Understanding Mineral Properties: Identification & Uses Guide

Geology 101 Laboratory 5 Minerals (94) Geology 101 Laboratory 2 Mineral Properties, Uses and Identification (Submit only pages 9,19 and 20) Introduction: Minerals: are defined as naturally occurring, inorganic, solids with a definite chemical composition and a regular, internal crystalline structure. The keys to this definition are the chemical composition and the crystalline structure. Different chemical compositions result in different minerals. A good example is the mineral plagioclase. Plagioclase is a member of the feldspar group, but there is more than one type of plagioclase. Albite and Anorthite are two examples. Albite has a chemical composition of NaAlSi 3 O 8 , while anorthite's chemical composition is CaAl 2 Si 2 O 8 . They are very similar, but different - therefore two different minerals due to small difference in chemical makeup. Quartz Crystals courtesy of Weller, Cochise College Different crystalline structures, or how the atoms and molecules are arranged, result in different minerals. A good example is diamond and graphite. Both minerals are composed of pure carbon (C). They have the same chemical composition, but two different crystalline structures - therefore, they are two different minerals. Graphite bonds in thin sheets and Diamonds bond in a 3 dimensional lattice which makes them very hard. 1

Geology 101 Laboratory 5 Minerals (94) From: http://www.chem.ufl.edu/~itl/2045/lectures/lec_h.html Physical Properties: Determination of the actual chemical composition and crystalline structure of a mineral is difficult without the proper equipment. In an introductory level lab it is impossible for us to determine these two aspects of a mineral. While many minerals share common physical properties, when all of a mineral's physical properties are examined, it often results in a unique set of physical properties which can be used to identify the mineral. The most common minerals properties (listed below) are used to identify hand specimen are the crystal form, color, streak, Luster, hardness, density and cleavage (fracture and parting). Mineral Physical Properties Chart PHYSICAL PROPERTY Definition* Testing Method Cleavage Breakage of a mineral along Examine the mineral for areas where the mineral is broken. Look for 2

Geology 101 Laboratory 5 Minerals (94) planes of weakness in the crystal structure. areas where the light reflects from planar surfaces. This can be easily confused with a crystal face and is the most difficult properties for students to master. Not all minerals have cleavage Color Visible light spectrum radiation reflected from a mineral. Look at the sample and determine its color - white, black, green, clear, etc. Be careful, the same mineral can come in many different colors. Crystal Form Geometric shape of a crystal or mineral. Examine and describe the geometric shape of the mineral - cubic, hexagonal, etc. Not commonly seen in most introductory lab samples. Fracture Breakage of a mineral, not along planes of weakness in the crystal structure. Examine the mineral for areas where the mineral is broken. Describe the breakage as either irregular or conchoidal (has the appearance of broken glass). Not all minerals have fracture. Hardness Resistance to scratching or abrasion. Use minerals of known hardness from the Mohs Hardness Kits. Scratch the unknown mineral with a known hardness to determine which mineral is harder. Continue doing this with harder or softer minerals from the kit until the hardness is determined. Luster Character of the light reflected by a mineral. Look at the sample to determine if the mineral is metallic in appearance (looks like a chunk of metal) or non-metallic (doesn't look like a chunk of metal). Magnetism Electromagnetic force generated by an object or electrical field. Use a magnet to determine if the magnet is attracted to the sample. Reaction to HCl Chemical interaction of hydrochloric acid and calcium carbonate (CaCO 3 ). Place one small drop of HCl on a sample and watch for a reaction - effervesces (bubbles). Specific Gravity Ratio of the mass of a mineral to the mass of an equal volume of water. Generally not determined in an introductory lab. Look this information up in your lab manual once the mineral has been identified. Streak Color of the mineral when it is powdered. Grind a small amount of a mineral into a powder on a porcelain streak plate and determine the color of the powder. Taste Nerve ending reaction in the tongue to different chemicals. Lick the mineral. (not recommended in an introductory lab - you don't know who has handled or licked the sample before you). Other Properties Fluorescence, Radioactivity Requires special equipment such as a UV lamp and geiger counter. These are not commonly tested for in an introductory lab. How to recognize minerals: 1. Crystal form Minerals are grouped into systems according to their crystal symmetry (regularity of form). The figure below shows the six main systems. 3

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Geology 101 Laboratory 5 Minerals (94) Modification of the basic shapes The six basic shapes can all be modified by cutting off corners or edges. For example, here are some of the ways that the cubic crystal can be shaped: 2. Color and Clarity Some minerals have more than one color for example; purple amethyst and yellow citrine are both varieties of quartz. In contrast, yellow is the only color of sulfur and is therefore a useful tool in identifying this mineral. Color is the least dependable way to identify most minerals. Mineral Crystals also vary in their clarity of color. They can be Transparent – see through Translucent – foggy like looking through a steamed up shower Opaque – impervious to light 3. Streak The streak is the color of the powder made by crushing a mineral. For example, hematite could take different forms and color but its streak is always reddish brown. Some minerals have may have to be crushed to a fine powder to obtain the streak. 4. Luster Is the way light reflects off the surface of a mineral. For example, pyrite and many sulfides have a metallic luster because they reflect most of the light hitting their surfaces. Types of luster are given below.  Metallic – looks like metal example: gold , silver ,copper etc  Vitreous (glassy) -- freshly broken glass example: quartz , tourmaline  Adamantine (brilliant) -- example: diamond  Resinous (like resin or sap from a tree) -- example: sphalerite  Greasy or waxy – oily or greasy looking example: turquoise  Pearly -- like a pearl example: talc  Silky – like strands of fine silk example: asbestos  Dull or earthy – no reflection like dry soil example: bauxite 5. Hardness 4

Geology 101 Laboratory 5 Minerals (94) We measure the hardness of a mineral by how easy we can scratch it using different tools like finger nails, piece of glass and piece of copper (usually a penny). Moh’s scale of hardness is used to test the hardness of an unknown mineral. Simply determine using the hardness kit what your mineral will scratch and what will scratch your mineral. 6. Cleavage Cleavage describes how a crystal breaks when subject to stress on a particular plane. If part of a crystal breaks due to stress and the broken piece retains a smooth plane or crystal shape, the mineral has cleavage. A mineral that never produces any crystallized fragments when broken off due to stress has no cleavage. Cleavage is measured by two factors: quality and number of sides exhibiting cleavage. Quality of cleavage can be categorized into four qualities:  Perfect  Good  Poor  None Minerals with perfect cleavage cleave (break neatly)without leaving any rough surfaces; a full, smooth plane is formed where the crystal broke. Minerals with good cleavage also leave smooth surfaces, but often leave some rough surface as well. In minerals with poor cleavage, the smooth edge is barely visible, since the rough surface is dominant. Minerals with no cleavage (none) never exhibit any cleavage, thus broken surfaces are jagged and rough. If a mineral exhibits cleavage, but it is so poor that it is hardly noticeable, it has "indistinct" cleavage. Minerals with good cleavage can be called "excellent" or "distinct". 5

Geology 101 Laboratory 5 Minerals (94) 7. Fracture: Fracture is the characteristic mark left when a mineral chips or breaks. Cleavage and fracture differ in that cleavage is the break of a crystal face where a new face (resulting in a smooth plane) is formed, whereas fracture is the "chipping" of a mineral. All minerals exhibit a fracture, even those that exhibit cleavage. If a mineral with cleavage is chipped a certain way, it will fracture rather than cleave. There are several terms used for various mineral fractures: Conchoidal - Fracture resembling a semicircular shell, with a smooth, curved surface. A good illustration of a conchoidal fracture is a large chip in a piece of glass. This fracture is also known as "shelly" in some references. Uneven - Fracture that leaves a rough or irregular surface. Hackly - Fracture that resembles broken metal, with rough, jagged, points. True metals exhibit this fracture. This fracture is also known as "jagged". Splintery - Fracture that forms elongated splinters. All fibrous minerals fall into this category. 6

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Geology 101 Laboratory 5 Minerals (94) Earthy or crumbly - Fracture of minerals that crumble when broken. Even or smooth - Fracture that forms a smooth surface. Subconchoidal - Fracture that falls somewhere between conchoidal and even; smooth with irregular rounded corners. Other Properties : Reaction to Acid -You can do an acid test by dropping a drop of dilute HCL onto a sample. If the acid fizzes then that is an indication that the mineral is probably Calcite or Dolomite. Sometimes these minerals will not fizz until they are powdered. Striations – are hairline like grooves on the cleavage surfaces of some minerals. Plagioclase feldspars have striations on surfaces of one of their cleavage planes. Potassium (orthoclase) feldspars have lines that resemble striations but are not. Look closely and you will see that they are thin and discontinuous and are actually exsolution lamellae . Magnetism – some metallic minerals may be highly to slightly magnetic. This is easily checked by testing with a magnet. Specific gravity – is the ratio of the density of a substance divided by the density of water. Since the density of water is 1 g/cm 3 the specific gravity of a mineral is equal to its density. Prelab 1. Which of the following terms are useful for describing the luster of nonmetallic minerals? (Select all that apply.) a. Vitreous b. Translucent c. Earthy d. Shiny e. Silky 2. True or False? A mineral with a lower Mohs Hardness number will scratch a mineral with a higher Mohs Hardness number. 3. ___ refers to the way the surface of a mineral reflects light. 4. Describe why it is not a good idea to use only color and clarity to identify a mineral. 7

Geology 101 Laboratory 5 Minerals (94) 5. A mineral cannot be scratched by a fingernail, but can be scratched by a copper coin. Using the hardness scales what would you estimate the hardness at? 6. Another mineral you find can scratch orthoclase feldspar, but it cannot scratch quartz. A porcelain streak plate has a hardness of 6.5. What would you do next to determine the hardness of your mineral sample? Explain. 7. A mineral sample has a mass of 30 grams and takes up 11.4 cm 3 . What is the density of this mineral? Show work. 8. What is this minerals specific gravity? Mineral Identification Below you will find photographs of 10 different minerals. On the last page of this lab you will find a chart that you will fill in for the characteristics of each mineral. Each mineral has a description next to it. Step 1: Read the description and examine the photograph. Step 2: Determine if this mineral is metallic or nonmetallic and then dark or light colored. Step 3: Find the correct mineral identification sheet (2A, 2B or 2C). These are located at the end of the lab. Step 4: Note the minerals hardness from the description. Step 5: If it is metallic, note the streak from the description. If it is nonmetallic, note whether or not it has cleavage and what type. Step 6: You will now read through the remaining descriptions and by process of elimination you will narrow down what the name of mineral is by matching it up as best as you can to the descriptions. Step7: Cross reference with the “Properties of Common Minerals Table”. Finish filling the chart with any information you can find. Mineral 1. This mineral has good cleavage. It reacts with acid. It can be scratched by glass but not by a fingernail. 8

Geology 101 Laboratory 5 Minerals (94) Mineral 2. This mineral can just barely scratch glass. It has good cleavage but only on two surfaces. Mineral 3. This mineral cannot scratch glass and cannot be scratched by a fingernail. It has a cubic crystal growth and a cubic cleavage. It has a dark gray streak. 9

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Geology 101 Laboratory 5 Minerals (94) Mineral 4. This mineral cannot scratch glass. It has a red streak and is attracted to a magnet. Mineral 5. This mineral can scratch glass. It has good cleavage in two directions. This mineral has fine lines through out it. Mineral 6. This mineral can easily scratch glass. It breaks like broken glass. It is commonly used as a gemstone. 10

Geology 101 Laboratory 5 Minerals (94) Mineral 7. A fingernail can easily scratch this mineral. It feels like a bar of soap in the hand. It has no cleavage. Mineral 8. This mineral can be scratched by a knife blade. It breaks easily in thin transparent sheets. The sheets are bendable. It leaves a white streak. Mineral 9. This mineral can scratched by glass. It has a blue streak. It grew into botryoidal crystals. It does not show any cleavage. 11

Geology 101 Laboratory 5 Minerals (94) Mineral 10. This mineral can scratch glass. It has a play of colors in the light. It has fine lines throughout. It has good cleavage planes on two sides. Properties of Common Minerals Amph ibole : Hardness value right around glass (~5.5-6); green to black, elongate, or rod-shaped crystals common; two cleavage directions not at 90°. Augite : Hardness value right around glass (~5.5-6); blocky green to black crystals common; two cleavage directions at 90°. Biotite : Dark colored mica mineral; sheet cleavage (1 direction); flexible. Calcite : Reacts with HCl; rhombohedral shape and cleavage. Corundum : hexagonal shape with flat ends; hardness of 9; many colors common, lab kits generally have dull, brown or purple varieties. If gem quality, red = ruby; blue = sapphire. Dolomite : Reacts with HCl only when powdered; rhombohedral shape and cleavage. Fluorite : Commonly cleaves in octahedral shapes. Hardness is less than glass, but greater than copper. Many colors are common. Galena : High heft; cube-shaped crystals common; silvery-blue color. Garnet : Typically dark red in color (although there are many varieties); may have hackly or uneven fracture; dodecahedron shape common. Graphite : Soft, greasy feel; will leave marks on paper. Gypsum : Soft, may be tabular or fibrous; transparent to opaque. Halite : Cubic crystals common; waxy feel; taste is distinct. Hematite : Brick-red streak; may be earthy or metallic in luster. 12

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Geology 101 Laboratory 5 Minerals (94) Hornblende : Hardness value right around glass (~5.5-6); green to black, elongate, or rod-shaped crystals common; two cleavage directions not at 90°. Kaolinite : Soft, earthy, white; massive; sticks to tongue. Kyanite : Blue color; bladed crystals; two different hardness values, H = 5, parallel to blade; H = 7 across blade. Magnetite : Magnetic; metallic; harder than glass. Muscovite : Colorless mica; sheet cleavage; transparent, flexible sheets. Olivine : Green color; granular or sugary feeling masses. Plagioclase feldspar var. Labradorite : Iridescent, dark gray; striations on cleavage faces; harder than glass. Plagioclase feldspar var. Albite : White to grey; striations on cleavage faces; harder than glass. Potassium feldspar : Commonly beige to pink or reddish, however Amazonite is blue-green; exsolution lamellae on cleavage faces; harder than glass. Pyrite : Metallic, brassy gold, cubic shape common; green-black streak. Pyroxene : Hardness value right around glass (~5.5-6); blocky green to black crystals common; two cleavage directions at 90°. Quartz : Harder than glass; glassy look; crystal shape, hexagonal prismatic; many colors common. Serpentine : Green, white, grey masses or fibrous. *Composes serpentinite, California’s state rock! Sphalerite : Yellow to black submetallic, vitreous, or resinous crystals with 6 cleavage faces; can form dodecahedrons; smells like rotten eggs when scratched, powdered, or treated with acid. Softer than glass (H=3.5-4). Sulfur : Yellow color and streak; light heft; brittle. Talc : Soft, greasy feel; pearly or dull luster common. Topaz : Harder than glass (H=8); adamantine luster, crystal shape, hexagonal prismatic; many colors common. Tourmaline : Crystals are long, slender to thick prismatic and columnar with a triangular cross-section; crystal faces may be striated. Color is commonly brown to black in color (Schorl), however, the pink version, watermelon tourmaline , can be found near San Diego, CA! 13

Geology 101 Laboratory 5 Minerals (94) Mineral Sample Color/ Clarity Luster Streak Hardness Cleavage Other Distinctive Properties Mineral Name 1 2 3 4 5 6 7 8 9 10 Post Lab 1. Marble is a rock that is composed primarily of the carbonate minerals calcite (CaCO 3 ) and dolomite (Ca,Mg(CO 3 ) 2 ). In this exercise, you are told that carbonate minerals react to a drop of acid. Based on this 14

Geology 101 Laboratory 5 Minerals (94) observation, why do you think people who care for monuments and sculptures made of marble are concerned about acid rain? 2. Using the maps below, answer the following questions. a. What minerals are mined in the state in which you are taking this course? List them. b. What region (West, Midwest, South or Northeast) of the United States supplies the most ore mineral deposits? Explain your answer. c. Which state mines the most gold and silver? d. Which state mines the most copper? 15

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Lab 2 Minerals for online geology

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