Lecture 1

Earth Materials - Minerals and Rocks

Minerals: basic compounds of the solid earth. ~ 3000 known; these 3000 contain all of the chemical elements of which the earth is made.

Rocks: physical mixtures of minerals. Most of the rocks that make up the earth are composed of only ~ 50 of the known minerals

Why study? Minerals occur in nearly all inorganic materials of everyday life, and are thus important economically, aesthetically and scientific terms.

ECONOMIC Many minerals have economic value - early on were used for pigments, gemstones for ornamentation, clay for bricks, iron for utensils. Not until the 19th century, however, were metals mined and extracted.

ore minerals - minerals from which valuable metallic elements can be extracted

industrial minerals - nonmetallic materials used manufacturing (EX: insulators, ceramics, glass, cement, fertilizers, etc.)

gangue minerals - those minerals not considered useful but which must be mined anyway

Some minerals are now considered "strategic" - [ex. Al and Cr in WWII]

Synthetic minerals are now important in materials science (Ex. high-T superconductors)

AESTHETIC Gemstones have been important throughout the history of most cultures, and precious metals, prized for their aesthetic values, form the basis of most economies.

SCIENTIFIC Minerals are the building blocks of all solid parts of the universe, and are therefore fundamental to all aspects of the geological sciences. Some examples:

- petrologists use individual minerals that form under restricted conditions of pressure and temperature to map physical conditions within the Earth

- geophysicists studying the Earth's interior need to understand how minerals behave under pressure, and how they transmit seismic waves

- geochemists use changes in water chemistry to understand processes of weathering and contamination by interaction with mineral surfaces

Minerals are also important to a number of related scientific disciplines, including

- agricultural sciences (soils)

- hydrology (important for understanding development of aquifers and aquitards)

- materials science (development of synthetic minerals with specific properties)

- medical science (e.g., asbestos)

- biological sciences (may have been important as templates for the origin of life)

Definition of a mineral:

A mineral is a naturally occurring, inorganic, homogeneous solid with a definite chemical composition and an ordered atomic arrangement

- naturally occuring - this modifier in the definition is considered superfluous by some, but is an important distinction to others (especially gemologists)

- homogeneous solid a single, solid substance (phase) that cannot be physically separated into distinct compounds; this distinguishes minerals from rocks that can be disaggregated into individual mineral constituents

- definite chemical composition -al'>ineral is a chemical element or compound whose composition can be represented by a chemical formula (e.g., SiO₂). The compositions of minerals are either fixed, or vary within specified limits (EX: the mineral olivine may vary in composition from Mg₂SiO₄ to Fe₂SiO₄). The composition dictates the mineral structure, which in turn controls the physical properties of the mineral

- ordered atomic arrangement - nearly all minerals are crystalline. It means that the internal structure is characterized by periodic or predictable arrays of atoms, ions, or molecules ... this ordered arrangement of atoms is often expressed in the symmetry of the external form. It also means that different minerals may have the same composition but different structures (EX: graphite and diamond).

The crystalline state of matter is the most fundamental property of minerals

In fact, the word 'crystal' is an anglicized version of the Greek word for ice, and was generally employed through the Middle Ages for describing quartz - 'rock crystal' - as quartz was considered to be permanently solidified water. The word has persisted for any quartz clear enough for ornaments (e.g., New Age uses) or glass.

Aspects of Earth Materials that we will cover this term:

- crystal chemistry - the chemical nature of minerals

- crystallography - the study of symmetry and the internal order of crystals

- mineral physics - the physical properties of minerals

- petrology -al'> study of rocks and the minerals that form them

A bit of history

Very early man recognized natural pigments and used them in cave paintings. Stone Age man knew something about the hardness of minerals, and about mineral cleavage (how they break) - jadeite commonly used as tools. The mining and smelting of minerals for iron, copper, bronze, lead, silver probably dates back to > 4000 yrs. However, there is no written record of any of this.

300 BC Theophrastus (pupil of Aristotle) wrote On Stones

23-79 AD Pliny the Elder recorded a great deal of natural history, and described a number of minerals mined as gemstones, ores, and pigments. His are the earliest comments on crystal form and the quality of crystal faces. Books 33-37 of his volume on Natural History cover such topics as precious metals (not only mining and uses but also man’s greed and exploitation of mineral resources); use of minerals for pigments and painting; mining and metallurgy; marble and other materials used in sculpture; and precious stones.

1556 AD Georgius Agricola (Georg Bauer; German physician) published De Re Metallica, in which "I have omitted all those things which I have not myself seen, or have not read or heard of from persons upon whom I can rely." He is considered the founder of geology as a discipline, and his work contributed to the fields of mining geology, metallurgy, mineralogy, structural geology and paleontology. This book became a handbook of mining practices for the next two centuries, and laid the framework of stratigraphic principles. In other books he developed a classification systems for minerals based on their physical properties, gave them standardized names and recorded where they could be found.

1611 AD Kepler speculated on the planar arrangement of spheres as an explanation for the symmetry of snowflakes.

1669 Nicolaus Steno recognized that the interfacial angles of quartz crystals were the same regardless of the size or shape of the crystal. This led to his hypothesis of the constancy of interfacial angles, with the basis that there is an underlying pattern of symmetry to crystal forms. He concluded that processes of crystal growth may allow different faces to dominate under different growth conditions - this results in crystals of the same composition having very different external forms. SIGNIFICANCE OF CRYSTAL FORM

1768 AD Carolus Linnaeus developed a mineral classification based primarily on external form. Throughout 18th century, studies of chemistry and mineralogy closely linked, as chemists worked with minerals as their raw materials. Saw the discovery of many new minerals and, with that, identification of many new elements (cobalt, nickel, manganese, tungsten, molybdenum, uranium ...)

1784 AD The French mineralogist Rene Hauy recognized that the perfection of external form, the symmetry possessed by crystals, and the existence of perfect cleavage must be manifestations of the internal structure of minerals (involving planes of atoms). In his study of calcite crystals Hauy defined axes of reference for a few crystals, and recognized that all faces on a crystal cut those axes at simple multiples - this led to his conception of crystal structures made up of identical "integral molecules" ... turns out that this concept is similar to the modern concept of a space lattice. This work provided the relationship between crystallography and mineralogy.

1830 AD Johan Hessel proved that geometric constraints limit the number of crystal classes to 32, and that only 2-, 3-, 4-, and 6-fold axes of rotational symmetry are possible in minerals.

1837 AD James D Dana (Yale) completed the 1st edition of A System of Mineralogy, a book which, in the 4th edition, introduced the chemically based classification system that we still use.

1848 Auguste Bravais proposed the existence of 32 crystal classes He also proposed that there were only 14 space lattices (that is, he demonstrated that only 14 regular patterns in space can result form the periodic arrangement of points) - his work was the forerunner of space group theory. He also perceived that the 14 space lattices consisted of 7 different lattice symmetries, which correspond to 7 crystal systems.

1870 AD Petrographic (polarizing) microscope invented in mid-1800s ... brought to this country by G. Huntington Williams (Johns Hopkins). This allowed determination of the optical properties of minerals, and also made possible the study of fine-grained rocks...

1895 AD Wilhelm Roentgen discovered xrays; he showed that xrays were able to pass through materials that are opaque to light, invisible to the human eye, and could be recorded on photographic paper. For this work Roentgen was awarded the first Nobel Prize in physics in 1901.

1912 AD The physicist Max von Laue first demonstrated that xrays were diffracted by a crystal structure (in his case, copper sulphate). The implications of this observation were that the wavelengths of xrays were comparable with the spacing between atoms in crystals. This was the start of an entire field devoted to the determination of crystal structure.

1913 AD The father-son team W. Lawrence and W. Henry Bragg used diffraction to analyze the first crystal structure, that is, the precise position of atoms in the structure was ascertained, along with the distances and angles between atoms. This work allowed inferences about atomic size and bond strengths, and thus won the Braggs the Nobel Prize in 1915.

The twentieth century has seen a dramatic increase in the 'tools' available for studying minerals - XRD, XRF, electron microprobe, electron microscope, spectroscopic techniques, and, most recently, computer-assisted tomography, TEM and atomic force microscopy, where individual atoms can be imaged...

We now realize that the internal structure of any mineral can be described as a 3-D array of positive (cations) and negative (anions) ions built up by a regular repetition of the unit cell. Symmetry and form of a crystal are determined by the shape of that unit cell. The chemical formula of a mineral expresses the relative proportions of its constitutive elements.