Earth Materials Lab 1 – the properties of minerals
Relevant info in Chapter 15, Wenk and Bulakh
The iron sulfide called pyrite (FeS2) is also known as fool's gold, implying that this common iron ore may be
confused with the rare and valuable native Au. The somewhat similar color of these two compounds may be the
source of confusion. However, hand specimens of the two minerals are readily distinguishable to those who
understand the physical characteristics of each material.
Today you will learn several useful physical properties of minerals that will help you to distinguish one mineral
from another and also will aid you in describing specimens to other people. Furthermore, the physical properties
provide insight into the composition and structure of each mineral, because it is the atomic-level attributes of
crystals that control their macroscopic characteristics. Below, you will find some information about these physical
properties, as well as an exercise on these topics. The completed exercise is due in two weeks. However, this lab
period will be your only opportunity to examine the specimens needed to complete the exercise. You may work
alone or as groups, but everyone in the class should examine every specimen prior to departing.
Crystal Habit
The shape or habit of a mineral grain (one single crystal) or a collection of grains (a rock or polycrystalline
specimen) is a function of energy along the surface of each mineral grain. This energy is controlled both by the
internal structure and composition of each grain and that of the medium on the other side of its boundary. Many
minerals may exhibit a range of habits depending on the physical conditions at which they formed and the nature of
the surrounding materials. But typically the range of habits may be limited by internal structure and therefore
diagnostic.
Below are frequently used terms for describing mineral habit. Because most of our specimens (indeed, most of the
specimens you are likely to encounter) are comprised of more than one crystal, it is important to be able to describe
both the form of single crystals and the texture of aggregate materials. The table below lists some of the more
commonly used descriptive terms for crystal habit.
Single crystals
Polycrystalline Materials
Acicular
Capillary
Bladed
Needlelike
Hair or threadlike
Elongate and flat in 1 direction
Massive
Granular
Radiating
Tabular or
platy
Columnar
High aspect ratio (equant in 2
dimensions and short in 1)
Low aspect ratio (equant in 2
dimensions and long in 1)
Columnar with well-defined
faces.
Equant in 3 dimensions
Fibrous
Solid mass with few features
Composed of many small grains
Crystals emanating from a
common point
Composed of small fibers
Lamellar
Composed of flat plates
Stellated
Forming star-like patters
Dendritic
Reticulated
Globular
Tree-like
Lattice-like form
Crystals radiating in 3
dimensions
Shaped like a bunch of grapes
Kidney-shaped
Made of small concentric
spheres
Forming soft, pliable fibers.
Prismatic
Blocky
Botryoidal
Reniform
Oolitic
Asbestosform
Hardness
Hardness is a measurement of an individual mineral grain’s resistance to abrasion and is a function of the atomic
bonding within a crystal. Hardness is a very useful diagnostic tool. Note because atomic bonding controls this
property, hardness may with orientation.
ERTH 2330
Lab 1 - Properties of Minerals
2
The Mohs hardness scale is composed of ten common minerals arranged in order of increasing hardness (see table,
right). Find a Mohs hardness kit and examine these minerals (sorry - no diamond).
Useful to us is the concept of relative hardness. We may use objects of known hardness to gauge that of an
unknown specimen. For instance, if someone were to mix up the Mohs scale samples, it would be easy to restore
their order even if you knew no other distinguishing properties for
Mohs hardness scale
these minerals. You would start by taking one specimen and
1 Talc
6 Feldspar
scratching it with the others, separating those that leave a mark on the
2 Gypsum
7 Quartz
specimen one pile, and those that are scratched by the specimen in
3 Calcite
8 Topaz
another. Then you would take another from one of the piles and use it
4 Fluorite
9 Corundum
to create two more piles, and continue until each mineral is between a
5 Apatite
10 Diamond
specimen that it can scratch and a specimen that scratches it.
Other objects are useful for hardness determination. Fingernails have a hardness of ~2.5 (try scratching the talc, the
gypsum, and the calcite with your nail), a penny (made of a copper alloy) has a harness of ~3.5, and stainless steel
(knife) and glass have a hardness of ~5.5. Knowing the hardness of such tools is particularly useful for identifying
outside the lab, where one typically doesn’t have access to all the Mohs minerals.
Breaking minerals
Cleavage and fracture
Cleavage is the tendency of a mineral grain to break parallel to atomic planes, resulting in planar rupture. Fracture
is the lack of any planar breaking in a specimen, occurring when atomic bonds are easily broken in any direction.
The way a mineral breaks is highly diagnostic. Furthermore, it is helpful in determining the structural
(crystallographic) orientation of a grain because cleavage is specifically oriented by atomic structure.
Unfortunately, testing for this property is rather destructive, as the mineral needs to be cracked or broken to
determine cleavage or fracture. For example, a diamond has one perfect (well-defined) cleavage in one direction,
but most people who own a solitaire will not employ this simple test to determine if their stone is indeed a diamond.
Fortunately for us, nature is cruel. The forces that bring specimens from the depths of the Earth to the surface
typically break and crack minerals. And as if that were not enough, they may suffer further damage at the hands of
the enterprising geologist or rock-hound collecting the specimen. The upshot of all this damage: careful observation
of existing cracks, breaks, and edges may tell you all you need to know without further destruction of the specimen.
Descriptions of cleavage are typically presented as a qualitative assessment with an orientation. We will forgo
describing orientation for the time being, but you should be able to describe cleavage as perfect - makes very
straight planes, poor - makes planes with numerous steps and breaks, or good - somewhere between perfect and
poor. Note that a mineral may have more than one direction of cleavage and that each direction has its own level of
quality dependant on the strength of molecular bonding in that direction.
Descriptions of fracture are somewhat similar to the descriptions of habit (it’s important to distinguish
morphological features imparted by formation (habit) and those resulting from strain). There are only a few terms
you need to know and use: conchoidal - breaks in curved surfaces, fibrous - breaks in fibers, splintery - breaks into
splinter fragments, and hackly - breaks with jagged edges.
Tenacity
Tenacity describes the deformation characteristics of a mineral. Determination of this property more or less requires
witnessing the destruction or deformation of a material. Some of the common terms are presented here: brittle easily broken or powdered, malleable - easily bent or deformed, sectile - easily shaved with a knife, flexible - easily
bent, and elastic - returns to its shape following bending.
We have provided a few specimens for you to damage and observe cleavage, fracture, and tenacity. After you
have broken these specimens, examine the pieces carefully. Note the exterior cleavage planes and fracture
surfaces and then see if you can observe these features as cracks on the sides of the mineral, or as planes or
curves on the exterior of the specimen.
ERTH 2330
Lab 1 - Properties of Minerals
3
Specific gravity
The density (mass / volume) of a mineral is diagnostic in many cases. Specific gravity (G) is density normalized to
that of water (pure H2O at 20ºC and 1atm pressure), in other words the ratio of the mass of the mineral to the mass
of an equal volume of water and is commonly given in written descriptions of minerals. For the purposes of this
class, you will not need to determine the exact specific gravity of minerals. However, you will find it useful to heft a
mineral and determine its relative density. For example, your instructor has selected several minerals of roughly
equal volume; heft these and note how they differ in weight.
Streak
The color of a finely powdered mineral is called streak, and this property is extremely useful in distinguishing many
of the transition-metal oxides and sulfides (most of which have similar hardnesses and G). Streak is easily
ascertained for these "soft" minerals by rubbing the grain or grains against a white ceramic plate (hardness ~6.5).
This process is somewhat destructive, as a small amount of the specimen is transferred to the ceramic plate. Streak
the specimens provided for this characteristic. Note that hematite’s (Fe2O3) streak is the same color
regardless of whether the specimen color is gray or red, with metallic or dull luster.
Color
Although it is a dominant characteristic used for identifying unknown items, color is a largely unreliable
characteristic for identifying minerals. For one thing, it is contingent on the wavelength distribution of the light that
is shining on the mineral. Even in daylight, color in many minerals varies due to trace amounts of impurities or even
structural discrepancies.
Look at the examples of quartz (SiO2). Notice that color varies from sample to sample, but all are made almost
entirely of SiO2. Some of the color variation is caused by trace impurities called chromophores. These are typically
transitional metals, such as Fe3+ and Cr3+ that absorb certain portions of the light spectrum as it passes through the
crystal. Other color-altering phenomena are more complex. In smoky quartz, for example, small amounts of Al3+
substitute for Si4+. The resulting charge imbalance means that Al couples must couple with another element with a
single positive charge (typically Na+ or H+). But it isn't the spectral absorption of the Al, Na, or H that produces the
color. The smoky color is the result of radiation damage. As the specimen is exposed to low-levels of common
radiation for millions of years, one of a pair of electrons in oxygens adjacent to Al ions is removed, creating a “hole”
which absorbs light and produces the foggy-dark color.
There are some minerals that commonly exhibit one color or a limited range of colors in daylight. Thus, color may
be diagnostic in certain minerals. Even so, it is most useful when it is combined with other physical properties.
Luster
The concept of luster can be difficult to grasp. While most terms applied to luster are familiar, their usage seems
subjective at times. This property describes how light (of any wavelength) is reflected back to an observer.
Important terms to know are: metallic - shines like polished metal, non-metallic - doesn't shine like metal, submetallic - somewhat shines like metal, vitreous - glassy, adamantine -sparkles, dull - flat, and resinous - oily.
Diaphaneity
This is the ability of a mineral to transmit light. There are three terms that you already know from everyday usage:
transparent - optically clear, translucent – scatters light as it passes through the crystal, and opaque - doesn’t allow
light to pass through. Diaphaneity is not particularly useful with hand specimens, as some minerals are translucent
when very thin, but appear opaque at greater thickness. The degree of scattering may vary as a function of small
inclusions of other materials within a crystal. This property will become more useful when we look at thin slices of
minerals through the microscope.
Today's exercise
First, read through the lab handout. Second, spend some time examining the specimens for each of the
properties covered by the lab and demonstrate these concepts to yourself. Third, go to the table in the front
of the room and examine each of the specimens. On a separate sheet of paper, list each sample and describe
all of its properties except for tenacity (see example below). Attempt to name the mineral represented by the
sample based on its characteristics.
ERTH 2330
Lab 1 - Properties of Minerals
4
Regarding cleavage, fracture, and tenacity: do not break any of the specimens except for those designated.
For the non-designated specimens: attempt to determine cleavage and fracture from features already present
on the mineral. Do not attempt to determine tenacity.
Specimen #example
Habit: Massive, made of multiple blocky, roughly cubic crystals.
Cleavage: Perfect in one direction, fractures are sub-conchoidal
Streak: dark gray
Color: Silver gray
Hardness: ~2.5.
Luster: Metallic
G: high.
Diaphaniety: opaque
Its identity: Galena
Additional questions:
1.
Can a sectile specimen be harder than 6.5 on the Moh’s relative scale?
2.
What happens if you streak quartz?
3.
Why is diamond a 10 on the hardness scale?
4.
What is the relationship between Moh’s hardness and Vicker’s indentation hardness?
5.
Which properties are controlled by atomic bonding?
6.
Corundum comes in many colors: name two common gemstones that are varieties of corundum that
differ in color?
7.
Corundum, garnet, and diamond are used as abrasives (sandpaper, polishing compounds, and
cutting tools). Why?
7a. What mineral is required to polish a diamond (such as a faceted solitaire stone)?