EARTH SCIENCE 11 CHAPTER 4 NOTES HOW TO KNOW THE MINERALS. Introduction. Background

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1 EARTH SCIENCE 11 CHAPTER 4 NOTES HOW TO KNOW THE MINERALS KEY Introduction Much of the Earth is covered with loose rock, soil, or sand. Beneath this loose material there is always unbroken solid rock which is attached to the entire mass of Earth. This is called BEDROCK. It is the outer part of the crust. BOULDERS are large unattached blocks of rock. Bedrock is probably the same kind of rock over a large area. It may be shale, granite, limestone, sandstone, or something else. Bedrock is composed of substances called MINERALS. MINERALS are elements or compounds that occur naturally in Earth's crust in solid, crystalline states. Not all substances are MINERALS. Minerals MUST BE naturally occurring, inorganic solids that have a crystalline structure (even if it is microscopic) with a specific chemical composition and definite physical properties. Minerals include quartz, mica, calcite, magnetite, and many others, ranging in size from tiny grains to pieces larger than pebbles. In some rocks, the minerals are CEMENTED together, in others they are MELTED together. LIMESTONE may only contain the mineral calcite. GRANITE always contains quartz, feldspar and at least one other mineral. Note: Limestone and granite are ROCKS not minerals. Background An element is a substance that cannot be broken down into simpler substances by ordinary means. There are 92 different elements which occur naturally on Earth. The Eight Most Common Elements in Earth's Crust Name Symbol % by Mass % by Volume Oxygen O Silicon Si Aluminum Al Iron Fe Calcium Ca Sodium Na Potassium K Magnesium Mg Total 98.5% 100.0% Usually these elements are combined with other elements. Sometimes elements, often called NATIVE minerals, are found uncombined, like gold, silver, platinum, copper, sulphur, and carbon. oxygen and silicon give QUARTZ aluminum, silicon, and oxygen give common CLAY

2 Distribution of Elements ATMOSPHERE HYDROSPHERE LITHOSPHERE O 2 21% O 85.79% O 46.6% N 2 78% H 10.67% Si 27.7% Ar 0.9% Cl 2.07% Al 8.1% CO % Na 1.14% Fe 5.0% other 0.07% Mg 0.14% Mg 2.1% other 2.26% Na 2.8% Ca 3.6% K 2.6% other 1.5% Crystals 1. The nature and size of atoms or ions will determine the SHAPES of molecules. 2. In minerals, the regular arrangement of these molecules will produce characteristic shapes called CRYSTALS. 3. There are 6 basic crystal shapes or CRYSTAL SYSTEMS: Cubic Orthorhombic Tetragonal Triclinic Hexagonal Monoclinic (see text page 39 for a summary chart with additional examples) Main Minerals and Metals Produced in Canada Many of the things that you see around the home are made from minerals and metals. This is just a partial list of important minerals and metals produced in Canada. Aluminum Copper Iron Ore Potash Chrysotile (a form of asbestos)* Diamonds Lead Salt Coal Gold Nickel Zinc *Scientific research has reaffirmed forcefully over the last few years the difference in risk between chrysotile fibres and other asbestos fibres called amphiboles. Studies on fibre biopersistance, numerous other recent scientific data, as well as modern production methods and safe work practices, have shown that without a doubt, when chrysotile is used in a safe manner, it does not present any significant risk to human health. (Rotterdam Convention 10/10/2006)

3 EARTH SCIENCE 11 CHAPTER 4 NOTES MINERAL IDENTIFICATION MINERAL IDENTIFICATION Minerals are the chemical compounds that make up the rocks of the Earth's crust. To identify minerals a series of properties are used. These properties are features of the substance that can be used to give clues to the identity of the minerals being investigated. A mineral may share a common property, such as color, but will have a UNIQUE property that will allow you to identify it. The properties used are: COLOR LUSTER HARDNESS STREAK CLEAVAGE DENSITY/specific gravity CRYSTAL HABIT/shape special properties: MAGNETISM RADIOACTIVITY FLUORESCENCE EFFERVESCENCE (bubbles in acid) TARNISH The main purpose of this chapter is to learn what the properties are and how to identify some of the common minerals of Canada. It is important that you learn the proper terms used to describe the properties, as these terms are in common usage worldwide. You are not expected to become a mineralogist, but it is expected that you will be able to use the properties to identify the most obvious of the common minerals. A list of the minerals in our sample sets and the minerals you are expected to know are included at the end of this handout. The set of minerals you will use was originally prepared by the Geological Survey of Canada and contains 30 common minerals. These are numbered and the name can be found on the list. Note that the samples used on the test will not be numbered so don't bother to learn the numbers. Each time you pick up a set and before you return it to the drawer please check that it is complete. As you work through this chapter pay attention to the properties discussed in these notes as they apply to the minerals you must learn. PROPERTIES OF MINERALS: COLOR Color is the first thing we notice about a mineral, and can be one of the most valuable properties for mineral identification. If a mineral has a fixed composition and is free of IMPURITIES, it will have a characteristic color. If a mineral has a composition which can vary, or tends to have impurities, then its color MAY NOT BE USEFUL for identification.

4 4 LUSTER The luster or "shine" of a mineral depends on the nature of its surface and the way it reflects light. The following terms are used to describe luster. Luster is independent of color, that is, the mineral will have a certain luster regardless of its color. Main Types of Luster: METALLIC NON-METALLIC There are other lusters, but these are the most common NON-METALLIC lusters. vitreous: shines like glass resinous: like wax earthy: like dry mud or clay, dull: adamantine: brilliant like diamond greasy oily STREAK The streak of a mineral is its COLOR when it is crushed into a fine POWDER. It is usually observed with the aid of a streak plate, which is a piece of hard white unglazed tile. The mineral is rubbed on the streak plate and the color of the trace or streak made on the tile is observed. Its streak may be the same color as that of the mineral itself or it may be quite different. Usually the streak is more dependable (shows less variation) than the actual color of the mineral. As a general rule, metallic minerals have a dark streak while nonmetallic have a lighter streak. The streak is often very DIFFERENT from that of the bulk (solid) mineral. With some minerals the streak is always the same color, but the bulk mineral may be a great variety of colors. For example, hematite in bulk is usually rust red, but may be orange, black or metallic black. Hematite always has a rust red streak. If the mineral you are testing is harder than the streak plate you will not be able to give a streak for it as it will simply scratch the tile. HARDNESS The hardness of a mineral is a measure of how resistant it is to scratching. This hardness is related to the chemical bonds between the atoms of the mineral. The hardness of a sample is found by comparing its resistance to scratching to that of a set of common minerals arranged in a scale from softest (most easily scratched) to hardest. The scale is called MOH S Scale of Mineral Hardness, after the man who devised it. Note that the hardness numbers are relative; they tell the ORDER, not how many times harder one mineral is than another. For example diamond is 1000X harder than talc, not 10X as the scale implies. To find the hardness of an unknown sample you SCRATCH it with samples from the hardness scale and scratch the hardness samples with the unknown until you find two of the scale that it falls between. That is, a scale mineral it will scratch and the scale mineral just above that will scratch it. Its hardness will be between those two scale numbers.

5 5 Example: A mineral that scratches gypsum (hardness 2) but is scratched by calcite (hardness 3) would have a hardness of 2 1/2. The hardness is not an exact measure but is useful to identify minerals whose other properties are similar. Unfortunately, a complete sampling of Moh s Hardness Scale minerals is not usually available for testing purposes so a simplified list of "scratch" samples is used. This is called the PROSPECTOR'S Scale. You must know both of these scales. (see combined chart below) MOH S SCALE OF HARDNESS Hardness Example Mineral very soft 1 talc everything will scratch it 2 gypsum fingernail will scratch it soft 3 calcite scratches gypsum, can be scratched by a penny semi-hard 4 fluorite will scratch a penny 5 apatite steel file or knife will scratch it hard 6 feldspar may scratch a knife 7 quartz scratches steel and glass easily 8 topaz scratches quartz very hard 9 corundum you can't scratch it hardest 10 diamond not in your set for obvious reasons PROSPECTOR S SCALE Hardness Test Item 2.5 fingernail 3.0 penny 5.5 knife 5.5 glass 6.5 steel file COMBINATION SCALE CHART Hardness Example Mineral very soft 1 talc everything will scratch it 2 gypsum fingernail will scratch it 2.5 fingernail

6 6 soft 3 calcite scratches gypsum and can be scratched by a penny 3.0 penny semi-hard 4 fluorite will scratch a penny 5 apatite steel file or knife will scratch it 5.5 knife 5.5 glass hard 6 feldspar may scratch a knife 6.5 steel file 7 quartz scratches steel and glass easily 8 topaz scratches quartz very hard 9 corundum you can't scratch it hardest 10 diamond not in your set for obvious reasons You must know the hardness of talc, gypsum, calcite, feldspar, quartz, corundum and diamond, and you should know HOW to determine the hardness of others. CLEAVAGE Cleavage is the tendency to SPLIT easily in certain directions. The planes in which a mineral will cleave or split is determined by the CRYSTAL STRUCTURE and the strength of the bonds between atoms in the crystals. Cleavage can occur in 1, 2, 3 or more directions and can usually be recognized by looking for the shiny faces on the sample. These are easily found when the sample is tilted back and forth in the light as the flat surfaces will sparkle. Some minerals have a characteristic angle between their cleavages, which allows us to distinguish them from minerals with otherwise similar properties. Amphiboles and pyroxenes, for example, are similar minerals, but amphiboles have two cleavages at 120 to one another, and pyroxenes have two cleavages at 90. DENSITY / SPECIFIC GRAVITY The density of a body is its mass per unit volume (g/ml or g/cm 3 ). It is numerically equivalent to the specific gravity (SG), which indicates how many times the body weighs more than an equal volume of water. For example, water always equals 1, so a mineral with a specific gravity of 5 weighs 5 times more than an equal volume of water. Generally, we can compare the specific gravity of minerals by noting their RELATIVE WEIGHTS (very heavy to very light). To estimate the density of a mineral, "heft" the sample, that is, toss it in your hand and compare its weight to the piece of massive quartz. You will say that the mineral has high density if it seems to be heavier than the quartz, medium if about the same and low if it seems to be lighter. More exact calculations of specific gravity can be made using a SPRING SCALE and the displacement of a volume of water but we will not be using that technique.

7 7 CRYSTAL HABIT The ARRANGEMENT of ATOMS in the mineral determines the shape of the crystal. Each crystal of a mineral is always the same shape and often unique from the shape of crystals for other minerals. Some of the most common crystal habits are listed below. You will only be asked to identify the most obvious of the types quartz, calcite, halite, galena, pyrite and fluorite. The usual appearance of the mineral and its crystal habit or shape can often be diagnostic of that mineral. DESCRIPTIVE terms for general appearance of a specimen include: crystalline - made up of groups of crystals granular -made up of small rounded or shapeless crystals of grains earthy - the consistency of hard dried clay massive appears to contain indistinctive crystals powdery forms or is covered with a fine dust flaky small thin layers fibrous thread-like If the mineral is crystalline, some general descriptive terms for the crystal habit include: tabular crystals stubby crystals long crystals cubes prisms See page 2 or 9 for a diagram of the six crystal systems. SPECIAL PROPERTIES: Some of the minerals have special unique properties that are useful in identifying them. MAGNETISM Magnetism occurs when there is an imbalance in the structural arrangement of the IRON ions. Iron is found in two principle ionic states called ferrous and ferric ions. The minerals that show this property are few. But the property is important because of this fact. Once a specimen is established as magnetic, identification becomes a rather routine exercise. The minerals that are magnetic range in magnetic strength from being capable of lifting steel rods to barely turning the needle on a compass. A compass needle is a good test device for testing magnetism, as is a magnet on a string that might sway near the specimen. Name the mineral that best exhibits this property. MAGNETITE

8 8 RADIOACTIVITY This is the spontaneous breakdown of atoms by giving off subatomic particles and energy. Elements which are radioactive are usually those with high atomic masses and they breakdown into lighter elements. The simplest method of detecting radioactivity is to use a GEIGER COUNTER. This device measures electricity produced as the subatomic particles pass through it. Which sample displayed radioactive properties? PITCHBLENDE FLUORESCENCE Some minerals will fluoresce (glow different colors) when placed under an ULTRAVIOLET light (UV or Black Light). List at least two samples that displayed fluorescent properties? CALCITE FLUORITE ACID TEST and EFFERVESCENCE CALCITE has a number of special properties. The most distinctive is its reaction with dilute acid. This reaction is called effervescence. When given the opportunity, perform this test at the ACID TEST STATION. Use only a few drops of hydrochloric acid (HCl) and DO NOT remove it from this area. Only the samples provided on the test tray may be tested. Calcite crystals may display another special property which is its ability to bend or refract light to form two images. What is this property called? DOUBLE REFRACTION TARNISH Some minerals will develop a FILM or coating when exposed to weathering. Identify the mineral that displays this property. BORNITE or Peacock Copper

9 9 Crvstal Habit: _:- =--,.,...,i; C I....,..,7. ;!: I ~ ;,_ :-.( (' CUBIC examples: halite galena pyrite COLUMNAR example: tourmaline TABULAR examples: barite corundum HEXAGONAL example: quartz RHOMBIC example: calcite or Iceland Spar FLAKY examples: mica (all types) FIBROUS example: asbestos Cleavage: A.C... v~~gelnon,~. Example: mueccivltit. B. Cleev111J8In two dlreotlona.t right ang!m. C. Cleavage In twodlnlcllone not et rlglllenglea. Example: leldejw. Eumple:.emphlbole.,D. CleavaGe In three dlrecti(li'la at right ang... Example: halite F. CleavaQeln lour dlrectlona. G. Cleav8ge In aia dlrec;tiont. Example: fluorite. Example: spbaler.lte.