Mineral Introduction Cube Fluorite

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 What is a mineral? ASK CLASS Geologists define a mineral as any naturally occurring inorganic solid that possesses an orderly crystaline structure and well defined chemical composition. READ SLIDE Inorganic solid halite are found naturally in the ground and are minerals, what other substance on your table has a crystalline structure? Sugar, but this is considered organic and therefore not a inorganic solid. Shells secreted by animals are considered inorganic Ordered internal molecular structure, their atoms are aligned or arranged in an orderly manner and have a regular shape quartz. Chemical compounds, pyrite FeS2 consists of one iron atom and two sulfur atoms Ice is a great substance, as some of you know I am a fan of water and ice, but another amazing thing about ice is that because minerals are solids within temperature ranges found on the planets surface, and thus ice is considered a mineral, liquid and gas water are not. Rock is defined as a solid mass of minerals or mineral like matter that occur on our planet. Few rocks are composed almost entirely of one mineral – limestone is an example made from impure masses of calcite HOWEVER most rocks are more common as AGGREGATES of several kinds of minerals, the term aggregate implies that the minerals are joined in such a way that that the properties of each mineral are retained.

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 What kind of mineral do you think this is? Most people would probably not think of ice as a mineral. However, many of us have seen magnified images of delicate snow flakes, and the typical six-sided form, even though much embellished by finer structures, is evidence of internal order. 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 H2O, transparent and colourless, with Mohs hardness 1.5 and calculated specific gravity 0.917. On this basis, ice is indeed a mineral: it is of hexagonal structure, of fundamental composition H2O, transparent and colourless, with Mohs hardness 1.5 and calculated specific gravity 0.917.

Classification of Mineral groups Nearly 4000 minerals have been named Rock-forming minerals Common minerals that make up most of the rocks of Earth’s crust Only a few dozen members Composed mainly of the 8 elements that make up > 98% of the continental crust So now that we know how to identify the minerals, how do they get named and classified? READ SLIDE Ones we will need to know about are the rock forming minerals Is interesting to note that 8 elements make up the bulk of the rock forming minerals, and reprsent over 98% (by weight) of the continental crust. In order of abundance oxygen, Silicon, aluminum, iron, calcium, sodium, potassium, and magnesium.

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 Each mineral has a unique crystalline and chemical composition which give it its unique properties, such as hardness, density, appearance etc and several physical properties can be used to identify hand samples of minerals Primary techniques READ SLIDE For some minerals there are a few secondary techniques – READ SLIDE

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 The appearance or quality of light reflected off a mineral is known as luster. There are 2 types Metallic and nonmetalic, minerals that appear metal regardless of colour are said to be metallic. Some minerals which initially appear to be metallic have a reaction with the atmosphere, such as copper of galena and become dull or tarnished and these are then classified as having a submetallic appearance. Most minerals have a nonmetallic luster, and are described by using adjectives such as vitreous or glassy, silky or earthy, greasy or pearly (like a pearl) Additionally the other optical property is the ability to transmit light, when no light is transmitted it is described as Opaque. When light is transmitted but no image is transmitted through a mineral it is said to be TRANSLUCENT When light and an image is transmitted through a mineral it is said to be transparent. 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

Quartz (SiO2) exhibits a variety of colours Generally unreliable for mineral identification Often highly variable due to slight changes in mineral chemistry Exotic colorations of certain minerals produce gemstones Quartz (SiO2) exhibits a variety of colours Even though colour is the most conspicuous characteristic of any mineral it is considered to be a diagnostic property of only a few minerals Any slight impurity within the mineral composition can give it a different colours Additionally other minerals such as tourmaline often display various hues with multiple colours sometimes being present in the same sample Multicoloured tourmaline

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 No visual colour of a mineral may not always be diagnostic, but the colour of the powdered mineral often is and this is known as streak. This is achieved by rubbing the mineral across a plate and seeing what colour it is. A useful diagnostic tool 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 Be careful when streaking as it can be a difficult thing to pull off and often get you arrested

Habit – crystal shape Refers to the common characteristics of shape of a crystal or an aggregate of crystals 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 The term habit is used to refer to the common characteristics of shape of a crystal or an aggregate of crystals Using habit is useful if you remember that some minerals exhibit specific shapes, READ SLIDE, and some like halite are cubic form, but others we must use other diagnostic features as they have more than one characteristic form like pyrites Many do not develop perfect geometric forms, but may do develop a characteristic shape that we can use for identification, some grow equally in all 3D, others are elongate in one direction or flattened if growth in one direction is surpressed. Commonly used terms include equant (equidimensional) bladed A – flattened in one direction, fibrous, tabular, prismatic B - faces that are parallel to a common direction, banded – C stripes of bands of different colours and textures, platy, blocky and botryoidal D – intergrown crystals representing a bunch of grapes,

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 Brittle Tenacity – describes a minerals toughness or resistance to breaking or deforming.

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 1 2 3 4 5 6 7 8 9 10 All minerals are compared to a standard scale called the Mohs scale of hardness which measures resistance to abrasion or scratching. Goes from soft to hard. All minerals are compared to a standard scale called the Mohs scale of hardness Hardest

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 As we saw when briefly looking at the mineral structures, many have a structure or lattice that is not the same in every direction, and therefore have bonds that vary in strength. As a result these minerals will fail or break with preference along these weaker bonds, these broken fragments are bounded by more or less flat PLANAR surfaces , and this is known as CLEAVAGE Cleavage can be recognised by rotating a sample and looking for smooth, even surfaces that reflect light like a mirror. However cleavage surfaces can occur in small flat segments arranged in stair-step fashion The simplest form of cleavage is exhibited by micas, because they have such a weak bond in one direction than others

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 Not all cleavage is as simple as micas, there are many forms, 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 Fluorite, halite, and calcite all exhibit perfect cleavage

Fracture & Density Fracture Density Absence of cleavage when a mineral is broken 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. Quartz fractures into a conchoidal pattern When a mineral has structures that are equally or nearly equal in all directions, when we apply pressure they FRACTURE. The fracture pattern varies, such a quartz fractures into a conchoidal pattern, others into splinters or fibrous but most just fracture into irregular fragments Density of a mineral is expressed as mass per unit volume. Often as grams per cubic cm We use a related measure called Specific gravity to describe the density of minerals. Specific gravity is a unitless number representing the ratio of a minerals weight to the weight of an equal volume of water. 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. This can be used to develop a rule of thump, if you have a mineral in your hand, compare it to rocks of a similar size you have handled and then the SG is about 2.5-3, if not it has a different SG

Physical properties of minerals Magnetism Reaction to hydrochloric acid - Effervesce Malleability Double refraction Taste Smell Elasticity In addition to these there are a number of additional secondary techniques READ SLIDE Lick halite Talc feels soapy whilst graphite feels greasy Sulfur smells like rotten eggs Transparent calcite has an optical property of double refraction Drop HCL on to carbon ates and it will EFFERVESCE - fizz

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 The unit cells of halite join together in a regular manner creating the whole crystal. The shape and symmetry of the entire crystal relate to the shape and symmetry of the whole crystal. However, it is rare to find perfect crystals in nature, but the angles between the equivalent crystal faces of the same mineral are remarkably consistent. This observation was first made by Nicolas Steno in 1669. He found that the angle between adjacent prism faces of quartz crystals are 120 degrees regardless of sample size or provenance of the crystal. This observation is known as Law of Constancy of Interfacial Angles . Because it applies to all minerals it states that “angles between equivalent faces of crystals of the same mineral are always the same” Therefore crystal shape is often a valuable tool in mineral identification. HOWEVER, minerals can be built of geometrically similar building blocks yet exhibit different external forms. – Fluorite, magnetite and garnet are call constructed by cubic unit cells, yet they exhibit different shapes, and because the building blocks are so small they reslut in smooth flat surfaces.