Lecture 1: Introduction to Crystallography and Mineralogy PDF

Introduction to Crystallography and Mineralogy Elements of Mineralogy Engr. Caryll Joy C. Franco CRYSTALLOGRAPHY ▪ Crystallography is the study of crystals. ▪ Crystallography is the branch of mineralogy that deals with the formation and prop...

Introduction to Crystallography and Mineralogy Elements of Mineralogy Engr. Caryll Joy C. Franco CRYSTALLOGRAPHY ▪ Crystallography is the study of crystals. ▪ Crystallography is the branch of mineralogy that deals with the formation and properties of crystals. It plays a huge role in our lives. This is because knowledge of crystals and their properties, and the technology that comes from them, are fundamental to electronics, and to modern living. CRYSTALS ▪ A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an ordered pattern extending in all three spatial dimensions. This ordered arrangement of atoms is called a crystal structure, and thus all minerals are crystals. If the crystal structure is different, then we give the mineral a different name. A solid compound that meets the other criteria, but has no definite crystal structure is said to be amorphous. CRYSTALS Crystals are bodies bounded by surfaces, usually flat, arranged upon a definite plan which is an expression of the internal arrangement of the molecules, and are formed by the change of a mineral from the gaseous or liquid state into the solid state, that is crystallization. CRYSTALS A crystal may be viewed either from the external morphology or from the internal structure: a. Homogenous solid bounded by natural planar surfaces. This is the definition based on the external morphology. b. Homogeneous solid with ordered internal structure. In this view, the internal crystal structure is said to be based on the orderly arrangement of the unit cell, a parallelepiped that is the smallest building block of the internal structure. CRYSTAL HABIT The crystal habit of a mineral describes its visible external shape. It can apply to an individual crystal or an assembly of crystals. The crystal habit of a mineral describes its visible external shape. It can apply to an individual crystal or an assembly of crystals CRYSTAL HABIT ❑ There is a complex terminology for crystal faces, but some obvious names for faces are prisms and pyramids. ❑ Crystals that commonly develop prism faces are said to have a prismatic or columnar habit. ❑ Crystals that grow in fine needles are acicular; crystals growing on flat plates are tabular. ❑ Crystals forming radiating sprays of needles or fibers are stellate. ❑ Crystals forming parallel fibers are fibrous, and crystals forming branching, tree-like growths are dendritic. CRYSTAL HABIT Prismatic Acicular Habit Globular (Hematite) Stellate Dendritic Calcite Crystal CONCEPTS OF CRYSTALLINITY a. Inert monoatomic gases have no regular ordering of atoms CONCEPTS OF CRYSTALLINITY b. Some materials, including water vapor, nitrogen gas, amorphous silicon and silicate glass have short range order CONCEPTS OF CRYSTALLINITY c. Metals alloys, many ceramics, and polymers have the regular ordering of atoms/ions that extends through the material CHARACTERISTICS OF CRYSTALS Form Faces A crystal having all of its Crystals are bounded by faces like faces is termed a surfaces which are usually simple form. For example, plane but may be curved, as in the cube and the octahedron Chalybite, diamond, etc. These are each of them simple surfaces are called faces. Like forms, since all the faces of faces have the same each have the same properties, and unlike faces properties. A crystal that have different properties. consists of two or more simple forms is called a combination. CHARACTERISTICS OF CRYSTALS Edge -An edge is formed by the intersection of any two adjacent faces. Solid Angle - A solid angle is formed by the intersection of three or more faces. Interfacial Angle - The angle between any two faces of a crystal is termed the interfacial angle CRYSTAL FACES AND SHAPES Crystals are commonly recognized by their shape, consisting of flat faces with sharp angles. A crystal is scientifically defined by its microscopic atomic arrangement, not its macroscopic shape but the characteristic macroscopic shape is often present and easy to see. There are three types: 1. Euhedral crystals are those with obvious, well-formed flat faces 2. Anhedral crystals do not, usually because the crystal is one grain in a polycrystalline solid 3. Subhedral crystals UNIT CELL ▪ The smallest possible structural unit that is repeated, 3 dimensionally ▪ Contains a full description of the structure as a whole ▪ Crystallographic analog of an atom LAWS OF CRYSTALLOGRAPHY Law of Constancy All crystals of any one mineral have of Symmetry the same symmetry The angle between two like faces of a given Law of Constancy form is constant for all crystals of a given mineral. The interfacial angle is therefore of of Interfacial Angle great use in the detection of minerals All crystal faces make intercepts on the Law of Rational Crystallographic axes, which are either infinity, or small rational multiples of the intercepts Indices made by the unit form MINERALOGY Mineralogy is a subset of geology specializing in the scientific study of chemistry, crystal structure, and physical (including optical) properties of minerals MINERALOGY ▪ In ancient times, people divided all things on Earth into the animal, vegetable, or mineral kingdoms, so a mineral was any natural inorganic substance ▪ Today, dieticians use the term “mineral” to refer to nutritional elements such as calcium, iron, or sodium. Miners often use the term “Minerals” for anything they can take out of the ground including coal, sand, or gravel MINERALS A mineral is a naturally occurring homogeneous solid with a definite (but not generally fixed, see example below) chemical composition and a highly ordered atomic arrangement, There are two types of minerals namely; inorganic (e.g. calcite, quartz, olivine, etc.) and organic (e.g. uricite, calclacite and hoelite) Example: Apatite is a group of phosphate minerals, usually referring to hydroxylapatite (Ca5 (PO4 )3 (OH)), fluorapatite (Ca5 (PO4 )3 (F)) and chlorapatite (Ca5 (PO4 )3 (Cl)) Strong clues to a mineral's identity: Crystal form Luster and Color and and habit transparency streak (shape). Cleavage, fracture, and Tenacity Density parting Hardness BRANCHES OF MINERALOGY Physical Chemical mineralogy Biomineralogy mineralogy Physical mineralogy is the Biomineralogy is a specific focus on physical cross-over field between attributes of minerals. mineralogy, Chemical mineralogy paleontology and Description of physical focuses on the chemical biology. It is the study of attributes is the simplest composition of minerals in way to identify, classify, order to identify, classify how plants and animals and categorize minerals, stabilize minerals under and categorize them, as and they include color, biological control, and well as a means to find crystal structure, crystal the sequencing of beneficial uses from them. habit, twinning, cleavage, mineral replacement of luster, streak, hardness those minerals after and specific gravity. deposition BRANCHES OF MINERALOGY Determinative Descriptive Optical Mineralogy Mineralogy mineralogy Determinative mineralogy is the actual scientific process of identifying minerals, through Descriptive mineralogy data gathering and conclusion. summarizes results of studies When new minerals are performed on mineral Optical mineralogy is a discovered, a standard substances. It is the scholarly specific focus of mineralogy procedure of scientific analysis and scientific method of that applies sources of light as is followed, including recording the identification, a means to identify and measures to identify a classification, and classify minerals. mineral's formula, its categorization of minerals, crystallographic data, its their properties, and their uses optical data, as well as the general physical attributes determined and listed. MINERALOIDS By definition, a mineral has to have an ordered atomic arrangement, or crystalline structure. There are some Earth materials that fit all other parts of the definition of a mineral, yet do not have a crystalline structure. Such compounds are termed amorphous (without form). Some of these amorphous compounds are called mineraloids. These usually form at low temperatures and pressures during the process of chemical weathering Example: Limonite and allophane are good examples of mineraloids POLYMORPHISM Polymorphic minerals are two or more minerals that have the same chemical composition, but different crystal structures. Examples: FeS2 Pyrite (cubic system) Marcasite (orthorhombic system) CaCO3 Calcite (trigonal system) Aragonite (orthorhombic system) ISOMORPHISM Isomorphism (Isostructuralism) Isomorphic minerals are crystalline mineral compounds having different chemical compositions but identical structures. Example: Galena (PbS) Cubic system Halite (NaCl) PSEUDOMORPH In mineralogy, pseudomorph is a mineral or mineral compound that appears in an a typical form (crystal system), resulting from a substitution process in which the appearance and dimensions remain constant, but the original mineral is replaced by another. The name literally means "false form". Examples: Anhydrite (CaSO4 ) pseudomorphed after gypsum (CaSO4.2H2O) Limonite [FeO. (OH).nH2O] pseudomorphed after pyrite (FeS2 ) METAMICT MINERALS Metamict minerals are minerals whose crystal structure has been partially destroyed by radiation from contained radioactive elements. Example: The mineral zircon (ZrSiO4 ) often has U and Th atoms substituting for Zr in the crystals structure. Since U and Th have radioactive isotopes, Zircon is often seen to occur in various stages of metamictization.