use of specific gravity, like hardness, involves comparing the “heft” or feeling of weight of approximately equal size pieces of different minerals.
Special properties
The properties, descriptions, and examples in Table 1-9 are unique to certain minerals and should not be used as common tests for all minerals.
| Properties | Description and examples |
|---|---|
| Magnetism | Magnetite is magnetic, and a variety of magnetite, called lodestone, also acts as a magnet. |
| Taste | Halite (salt) has its own unique taste. |
| Effervescence | Effervescence is common in carbonates. Calcite effervesces (bubbles) vigorously when a drop of dilute hydrochloric acid (HCl) is applied to its surface. Dolomite must be powdered or scratched to show very slight effervescence. |
| Smell | Sphalerite, if rubbed briskly with a knife, smells like sulfur. |
