Minerals: Introduction and Identification PDF

Minerals Introduction and Identification  How is a mineral different from a rock? A mineral is to a rock what an atom is to a molecule O and H Atoms Water Molecule ….sort of  How is a mineral different from...

Minerals Introduction and Identification  How is a mineral different from a rock? A mineral is to a rock what an atom is to a molecule O and H Atoms Water Molecule ….sort of  How is a mineral different from a rock? A rock is a solid consisting of an aggregate of mineral grains, pieces of older rocks, or a mass of natural glass. So rocks are made of minerals…  What is a rock? A Rock Is an Aggregate of Minerals Credit: mineralseducationcoalition.org So what are minerals? Minerals are the building blocks of our planet They make up most rocks and sediment Over 4000 known minerals Useful resource Can be dangerous.. and fascinating!  Different definitions Different people have different definitions In food science minerals help our bodies function normally Calcium Fluoride Zinc Iron etc. The most basic definition of a mineral is something that is neither Harvard Health Publishing plant or animal  Minerals in geology A homogeneous, naturally occurring, solid, inorganic* substance with a definable chemical composition and an internal structure characterized by an orderly arrangement of atoms, ions, or molecules in a lattice  Naturally occurring To be a true mineral it must form naturally by Earth processes Man made materials like steel and plastic are not minerals Lab grown minerals are known as Synthetic minerals  What are “Earth processes?” Solidification Forming from melts Precipitation Usually due to evaporation Some are formed by organisms “Biogenic minerals” Chemically identical to naturally occurring non organic minerals Calcite Apatite  Must be solid A material that can maintain its shape indefinitely Minerals can not be liquids  Inorganic There is a difference between being made by an organism and being an organic molecule Organic means the molecule contains carbon-carbon or carbon-hydrogen bonds An example is sugar (C12H22O11) It’s a gray area….  Definable chemical composition A mineral must have a definable chemical formula Some minerals contain only one element Graphite or diamond = C Most are compounds or two or more elements Quarts = SiO2  Crystalline structure Minerals must have a crystalline structure Atoms in a mineral must occupy fixed positions in a grid called a lattice Solids lacking internal atomic order are called glasses. Most solid materials are crystalline.  Crystals A crystal is a single, continuous piece of crystalline solid typically bounded by flat crystal faces. Crystal faces grow naturally as the mineral forms and reflect atomic structure. Equivalent faces found on two samples of the same mineral always bear the same angular relationship. Crystal Shapes  Atomic bonding The geometry of the atomic packing and the nature of chemical bonding determine the mineral properties.  Atomic bonding The way elements are packed into a mineral crystal lattice depends upon the size and the charge of the ions of that element. Anions are usually larger than cations. A large central cation requires a larger number of anions; a smaller central cation, fewer anions.  Atomic bonding The two polymorphs of carbon are diamond and graphite The diamond lattice is arranged in a tetrahedron Graphite lattice is arranged in sheets These minerals have the same chemical formula but very different properties A result of different types of atomic bonds.  Crystal growth Growth occurs as atoms attach to the outer surface. In an open cavity, crystal faces grow perfectly. Early crystals act as a seed for further growth of the mineral.  Crystal growth As crystals grow, they often encounter other growing crystals. Three types of crystals; euhedral, anhedral, subhedral  Mineral identification Minerals are identified based on physical and chemical properties These properties depend on both the chemical composition and crystal structure Mineral identification is a skill. It requires learning mineral properties and ways to test for them.  Mineral identification Minerals are identified based on their color streak luster hardness specific gravity crystal habit cleavage (or fracture) reaction to acid special properties*  Color Color is the part of the visible spectrum of light that is not absorbed by the mineral. Color is diagnostic of some minerals (malachite) but a poor indicator for others (quartz). Don’t use one mineral to describe another.  Hardness Hardness is the scratching resistance of a mineral Hard minerals can scratch soft minerals Directly linked to atomic bond strength The Mohs hardness scale is used as a guide to determine a minerals harness Hardness  Streak Streak is a property whereby a mineral leaves a crushed powder on an unglazed porcelain plate Minerals may leave a different streak than their outward color Streak  Luster Luster is a property that refers to the way that a mineral surface scatters light. The two main subdivisions of luster are metallic and nonmetallic (which has many subdivisions).  Specific gravity The density of a mineral as defined by the ratio of the weight of a volume of the mineral and the weight of an equal volume of water at 4°C You can develop a sense of specific gravity by “hefting” minerals in your hands A piece of lead “feels” heavier than a similar sized piece of quartz….  Crystal Habit Refers to the shape of a single crystal with a well formed faces, or to an aggregate of many well formed crystals. Common geometric shapes describe crystal habit Cubic, prismatic bladed, platy, needle like, fibrous, etc.  Special properties Other less common physical properties that are useful for identifying minerals: Effervescence (Reactivity with acid) – Calcium carbonate Magnetism- Magnetite, Pyrite, Hematite Taste- Halite Smell- Sulfur Feel (tactile response) - Talc Striations- Plagioclase Marking on paper- Graphite  Fracture and cleavage Different minerals break in different ways  Fracture and cleavage Cleavage is the tendency for a mineral to break along lattice planes with weaker atomic bonds If a mineral breaks to form distinct planar surfaces that have a specific orientation then we say the mineral has cleavage This surface is referred to as a cleavage plane. Fracture and cleavage  Fracture and cleavage Cleavage planes may be hard to distinguish from crystal faces  Fracture and cleavage Minerals that have no lattice planes of weakness—bonds equally strong in all directions—will fracture instead of cleave  Summary Minerals are the basic constituents of rocks They have a specific definition in geology Minerals are identified based on physical and chemical properties These properties depend on both the chemical composition and the crystal structure By observing these properties you will be able to identify minerals in the lab!