Crystallography - Wikipedia
American Crystallographic Association · Learning Crystallography Web Portal of Open Access Crystallography Resources. JAVA crystal examples on the individual crystal classes for detailed study. Crystal Axes, Example Form, Java Example Forms and Links to Mineral Listings. Course Title: Crystallography & Mineralogy Crystallography is defined as the study of crystals and their structural Links to Mineralogy & Crystallography.
Dana published his first edition of A System of Mineralogy inand in a later edition introduced a chemical classification that is still the standard.
It, however, retains a focus on the crystal structures commonly encountered in rock-forming minerals such as the perovskitesclay minerals and framework silicates. In particular, the field has made great advances in the understanding of the relationship between the atomic-scale structure of minerals and their function; in nature, prominent examples would be accurate measurement and prediction of the elastic properties of minerals, which has led to new insight into seismological behaviour of rocks and depth-related discontinuities in seismograms of the Earth's mantle.
To this end, in their focus on the connection between atomic-scale phenomena and macroscopic properties, the mineral sciences as they are now commonly known display perhaps more of an overlap with materials science than any other discipline. Calcite is a carbonate mineral CaCO3 with a rhombohedral crystal structure. Aragonite is an orthorhombic polymorph of calcite.
An initial step in identifying a mineral is to examine its physical properties, many of which can be measured on a hand sample. These can be classified into density often given as specific gravity ; measures of mechanical cohesion hardnesstenacitycleavagefractureparting ; macroscopic visual properties lustercolor, streakluminescencediaphaneity ; magnetic and electric properties; radioactivity and solubility in hydrogen chloride H Cl.
In the Mohs scalea standard set of minerals are numbered in order of increasing hardness from 1 talc to 10 diamond. A harder mineral will scratch a softer, so an unknown mineral can be placed in this scale by which minerals it scratches and which scratch it. Once a crystal is obtained, data can be collected using a beam of radiation. Although many universities that engage in crystallographic research have their own X-ray producing equipment, synchrotrons are often used as X-ray sources, because of the purer and more complete patterns such sources can generate.
Synchrotron sources also have a much higher intensity of X-ray beams, so data collection takes a fraction of the time normally necessary at weaker sources. Complementary neutron crystallography techniques are used to identify the positions of hydrogen atoms, since X-rays only interact very weakly with light elements such as hydrogen.
Producing an image from a diffraction pattern requires sophisticated mathematics and often an iterative process of modelling and refinement. In this process, the mathematically predicted diffraction patterns of an hypothesized or "model" structure are compared to the actual pattern generated by the crystalline sample. Ideally, researchers make several initial guesses, which through refinement all converge on the same answer.
Models are refined until their predicted patterns match to as great a degree as can be achieved without radical revision of the model. This is a painstaking process, made much easier today by computers. The mathematical methods for the analysis of diffraction data only apply to patterns, which in turn result only when waves diffract from orderly arrays.
Hence crystallography applies for the most part only to crystals, or to molecules which can be coaxed to crystallize for the sake of measurement.
- Crystalography & Mineralogy (ERSC 3001)
In spite of this, a certain amount of molecular information can be deduced from patterns that are generated by fibers and powderswhich while not as perfect as a solid crystal, may exhibit a degree of order.
This level of order can be sufficient to deduce the structure of simple molecules, or to determine the coarse features of more complicated molecules. For example, the double-helical structure of DNA was deduced from an X-ray diffraction pattern that had been generated by a fibrous sample.
In materials science[ edit ] Crystallography is used by materials scientists to characterize different materials. In single crystals, the effects of the crystalline arrangement of atoms is often easy to see macroscopically, because the natural shapes of crystals reflect the atomic structure.
In addition, physical properties are often controlled by crystalline defects.
The understanding of crystal structures is an important prerequisite for understanding crystallographic defects. A crystal made up of like faces is termed simple form. Combined form is characterized by tow or more simple forms.
Mineralogy - Wikipedia
Simple forms can enclose space by themselves and considered as closed forms. An edge is formed by the intersection of any two adjacent faces. This the angel formed by the intersection of three or more faces. The angel between any two faces of a crystal. It is formed by the intersection of two normal lines taken along two adjacent faces.
Law of constancy of interfacial angle The interfacial angles are constant for all crystals if a given mineral with identical composition at the same temperature. Interfacial angles are measured by a contact goniometer. Crystallographic Axis Crystallographic axis are imagenary lines taken parallel to the major faces in the crystal. The position of a plane in space is given by the interceptd the lengths cut off that the plane makes on three given lines called the axis. The crystallographic axes intersect at the origin.