3 Optical Crystallography
The discoverers of X-ray diffraction were able to draw on the knowledge and
expertise which had been developed by geologists and mineralogists over several
centuries in universities, but the study of crystals has a much longer history. The
occasional discovery of crystalline materials by primitive man was probably looked
upon as a generous gift from the Gods, and they were incorporated into beautiful
jewellery. Their colours, magnetic properties and sharp edges were utilised to enrich
life, simplify daily tasks and enable them to explore the larger world. The properties
of quartz in the natural state led the Greeks and Romans to define the more general
term “crystal”, and they were attracted by their obvious symmetry properties which
resonated with the mathematically derived regularity of the Platonic solids. The
mathematical basis of crystallography therefore had its origins in ancient Greece and
Rome, and Pliny devoted five entire volumes of his work Naturalis Historia (77 CE)
to the classification of “earths, metals, stones and gems”. He described many
minerals not known to the Greeks and discussed their applications and properties.
He laid the fundamentals of crystallography by discussing crystal habits and was
particularly taken by the octahedral shape of diamond crystals. It developed into a
rigorous scientific discipline in the Renaissance with the discovery of the microscope
which enabled mineralogists to examine and measure in a scientific manner a wide
range of minerals and appreciate the great beauty of hexagonal symmetries in the
diverse range of ice crystals. This eventually formed the basis of the following
qualitative classification of crystalline minerals in the nineteenth century [28]:
1. Predominant crystal faces (prism-prismatic, pyramid-pyramidal and pinacoidplaty)
2. Crystal forms (cubic, octahedral, dodecahedral)
3. Aggregation of crystals or aggregates (fibrous, botryoidal, radiating, massive)
4. Crystal appearance (foliated/lamellar (layered), dendritic, bladed, acicular, lenticular, tabular (tablet shaped))
Perhaps the most influential text in the nineteenth century was the Manual of
Mineralogy by James Dwight Dana, first published in 1848. The fourth edition was
entitled Manual of Mineralogy and Lithology (1887), and it became the standard
college text. It has been successively revised and updated in its subsequent
23 editions.
The atomistic view of the universe associated with Democritus (370 BCE) led to
the proposal that crystals were made up of indivisibly small regularly shaped and
identical “units”. The Pythagorean school had defined around 550 BCE the five
regular Platonic solids which bore some resemblance to the crystalline forms found
in nature. The problem was related to the principles familiar to masons, who had
successfully built pyramids in ancient Egypt and South America. Aristotle speculated that a crystal could be assembled if very small polyhedral “bricks” were packed
together without leaving any space between, thereby avoiding vacuum cavities. In
two dimensions all space could be tiled in this way using triangles, rectangles and
Early History of X-Ray Crystallography
9
The discoverers of X-ray diffraction were able to draw on the knowledge and
expertise which had been developed by geologists and mineralogists over several
centuries in universities, but the study of crystals has a much longer history. The
occasional discovery of crystalline materials by primitive man was probably looked
upon as a generous gift from the Gods, and they were incorporated into beautiful
jewellery. Their colours, magnetic properties and sharp edges were utilised to enrich
life, simplify daily tasks and enable them to explore the larger world. The properties
of quartz in the natural state led the Greeks and Romans to define the more general
term “crystal”, and they were attracted by their obvious symmetry properties which
resonated with the mathematically derived regularity of the Platonic solids. The
mathematical basis of crystallography therefore had its origins in ancient Greece and
Rome, and Pliny devoted five entire volumes of his work Naturalis Historia (77 CE)
to the classification of “earths, metals, stones and gems”. He described many
minerals not known to the Greeks and discussed their applications and properties.
He laid the fundamentals of crystallography by discussing crystal habits and was
particularly taken by the octahedral shape of diamond crystals. It developed into a
rigorous scientific discipline in the Renaissance with the discovery of the microscope
which enabled mineralogists to examine and measure in a scientific manner a wide
range of minerals and appreciate the great beauty of hexagonal symmetries in the
diverse range of ice crystals. This eventually formed the basis of the following
qualitative classification of crystalline minerals in the nineteenth century [28]:
1. Predominant crystal faces (prism-prismatic, pyramid-pyramidal and pinacoidplaty)
2. Crystal forms (cubic, octahedral, dodecahedral)
3. Aggregation of crystals or aggregates (fibrous, botryoidal, radiating, massive)
4. Crystal appearance (foliated/lamellar (layered), dendritic, bladed, acicular, lenticular, tabular (tablet shaped))
Perhaps the most influential text in the nineteenth century was the Manual of
Mineralogy by James Dwight Dana, first published in 1848. The fourth edition was
entitled Manual of Mineralogy and Lithology (1887), and it became the standard
college text. It has been successively revised and updated in its subsequent
23 editions.
The atomistic view of the universe associated with Democritus (370 BCE) led to
the proposal that crystals were made up of indivisibly small regularly shaped and
identical “units”. The Pythagorean school had defined around 550 BCE the five
regular Platonic solids which bore some resemblance to the crystalline forms found
in nature. The problem was related to the principles familiar to masons, who had
successfully built pyramids in ancient Egypt and South America. Aristotle speculated that a crystal could be assembled if very small polyhedral “bricks” were packed
together without leaving any space between, thereby avoiding vacuum cavities. In
two dimensions all space could be tiled in this way using triangles, rectangles and
Early History of X-Ray Crystallography
9