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Mineral Introduction

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Minerals: Building blocks of rocks By definition a mineral is Naturally occurring (synthetic diamonds not a mineral) Inorganic solid Ordered internal molecular structure Definite chemical composition Rock A solid aggregate of minerals Few rocks are composed almost entirely of one mineral calcite Obsidian & Pumice nonmineral matter both are crystalline glassy substances and coal solid organic matter

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Most unusual mineral? The basic definition of a mineral is: a material, composed of one or more chemical elements with a definite crystal structure and a chemical composition which is either fixed or variable within identifiable limits On this basis, ice is indeed a mineral: it is of hexagonal structure, of fundamental composition H 2 O, transparent and colourless, with Mohs hardness 1.5 and calculated specific gravity 0.917.

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Classification of Mineral groups Nearly 4000 minerals have been named Rock-forming minerals Common minerals that make up most of the rocks of Earths crust Only a few dozen members Composed mainly of the 8 elements that make up > 98% of the continental crust

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Physical properties of minerals Several physical properties are used to identify hand samples of minerals Observation of minerals Primary techniques: Luster Colour Streak Crystal Shape (habit) Tenacity Hardness Cleavage Fracture Specific gravity (density) Observation of minerals Secondary techniques: Taste Feel Smell Double refraction Chemical reaction to 10% HCL

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Luster Appearance of a mineral in reflected light Two basic categories Metallic - submetallic Nonmetallic - vitreous or glassy, silky, or earthy, greasy, pearly Galena (PbS) displays metallic Additionally the other optical property is the ability to transmit light, No light is transmitted it is described as Opaque. Light but no image is transmitted through a mineral it is said to be Translucent Light and an image is transmitted through a mineral it is said to be Transparent

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Colour Generally unreliable for mineral identification Often highly variable due to slight changes in mineral chemistry Exotic colorations of certain minerals produce gemstones Quartz (SiO 2 ) exhibits a variety of colours Multicoloured tourmaline

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Streak Color of a mineral in its powdered form Streaking a sample can also help determine between metallic and nonmetallic, Metallic minerals tend to have a dark streak whereas nonmetallic minerals tend to have a light streak Hardness if the mineral is harder than the streak plate = no streak

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Habit crystal shape Commonly used terms include A equant (equidimensional) bladed flattened in one direction, fibrous, tabular B prismatic faces that are parallel to a common direction C banded stripes of bands of different colours and textures, platy, blocky D botryoidal intergrown crystals representing a bunch of grapes Refers to the common characteristics of shape of a crystal or an aggregate of crystals

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Tenacity Describes a minerals toughness or resistance to breaking or deforming Some are BRITTLE such as the ionically bonded halite and fluorite, and these shatter when we hit them. Minerals with metallic bonds such as native copper are MALLEABLE and are easily deformed and hammered into different shapes Minerals that can be cut into thin shavings such as talc, gypsum are described as SECTILE While others most notably the micas are ELASTIC as they will bend and snap back into their original shape once the pressure is released

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Mohs Mineral Hardness Scale 1) Talc 2) Gypsum 3) Calcite 4) Fluorite 5) Apatite 6) Feldspar 7) Quartz 8) Topaz 9) Corundum 10) Diamond Softest Hardest 1 2 3 4 5 6 7 8 9 10 All minerals are compared to a standard scale called the Mohs scale of hardness

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Cleavage Tendency to break along planes of weak bonding Produces flat, shiny surfaces Described by resulting geometric shapes Number of planes Angles between adjacent planes

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Common cleavage directions When minerals break evenly in one or more directions then they are described by the number of cleavage planes and angle(s) at which they meet Fluorite, halite, and calcite all exhibit perfect cleavage

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Fracture & Density Fracture Absence of cleavage when a mineral is broken Quartz fractures into a conchoidal pattern Density Specific Gravity Weight of a mineral / weight of an equal volume of water Average value = 2.7 Most common minerals have a SG of between 2-3 Quartz has a SG of 2.65, the metallic mineral of Galena has a SG of roughly 7.5 and 24 karat gold has an SG of 20.

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Physical properties of minerals Magnetism Reaction to hydrochloric acid - Effervesce Malleability Double refraction Taste Smell Elasticity

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Structure of minerals Nicolas Steno (1638-1676) Law of Constancy of Interfacial Angles : angles between equivalent faces of crystals of the same mineral are always the same HOWEVER, minerals can be built of geometrically similar building blocks yet exhibit different external forms