Igneous and Metamorphic Petrology Lecture 3 Fall 2024 PDF

Ms. Nouf S. Alteneiji Chemical bonding crystal structure Crystal structure crystal shape Crystal structure mineral properties Graphite is made up of sheets of carbons bounded to each other by...

Ms. Nouf S. Alteneiji Chemical bonding crystal structure Crystal structure crystal shape Crystal structure mineral properties Graphite is made up of sheets of carbons bounded to each other by van der Waals bonds (weak bonds). Soft and greasy feel. Crystal structure mineral properties Diamond is made up of carbons bounded to each other by Covalent bonds (typically quiet strong). Crystals have high melting points and are hard. Ions as spheres Stick- and ball polyhedron Polyhedron + stick- and ball  Isostructural Minerals - Two or more minerals whose atoms are arranged in same type of crystal structure are isostructural. - Halite (NaCl) and galena (PbS), for example, are isostructural because the arrangement of Pb and S in galena is identical to the arrangement of Na and Cl in halite. - The minerals are dramatically different in many physical and chemical properties, but the fact that they are isostructural is reflected in an identical symmetry, cleavage (three perfect cleavages at right angles) and crystal habits (cubes). Halite (NaCl) Galena (PbS)  Isostructural group -An isostructural group of minerals is a group of minerals that is isostructural and chemically related by having a common anion or anionic group (ex. calcite group of carbonates). Calcite Rhodochrosite Smithsonite Magnesite CaCO3 MnCO3 ZnCO3 MgCO3  Polymorphism ❖ The ability of a chemical compound to crystallize with more than one structure is known as polymorphism. ❖ The different structures of a given chemical compound are known as polymorphic forms or polymorphs. ❖ The set of different minerals with the same chemical composition is known as a polymorphic group. ❖ Why do minerals form polymorphs?  Polymorphism ❖ Different polymorphs of the same substance are stable under different sets of T-P conditions.  a mineral is natural a mineral is solid a mineral is crystalline (has an orderly and repetitive atomic structure) a mineral has a well-defined chemical composition (varies within limits and can be represented by a chemical formula) a mineral is inorganic (mostly!) Compositional variation in minerals = solid solution Cations in crystal lattice are interchangeable in different ways. Range of composition produced by solid solution is a given mineral is called substitution series, the compositional extremes are the end members. Three types of solid solution: 1. substitutional solid solution 2. Omission solid solution 3. Interstitial solid solution Substitutional solid solution – simple Two requirements control whether substitution can occur. 1. Ion sizes must be similar. 2. Charge neutrality must be maintained. Substitution involves interchanging one cation for another in a structural site with identical charges = simple substitution. The mineral olivine, (Mg,Fe)2SiO4, forms a continuous solid solution series whose end members are forsterite (Mg2SiO4) and fayalite (Fe2SiO4). Substitutional solid solution – simple One cation replaced by another of similar size and electric charge (e.g. olivine, between Fe and Mg) OLIVINE GROUP (Mg,Fe)2SiO4 Forsterite (Mg2SiO4 ) Fayalite (Fe2SiO4)  Substitutional solid solution - coupled Coupled substitution maintains charge balance by coupling one substitution that increases the charge with another that reduces the charge. In the mineral plagioclase, the end members are albite (NaAlSi3O8) and anorthite (CaAl2Si2O8). The Ca2+ and Na+ both occupy a distorted 8-fold coordination site and Al3+ and Si4+ both occupy tetrahedral coordination sites. To substitute a Ca2+ for a Na+ it is necessary to also substitute an Al3+ for a Si4+ to keep the charge balance Substitutional solid solution - coupled PLAGIOCLASE GROUP (Na,Ca)[Al[Al,Si)2SiO8] Albite (NaAlSi3O8) Anorthite (CaAl2Si2O8 ) Omission solid solution  Interstitial solid solution Interstitial solid solution is a variation of coupled substitution in which charge balance is maintained by placing ions in sites that normally are vacant. Common in minerals with structures that provide large openings. The mineral beryl (Al2Be3Si6O18), for example, is constructed of rings of silicon tetrahedra that are stacked atop each other to form channel-like cavities in the center of the rings. Various monovalent cations, including K+, Rb+ and Cs+, can be inserted into these cavities. Charge balance is maintained by substitution of Al3+ and Be2+ for Si4+ in tetrahedral sites:  Graphical representations A binary diagram is used for A ternary diagram is used when there minerals whose composition can are three end-member compositions. be described in terms of two end The diagram is plotted as an equilateral members; compositions are triangle with an end member placed at represented on a straight line. each corner.  Graphical representations - Binary diagrams: solid - Ternary diagrams: solid solution series with 2 solution series with 3 end members end members Phase diagram Phase diagram of Plagioclase  Mineral classification - Systematics based on chemical composition. - Mineral group: minerals with same anion or anionic group.