Laboratory 4 Minerals A mineral: a) is naturally occurring; b) is solid; c) is inorganic; d) has a specific chemical composition which may vary within certain limits; and e) has an orderly internal atomic arrangement. All the above characteristics of a mineral determine its physical properties and outward appearance. There are many minerals that geologists and rock hounds can recognize based completely on one or two of its properties. There are many others that geologists need advanced tools to recognize. Some of the methods used for identifying minerals are x-ray diffraction, scanning electron microscopy, optical microscopy, and x-ray fluorescence. Because we are not equipped with these devices in our lab, we are going to use the properties that we can see to identify some common minerals. Properties of Minerals Luster is how a mineral surface reflects light. The two main types of luster are metallic and non-metallic. A mineral with a metallic luster typically reflects light and has a metallic look to it. Minerals with a non-metallic luster may have many different appearances. Some of the adjectives that may be used to describe them are glassy, waxy, pearly, milky, earthy, and resinous. When determining if a mineral is metallic or non-metallic be aware that darker minerals may appear to reflect light but are not always metallic. Thick pieces of biotite are an example of this. Color is a good diagnostic tool for a few minerals, one example is sulfur. There are, however, many minerals that occur in a wide range of colors. So using color alone is often not the best way to identify a mineral. An example would be fluorite, which may appear colorless, green purple, yellow or blue. Color would not be the primary property that you would want to use to identify fluorite. Streak is the color of the mineral when it has been ground into a fine powder. Streak is more useful than color when identifying a mineral such as fluorite. The numerous colors that fluorite may exhibit were listed above. Each of these colors will leave a white streak when powdered on a porcelain streak plate. In order for the streak plate to work the mineral must be softer than the streak plate (streak plates have a hardness of 6.5). Those minerals that are harder will not leave a streak. Hardness (H) is the measurement of how resistant the mineral is to scratching or abrasion. This can be measured by taking minerals or objects of a known hardness and attempting to scratch them. The Mohs Hardness Scale (Table 4.1, below) assigns minerals to a hardness value from 1-10. The minerals listed below for each hardness value are found in our hardness kits and are labelled with a number that reflects their hardness. The hardness of other available materials is also listed below. Table 4.1: Mohs Hardness Scale, with minerals and common materials Hardness 10 9 8 7 6 5 4 3 2 1 Mineral of that Hardness Diamond (hardest) Corundum Topaz Quartz Feldspar Apatite Fluorite Calcite Gypsum Talc (softest) Hardness blank blank blank 6.5 blank 5.5 blank 2.5 blank blank Common material of that hardness blank blank blank Porcelain plate, steel file blank Glass plate blank Fingernail blank blank The first step to determine hardness is to determine whether the mineral is harder or softer than glass (since a hardness of 5.5 is near the center of Mohs Hardness Scale). To compare your mineral to glass, hold the mineral firmly in one hand and attempt to scratch a corner or edge across the glass plate. If there appears to be a scratch on the surface of the glass, try to wipe it away. If it wipes away the mineral left a powder behind and it is softer than glass. If the scratch remains, the mineral is harder than glass. You can also double check the hardness by trying to scratch the mineral with the glass: if the glass scratches the mineral, the glass is harder, if not, the glass is softer. Some minerals have a range of hardness. A mineral with a hardness of 5 may be 5.5 in some samples or 4.5 in others, so one sample may scratch glass while another may not. When using your fingernail, attempt to scratch the mineral with your fingernail, and not your fingernail with the mineral. Cleavage is the tendency of a mineral to break along planes of weakness. Perfect cleavage describes minerals that tend to break along very smooth, flat surfaces. Poor cleavage does not break as smoothly. Cleavage is an excellent diagnostic tool. Unfortunately, we cannot turn every student loose with a rock hammer and a mineral to examine how the mineral cleaves or breaks. Therefore, it will be useful on some minerals and not as observable on others. Just because you may not see the cleavage on your specific sample does not mean that the mineral does not display cleavage. Minerals that do not display cleavage are said to fracture. Quartz is an example of a mineral which does not have cleavage and breaks along irregular surfaces. Your instructor will supply you with a sheet showing the different types of cleavage and their outward appearance, or you may refer to the chapter on minerals in your textbook. Number of Cleavage Directions 1 2 3 at 90° 3 not at 90° 4 Cleavage Type basal cleavage prismatic cleavage cubic cleavage rhombohedral cleavage octahedral cleavage Crystal habit is the outward shape of a crystal. A crystal is a solid bounded by smooth crystal faces which reflect the internal atomic arrangement. Perfect crystals are not common because minerals are often inhibited by space when growing. Specific Gravity represents the ratio of the mineral’s weight compared to an equal volume of water. It is similar to density. Compare the weight of several minerals by holding one in each hand. Minerals such as galena which contain metals tend to have a higher specific gravity. Other Physical Properties Magnetism- Minerals will attract a magnet. The most common example is magnetite. Double Refraction- Text or objects viewed through a transparent mineral may be seen as double. Clear samples of calcite demonstrate double refraction. Reaction to acid- Mineral will effervesce (bubble, fizz) when treated with dilute hydrochloric acid (HCl). Odor- Minerals which contain sulfur will often display a weak rotten egg smell when warmed between the hands. Examples of these would be sulfur, galena, and sphalerite. Feel- Some minerals may feel greasy, soapy, rough or smooth. Taste- This is not a tool you would want to use in lab, but minerals such as halite will have salty taste. Steps for determining mineral names Record your observations on the Mineral Identification Worksheets as you establish the physical properties of each mineral, and use those data to compare your samples with those in the mineral charts using the steps outlined below and the flowchart on the next page. Step 1: Determine if the mineral is metallic or non-metallic and use the appropriate ID chart. Step 2: Determine approximate hardness (harder or softer than glass, fingernail, etc.). Step 3: Determine the streak of the mineral. Remember to wipe the streak plate after each use because white streaks can only be seen by running your finger over the streak plate. Step 4: Once you have the data from Steps 1 through 3, use the other diagnostic properties listed on the mineral charts to determine which mineral you have. Mineral Identification Flowchart Table 4.2: Mineral ID Charts for Metallic Luster Luster Scratches Glass? (Hardness Value) Streak Cleavage Metallic Sometimes and (H=2-6) NonMetallic varieties exist Metallic Yes (H=6) Metallic Yes (H=6-6.5) Redbrown None Black None Metallic Brown None Metallic No and (H=3.5) NonMetallic varieties exist White to pale yellow to brown 6 directions, hard to determine Metallic No (H=3.5-4) Dark gray None Metallic No (H=2.5) Dark gray 3 directions at 90 Metallic No (H=1) Dark gray Metallic No (H=3) Copper red 1 direction, hard to determine None Sometimes (H=5.5) Greenish None black to black Color and Other Properties Dull (non-metallic luster) to sparkling gray (metallic luster), metallic varieties tend to be harder Dark gray to black, attracts magnet Brass yellow, often displays cubic crystals, also called ‘fool’s gold’ Black to brownish black, using the hand lens small crystals can be observed Color usually yellow-brown to brown or black, resinous, glassy, or metallic luster, when streaked or warmed between hands may get a slight rotten egg smell from sulfur Golden yellow or greenish yellow, often has tarnished brass appearance Silvery gray, very dense, cubic crystals common Gray to black, greasy feel, will write on paper Copper to brown, it may have a green coating, malleable Mineral Hematite Fe2O3 Magnetite Fe3O4 Pyrite FeS2 Chromite FeCr2O4 Sphalerite ZnS Chalcopyrite CuFeS2 Galena PbS Graphite C Native Copper Cu Table 4.3: Mineral ID Charts for Non-Metallic Light-Colored Minerals that are Hard
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