Phisical properties of Minerals

LAB

Physical Properties of Minerals OBJECTIVES

To determine these properties for unknown minerals

DEFINITION OF A MINERAL

M

any people think of a mineral as something contained in a multivitamin capsule. Mineral in this sense is really an abbreviation of “mineral salt,” something that is derived from a mineral, in the geoscientific sense of the word. Geoscientists have a very precise definition of what a mineral is. A mineral is a naturally occurring, usually inorganic, chemically homogeneous crystalline solid with a strictly defined chemical composition and characteristic physical properties.

Let’s consider the idea that a mineral is crystalline.

In a crystalline solid, atoms are arranged in an orderly way with a distinct structure (■ Figure 1.1a). In contrast, an amorphous solid such as glass has atoms arranged haphazardly, in no particular order (■ Figure 1.1b). A crystal is a single grain of a mineral in which the structural planes of atoms extend in the same directions throughout the grain.

The orderly arrangement of atoms controls many of the properties of the mineral, such as the external shape of the (a)

mineral if it was free to grow, and the way the mineral breaks. The crystalline structure can even influence the hardness and density of a mineral. For example, diamonds and graphite are both pure carbon, but their crystalline structures are very different. These differences give rise to diamonds, the hardest natural substance (density 3.51 g/cm3), and graphite, a substance (used as pencil lead, density 2.23 g/cm3) that is so soft we can write with it.

Crystallization of Minerals Minerals may crystallize in any one of four geological environments. We discuss these specific environments in later chapters, as indicated.

1. From molten rock. This is the typical mode of formation of minerals in igneous rocks (Lab 3).

2. From solution. This typically happens in various stages of the formation of sedimentary rocks (Lab 4).

3. Within living cells. Many creatures precipitate crystalline substances within their cells to build a skeleFigure 1.1 Models using acrylic spheres as atoms demonstrating (a) orderly crystalline arrangement of atoms and (b) disorderly amorphous arrangement of atoms

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To become familiar with the physical properties minerals possess

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To learn the definition of a mineral

(b)

Physical Properties of Minerals

1

ton or shell. Shellfish, for example, precipitate calcite or aragonite in their shells. We humans precipitate a calcium phosphate, called apatite, in our bones and teeth. Although these materials are generally not considered minerals while part of the living organism, they are an important source of minerals in certain kinds of sedimentary rocks once the organism dies (Lab 4).

4. By recrystallization. As rocks undergo changing conditions of pressure and temperature in the Earth, minerals may break down and re-form into the same or new minerals while in the solid state. This process is typical in the formation of metamorphic rocks (Lab 5). The remains of organisms may recrystallize (Lab 4), or minerals may precipitate from solution during the formation of sedimentary rocks.

1. Examine the rock samples your instructor provides that show minerals formed in these four ways. a. Pick out the rock with minerals formed from living organisms and record its sample number: _______________ b. Without training, can you tell which minerals formed by the other methods: from molten rock, from solution, or by recrystallization? Yes / No If so, how can you tell? If not, why not?

Acicular (needle-like)

Equant or stubby (nearly equal width, depth, height)

Bladed (like a knife blade)

Tabular (like a tablet of paper)

Blocky (block-shaped)

Columnar or prismatic (column-shaped)

Figure 1.2 Crystal habits of individual grains

2. Look at an aggregate of several pyrite crystals: a. Describe the shape of the individual crystals, using the illustrations and shapes shown in Figure 1.2.

b. Describe the aggregate type by referring to Figure 1.3.

PROPERTIES Crystal Habit

C

rystal habit is the external shape a mineral exhibits given favorable conditions when a crystal was free to grow and becomes well formed and remains unbroken. Habit is determined by the crystal’s internal structure; the external shape and symmetry of a crystal reflect its internal atomic order. We can analyze the symmetry of these shapes and use this analysis to help identify a mineral. ■Figure 1.2 illustrates some common terms used to describe crystal habits. If you wish to describe the habit of a crystal, you first look at the individual grains and determine their external shape. If the crystal is grouped with others, the sample is an aggregate. In this case, the next step would be to use ■Figure 1.3, which shows some typical aggregate types.

2

L ab 1

Bladed

Granular massive

Reticulate

Botryoidal

Stalagtitic

Fibrous massive

Micaceous foliate

Figure 1.3 Mineral aggregate habits, samples with multiple grains

Dendritic

metallic (galena) and submetallic (magnetite and graphite, Figures 1.10f and j, on pp. 15 and 16). Nonmetallic lusters are many and varied. The most common nonmetallic lusters are adamantine (garnet), vitreous (quartz), waxy, resinous (biotite), greasy, pearly (talc), silky (satin spar gypsum), and dull/earthy (kaolin). You will understand these terms better after you do Exercise 4, in which you associate the luster terms with simple English descriptions of the terms by looking at actual examples of each luster.

Table 1.1

Mineral Habits (Exercise 3) Sample Number

Habit

3. Your instructor will provide some samples showing a variety of crystal habits, some individual crystals and some aggregates. In ■ Table 1.1, list each sample number and one or more terms from Figures 1.2 and 1.3. Keep in mind that single crystals will only have habits from Figure 1.2, but for aggregates you should consider both figures.

Luster Luster describes how light reflects from a fresh surface.

You can observe luster on samples provided and in the photographs in Figures 1.9 and 1.10, on pp. 12–14 and 15–16. There are two broad classifications of luster: metallic and nonmetallic. Metallic lusters include

4. Examine the set of samples provided to demonstrate luster. Your instructor will supply information about each sample’s luster.

Identify which lusters and samples correspond to the descriptions in ■ Table 1.2.

Remember that you are looking for surface shine or appearance, not color, transparency, or opaqueness.

Color On a fresh, unaltered surface, the color of a mineral may help in identification, but beware: Color is a very unreliable property to use in identifying minerals. Impurities within a mineral may give rise to a variety of colors. Quartz, for instance, has many varieties: amethyst (purple), rock crystal (colorless), rose quartz (pink), smoky quartz (gray), citrine (yellow), and milky quartz (white) (Figure 1.9 l–p, p. 14). In addition, more than one mineral may have the same color. For example, amethyst and some varieties of fluorite are both

Table 1.2

Examples of Luster (Exercise 4) Luster Description

Luster

Sample

Like a bright shiny metal Shines like glass Like a tarnished metal Like wax Looks like it is coated with grease Not shiny, like chalk or dirt Very bright and gemlike shine Like pearls Shines like plastic Shimmers like silk fibers

Physical Properties of Minerals

3

Table 1.3

Table 1.4

Minerals with the Same Color (Exercise 6)

Mohs Scale of Hardness

Sample Number

Color

Mineral Name

Mohs Number

Mineral

1 2

Talc Gypsum

3 4 5

Calcite Fluorite Apatite

Common Object

Fingernail ( 21⁄2) Copper penny ( 3)

Pocket knife ( 51⁄2) Glass ( 51⁄2) 6 7 8 9 10

purple (Figures 1.9p and 1.10h, pp. 14 and 16), and quartz, K feldspar, calcite, and gypsum all have pinkish varieties.

5. Examine the samples provided that show the same mineral with more than one color.

What mineral is it? _______________ List the colors of the samples.

6. Some additional samples are grouped having more than one mineral with the same color. Enter information about these minerals and their colors in ■ Table 1.3.

Hardness The resistance of a mineral to abrasion (scratching) is termed hardness. The Mohs scale of hardness (H), established by mineralogist F. Mohs, has ten minerals arranged in an order of increasing relative hardness, each of which will scratch the mineral of lower hardness on the scale (■ Table 1.4). To determine the relative hardness of a specimen, you scratch an unweathered surface of the unknown mineral with an object of known hardness. For example, if glass of hardness 51⁄2 scratches the mineral, then try to scratch the mineral with a penny (H 3); continue until you no longer mark the surface or until you have no softer object with which to test it.

You can also check to see whether the unknown sample can scratch the object of known hardness. When you think you see a scratch, check to make sure you have not simply left powder of the scratching object behind; wipe

4

L ab 1

Orthoclase (a feldspar) Quartz Topaz Corundum Diamond

Streak plate ( 61⁄2)

Note: Hardness may vary in some minerals (1⁄2 to 2 points) from crystal face to crystal face as seen in kyanite, which has a hardness of 5 parallel to its length and 7 across the length.

away any powder, and look at the surface closely. Also, the physical nature of a mineral specimen may prevent correct determination of hardness if, for example, a mineral is splintery or granular and falls apart when tested.

Imagine that you are recording the properties of a sample you think is fluorite. You might have noticed that fluorite has a hardness of 4 on the Mohs scale. However, this does not mean you should write down 4 for its hardness. Instead, you should use the evidence obtained by the scratching tests to provide a range of possible hardness. You should not jump to conclusions when testing minerals but carefully record your observations.

What if you thought the mineral was a purple variety of quartz instead of fluorite and decided — knowing the hardness of quartz is 7 — that you would just write down 7? This conclusion would mislead you in the mineral identification. In fact, it would be an example of formulating a hypothesis and then changing the data to fit the hypothesis — a definite scientific no-no.

7. Test the hardness of the samples provided using common objects and enter the information in ■ Table 1.5.

Streak Streak is the color of a mineral when powdered. The color of the powder is less variable than the color of a mineral, so streak is a more reliable property than color.

Table 1.5

Hardness (Exercise 7) Sample Number

Testing object of nearest hardness that is harder than the mineral

Testing object of nearest hardness that is softer than the mineral

Mineral hardness range

Example: quartz

None

Streak Plate*

6.5

*Note: Scratches are not easily visible on a streak plate, but it will not scratch the quartz.

A streak plate made of porcelain is used to obtain a small amount of powder from a specimen. Look at the example of the streak for hematite shown in Figure 1.10g, on page 16. Notice that although one sample of hematite is silvery gray, both samples have a brown streak. Since the hardness of porcelain is about 6 to 7 on the Mohs scale, the streak of a mineral with hardness greater than 6 cannot be easily determined and can be said to have no streak. Softer minerals should each have streak, but keep in mind that it may be hard to see a white or colorless streak on a white streak plate (some people use black streak plates to solve this problem). Check to see whether you have powdered the mineral or the streak plate.

8. Test the streak of three minerals. Write your results in ■ Table 1.6.

Cleavage If a mineral breaks along parallel planes, the mineral has cleavage. The way that a mineral cleaves or splits depends on the crystal structure. A cleavage plane is a plane of weakness in the structure of the mineral (■ Figure 1.4). A mineral may have many or no cleavage planes, and these planes may be perfectly planar, as in muscovite, or slightly irregular, as in pyroxene or hornblende. A mineral with no planes of weakness (such as quartz) will fracture. This does not necessarily mean that the mineral is exceptionally hard or tough, only that inherent weaknesses in the crystal structure are not planar.

A complete description of cleavage includes the relationship of the cleavage planes. For instance, galena and halite both have cubic cleavage (Figure 1.10c and i, on pp. 15 and 16), which is equivalent to three cleavage planes at 90º to each other (■ Figure 1.5). In some cases, the angle between the cleavages helps to distinguish two minerals. Augite pyroxene and hornblende amphibole both have two cleavages, but the angle between them is

Broken Surfaces of Minerals The broken surfaces of a mineral often have characteristics that can help in mineral identification.

Table 1.6

+

Testing Streak (Exercise 8) Sample Number

Streak (color of the powder)

Mineral Name (if provided)

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

Because of weak bonds, mica splits easily between “sandwiches”

+

+

+

+

+

+

+

+

+

Positive ions, sandwiched between two sheet silicate layers

Figure 1.4 Cleavage in minerals occurs where weak bonds in planes of atoms allow the mineral to split apart easily, such as the mica shown here.

Source: From Earth Science Today, by B. Murphy and D. Nance, p. 25. Copyright © 1999, Brooks/Cole. All rights reserved.

Physical Properties of Minerals

5

One cleavage

of obsidian (a volcanic glass; Figure 3.14). Quartz displays conchoidal fracture on its broken surfaces (Figure 1.9 l-p, p. 14).

Fibrous: Fracture surface has a threadlike appearance, similar to the coarser, splintery appearance of wood. A good example is asbestos, another is satin spar gypsum.

Two cleavages at 90°

Two cleavages not at 90°

Three cleavages at 90°

Three cleavages not at 90°

Hackly: A sharp, irregular surface, the same as jagged. Both garnet and wollastonite may produce a hackly fracture.

Uneven/Irregular: A general term that can be applied to the fracture of many different minerals that otherwise defies definition.

Parting: Generally not a common characteristic, parting is a roughly planar break in a mineral that is not as well developed as cleavage. It is frequently produced by deformation of the mineral. A clear example of parting can most often be seen in hand specimens of corundum.

9. Examine the samples with unknown fracture or cleavage. In ■ Table 1.7, list the minerals that show cleavage in the appropriate place in the table; then list those with fracture. Describe the cleavage and fracture in more detail. Refer to earlier information and Figure 1.5, if necessary.

Four cleavages Table 1.7

Cleavage and Fracture (Exercise 9) Samples with Cleavage

Describe the Cleavage

Samples with Fracture

Describe the Fracture

Six cleavages Figure 1.5 Types of cleavage

near 90º for augite and much more oblique (at an angle other than 90º; in this case 56º and 124º) for hornblende (■ Figure 1.6).

Fracture If the broken surfaces of a mineral are irregular and nonplanar, the mineral is said to have fracture. When fracture is determined, it should be described with one of the following terms: Conchoidal: A smooth, curved surface that looks like the inside of a clam shell. See the photograph

6

L ab 1

Distinguishing Cleavage from Crystal Faces A crystal face is a planar surface of a well-formed crystal that grew when the crystal grew (Figure 1.6). Many students look at a well-formed crystal of quartz with flat crystal faces and say the sample has cleavage. This is wrong. By now you realize that quartz has conchoidal fracture, not cleavage. The flat surfaces on quartz did not break along planes of weakness but grew that way when the mineral formed.

How can you tell the difference between cleavage and crystal faces? Since cleavage is an inherent planar weakness in the mineral, you will almost always see multiple examples of a particular cleavage plane exhibited. These may show up as a step-like surface feature (augite, Figure 1.9d, p. 12; potassium feldspar, Figure 1.9i, p. 13; and galena, Figure 1.10c, p. 15). Look closely at samples for these steps — you may want to use a hand lens.

10. Examine a quartz crystal. Notice the flat planes on this sample. Since all quartz has fracture, not cleavage, how would you account for the flat planes in this natural sample of quartz?

11. Examine the two samples provided. For each sample, sketch the planar features and show any cleavage steps in your sketch.

93° 87°

Cross section

(a) Cleavage

Crystal faces

56° 124° (b) Figure 1.6 Distinctive cleavage angles. The two cleavage planes in (a) augite pyroxene form a nearly 90º angle, but in (b) hornblende amphibole, the planes meet at more oblique angles (56º and 124º). This figure also illustrates the difference between crystal faces and cleavage. Source: From Physical Geology: Exploring the Earth, 3d ed., by J. S. Monroe and R. Wicander, p. 45. Copyright © 1998, Brooks/Cole. All rights reserved.

the weight of an equal volume of water. In hand specimens, we are usually concerned more with relative density, which is crudely comparing the density of one mineral specimen to another. You can make general comparisons by using the “heft test,” in which you compare two specimens of about the same size by picking them up, to say which is heavier. This is apparent when you compare, for example, quartz, which has a fairly normal or average density, with a metal sulfide such as pyrite, which has a high density, or with graphite, which has a low density.

a. Which sample has fracture and crystal faces? _______________ b. Which sample has cleavage? _______________

Density (Specific Gravity) Density is defined as the mass per unit volume of a substance. Another way to express this property is specific gravity: the ratio of the weight of a mineral specimen to

12. You can approximately estimate the density of a sample. Examine and heft the samples provided. While looking at them, judge their relative density. Looking at the samples allows your eyes to judge the size while your hands judge the weight; together, the two give you an estimate of the density. Now rank the samples in order of their density from most dense to least dense and enter the sample numbers in ■ Table 1.8.

Physical Properties of Minerals

7

Table 1.8

lines (or use a graduated cylinder to measure the change in level). Record the new level as second level. Next, find the volume of the sample by subtracting the second level from the first, and record the volume. Note that milliliters are equivalent to cubic centimeters. Determine the density by dividing the mass from Exercise a by the volume just measured.

Density (Exercises 12, 13, 14) Relative Density

Sample Number

Measured Density (g/cm3)

Density Given

by Instructor (g/cm3)

Highest

first level

minus second _______________ ml level

Lowest

13. If a balance is available, measure the density of samples provided. To obtain density, you need to divide the mass by the volume. a. Measure the mass in grams on the balance. _______________ g b. Now measure the volume by displacing a volume of water. Carefully place the sample being measured in a beaker, graduated cylinder, or metric measuring cup. Fill the container with water to a line above the sample, being as accurate as possible. Read the water level at the bottom of the meniscus (â– Figure 1.7). Record this level as the first level in the space provided. Now remove the sample without spilling any of the water, and allow the sample to drip into the beaker for a few seconds.

Read the new level as accurately as possible by interpolating between the

Meniscus Read level here Figure 1.7 Beaker of water showing the meniscus. You should read the water level at the bottom of the meniscus.

8

_______________ ml

L ab 1

volume

_______________ ml,

density

_______________ g/cm3 (1 ml 1 cm3)

Follow this procedure for each sample and enter the density in Table 1.8.

14. Your instructor will provide the densities of these minerals so you can check your answers. Place these values in Table 1.8. Describe your ability to estimate relative densities by heft. Did it make a difference if the samples were nearly the same size or quite different in size?

Special Properties Some minerals have special properties, such as effervescence in acid, magnetism, unusual visual properties, or a distinctive smell, taste, or feel.

Effervescence Seen in carbonates (primarily calcite; Figure 1.10a, p. 15), effervescence is a fizzing or bubbling that takes place when a dilute solution of hydrochloric acid (HCl) is applied (please use sparingly, and clean off the sample with a tissue or paper towel afterward). This property is especially useful in distinguishing calcite from other common rock-forming minerals. Dolomite will effervesce when it is powdered; aragonite will do so easily without powdering, but is not so often seen in hand specimens. The chemical reaction that occurs when HCl is applied to calcite or aragonite (CaCO3) is: 2H (from HCl)

CO3 (from CaCO3)

⇒ H2O (water)

CO2 (escaping gas in fizzing)

Magnetism The mineral is attracted to a magnet.

Magnetite (Fe3O4) is strongly magnetic (Figure 1.10f, p. 15). Magnetism is especially useful in distinguishing magnetite from other common rock-forming minerals.

Hematite (Fe2O3) and magnetite often occur together in specular hematite samples, making what looks like a pure hematite sample magnetic.

Table 1.9

Special Properties (Exercise 15) Sample Number

Special Property

Mineral Name

Feel Some minerals have a diagnostic feel (such as the greasy feel — not greasy luster — of graphite).

Fluorescence Minerals may fluoresce when they are put under ultraviolet (UV) light. The short-wave radiation of the UV is absorbed by the mineral and radiated back as longer-wave visible radiation. One mineral that often shows fluorescence is calcite; and another, of course, is fluorite.

Double Refraction Visible in some calcite crystals, double refraction occurs when light entering a crystal is broken into two rays (■ Figure 1.8). You see a double image when looking through the crystal. If you rotate the crystal, one image will stay fixed and the other will move around it.

samples. Taste is a valid mineral property and can be quite helpful in identifying some minerals.

15. Use samples provided to test special properties. Fill in ■ Table 1.9 with results of your special properties tests.

Smell Some minerals, such as sulfur, possess a distinctive odor. (Students in a lab setting may have been putting acid on samples for years; this tends to give all of the samples a smell.)

Tenacity

Taste A few minerals have a characteristic taste.

Halite tastes salty. Sylvite tastes bitter. Please taste only the samples provided for tasting: Students have been putting acid on other samples. Place tasted samples where instructed; do not put them back with the fresh

Tenacity is the cohesiveness of a specimen, a description of a mineral’s resistance to mechanical deformation (breaking, bending, crushing, and so on). These tests are destructive, so please do not perform them on the lab mineral samples unless your lab instructor supplies tenacity test samples. The following are examples of this property: Brittle: This is the most common type of tenacity; a brittle mineral breaks in the same way as a hard candy.

Ductile: A specimen can be pulled out into an elongated shape.

Elastic: A sample bends when force is applied and resumes its previous shape when force is released.

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Flexible: A specimen bends without breaking but will not spring back to its original shape.

Figure 1.8

16. Test the tenacity of the samples provided for this activity and enter your results in ■ Table 1.10.

Double refraction. Calcite (Iceland spar) has double refraction.

Physical Properties of Minerals

9

10

Lab 1

Hardness, cleavage, twinning — appears as fine parallel grooves on cleavage surface 2.6–2.8

2.5–2.6 6

None NaAlSi3O8 to CaAl2Si2O8

CaCO3 Calcite

Feldspar (plagioclase)

K(Mg,Fe)3AlSi3O10(OH)2

Feldspar KAlSi3O8 (microcline or orthoclase)

Colorless to white

Light brown

Vitreous

21⁄2–3

2 CL directions at nearly 90°

Tabular to blocky

Hardness, cleavage Tabular to blocky

Rhombohedral 2.8–3.2

Hardness Color

Streak

Luster

Vitreous to waxy

Density Cleavage (CL) or (g/cm3) Fracture (F)

Biotite mica

Table 1.11

Text continued on p. 16.

Name

Materials needed: Known minerals (see also Figures 1.9 and 1.10): biotite mica, calcite, microcline feldspar, plagioclase feldspar, fluorite, galena, almandine to pyrope garnet, gypsum, halite, oolitic or red ocher hematite, specular hematite, hornblende, kaolin, magnetite, muscovite mica, pyrite, augite pyroxene, quartz.

Mineral testing kit Squeeze bottle of 10% hydrochloric acid solution. (Use as little as possible — one drop is enough — and clean off the sample with a tissue or paper towel when you finish.)

Chemical Formula

17. Complete ■ Table 1.11 by testing the mineral properties of known samples (ones with the names labeled) and filling in the blanks in the table. ■ Figure 1.9 (see pp. 13–15) shows examples of silicate minerals — those with chemical compositions containing silicon and oxygen. ■ Figure 1.10 (see pp. 16–17) has photographs of nonsilicate minerals.

Mineral Identification Chart of Known Samples (Exercises 17)

DETERMINING THE PROPERTIES OF MINERALS

2.7

Mineral Name

White

Tenacity

Crystal Habit and Form

Sample Number

Distinguishing Characteristic or Special Property

Tenacity (Exercises 16)

Color, luster and cleavage, thin sheets, elastic

Table 1.10

11

CaSO4 2H2O

NaCl

Fe2O3

Fe2O3

(Ca,Na)2-3(Mg,Fe,Al)5Si6(Si,Al)2O22(OH)2

Al2Si2O5(OH)4

Fe3O4

KAl3Si3O10(OH)2

FeS2

(Ca,Na)(Mg,Fe,Al) (Si,Al)2O6

SiO2

Gypsum

Halite

Hematite (oolitic or red ocher)

Hematite (specular)

Hornblende

Kaolin

Magnetite

Muscovite mica

Pyrite

Pyroxene (augite)

Quartz

Dark green; gray to black

Black

Colorless

None

White to colorless

Black

None

White

White

(Mg,Fe)3Al2Si3O12

Garnet (almandine to pyrope) Colorless to white to pink

None

PbS

White

Galena

Varied: purple, yellow, green, to colorless

CaF2

Fluorite

Vitreous

Vitreous to waxy

Vitreous to pearly

Vitreous

5–6

6–61⁄2

3.2–3.4

5.0

2.8–2.9

5.2

2.2–2.6

1–21⁄2 6

3.0–3.4

5–6

F — conchoidal

F — uneven

F — uneven

F — earthy

Good crystals are rare, stubby

Tabular

Equant

Very fine-grained aggregate

Prismatic

Aggregate of tabular crystals, micaceous

Hardness, fracture

Cleavage and _________________

“Fool’s gold”

Smell it: _________________

Color, luster and cleavage

5.3

51⁄2–61⁄2 F — uneven

Small (egg-shaped) Color and streak ooids or fine-grained aggregate

F — uneven

5.3

1–61⁄2

Salty taste

Equant

Sectile

2.2

1 good CL direction, Tabular 2 poor

Habit, color, and luster

Density, luster, color

Fluorescent, phosphorescent

21⁄2

2.3

Equant, dodecahedrons

3.5–4.3

61⁄2–71⁄2

Equant

Equant

4 CL directions (octahedral)

21⁄2

3.2

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(c) Augite (pyroxene): a fragment showing two cleavages at nearly 90ยบ. See also Figure 1.6a.

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(b) Almandine garnet: Left: a trapezohedral single crystal, equant habit. Right: a broken sample showing hackly fracture and adamantine and waxy luster.

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(a) Olivine: an aggregate of grains showing the typical green color.

(e) Talc aggregate: Talc has pearly luster. The distinction between pearly and vitreous luster is apparent in hand sample but does not show in a photograph. Inset: With a hardness of 1, talc is very easily scratched with a spatula (inset) and even with a fingernail.

Figure 1.9 Silicate Minerals

12

L ab 1

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(d) Hornblende (amphibole): fragments illustrating vitreous luster and two cleavages at an oblique angle. See also Figure 1.6b.

(f) Kaolin: an extremely fine-grained aggregate showing earthy luster and no visible cleavage.

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(j) Plagioclase feldspar of different chemical compositions: Ca-rich ones are usually darker. Upper right: dark gray Ca-rich. Left: light gray Na-Ca. Lower right: white Na-rich with twinning (stripes of reflected light). Twinning, if visible, distinguishes plagioclase from potassium feldspar.

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(i) Potassium feldspar (orthoclase or microcline): varies in color from white to salmon pink and has two cleavages at 90ยบ.

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(h) Muscovite mica: Top: pearly luster on the flat cleavage surface.

Bottom: cleavages seen edge on. Inset: thin cleavage sheets illustrate the one perfect cleavage of mica.

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(g) Biotite mica (black): has resinous luster and one perfect cleavage. The white grains in this sample are quartz. Inset: cleavage sheets.

(k) Quartz: hexagonal quartz crystals: Left: Colorless quartz crystals (rock crystal) with columnar habit. Right: smoky quartz crystals with columnar habit.

Figure 1.9 Silicate Minerals Continued

Physical Properties of Minerals

13

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(o) Rose quartz: a broken fragment of quartz with pink color and vitreous luster.

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(n) Smoky quartz or cairngorm stone: gray translucent quartz; may be nearly black, as in Figure 1.9k. Inset: typical smoky quartz (circled) in granite.

(p) Amethyst: a broken fragment of quartz with purple color. Inset: amethyst crystals from a geode.

Figure 1.9 Silicate Minerals Continued

14

L ab 1

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(m) Milky quartz: a broken fragment of quartz with white color and vitreous luster. Inset: milky quartz crystals from a geode.

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(l) Rock crystal: a broken fragment of colorless quartz showing excellent conchoidal fracture.

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(e) Sphalerite — zinc ore. Sphalerite has adamantine luster and is mixed yellows, browns, and black. Left: shows four of its six cleavages.

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(d) Pyrite: three samples showing brassy color and metallic luster.

Upper left: Granular massive aggregate of pyrite. Right: aggregate of several crystals. Lower left: pyrite cube.

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(c) Galena — lead ore: a fragment with three cleavages at 90º (cubic cleavage). Left: Metallic luster shows where freshly cleaved.

Right: Weathered surfaces are submetallic.

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(b) Gypsum: common habits. Upper left: massive aggregate, alabaster. Right: fibrous satin spar with silky luster. Lower left: a single crystal cleavage fragment, selenite.

D. Pirie

(a) Calcite: a broken fragment showing three cleavages at an oblique angle. The bubbles result from reaction with dilute hydrochloric acid. See also Figure 1.8.

(f) Magnetite — iron ore: granular aggregate, strongly magnetic with submetallic luster.

Figure 1.10 Nonsilicate minerals

Physical Properties of Minerals

15

D. Pirie

D. Pirie

(i) Halite: transparent to translucent with three cleavages at 90º (cubic cleavage).

D. Pirie

(h) Fluorite showing four cleavages and displaying a variety of colors. Its colors and vitreous luster are similar to quartz, but its cleavage distinguishes it from that harder mineral.

D. Pirie

(g) Hematite — iron ore. Left: Oolitic hematite—an aggregate of spheres of fine-grained hematite crystals. The grains and streak are bright reddish brown. Right: Specular hematite is silvery yet has a reddish brown streak, although darker than the oolitic sample.

(j) Graphite — used in pencil leads: submetallic luster, gray streak, and low density. Inset: This photo shows graphite’s gray streak on paper and its friability (ease of breaking).

Figure 1.10 Nonsilicate minerals Continued

18. Begin testing the mineral properties of the unknown samples (ones with numbers only) and filling in the information in ■ Table 1.12 (see pp. 17–18). The last two columns are reserved for Lab 2. (If you feel you have identified a mineral based on your observations of Table 1.11 and Figures 1.9 and 1.10, you may lightly pencil it in; but remember that this is only a hypothesis (see Appendix) and may change as you work Lab 2.)

16

L ab 1

Materials needed: Assorted unknown minerals Mineral testing kit Squeeze bottle of 10% hydrochloric acid solution. (Use as little as possible — one drop is enough — and clean off the sample with a tissue or paper towel when you finish.)

17

Sample Number

Color

Streak

Luster

Hardness

Cleavage/Fracture

Crystal Habit/Form (if applicable)

Mineral Identification Chart of Unknown Samples (Exercise 18)

Table 1.12

Distinguishing Characteristic or Special Property Mineral Name (Lab 2)

Mineral Group (Lab 2)

18

Sample Number

Color

Streak

Luster

Hardness

Cleavage/Fracture

Crystal Habit/Form (if applicable)

Distinguishing Characteristic or Special Property

Mineral Identification Chart of Unknown Samples (Exercise 18)—cont’d

Table 1.12

Mineral Name (Lab 2)

Mineral Group (Lab 2)