Lab 2 Minerals for online geology

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Cochise County Community College District *

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101

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Geology

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Dec 6, 2023

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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

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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