Minerals

1. Minerals A mineral is a naturally occurring, inorganic, solid which possesses a characteristic internal atomic structure and a definite chemical composition

2. Characteristics of Minerals • Minerals must occur naturally. This means man-made substances such as steel would not be considered to be minerals • Inorganic substances are those substances that are not living and are not formed by living processes • Crystalline solids are those solids in which the atoms composing the solid have an orderly, repeated pattern • Minerals will have definite chemical compositions, but these compositions may vary within given limits

3. Characteristics of Minerals (continued) • Substances that have these features: • Has a definite composition • Composed of Inorganic material • Has a crystalline structure • Occur Naturally. Not made by man. -----will also have distinctive physical properties such as color, crystal form, cleavage, luster, streak, etc

4. Properties of Minerals(p.16 of your reference table) • Properties of Minerals

5. Hardness • Friedrich Mohs, a German mineralogist, developed a hardness scale over 100 years ago. The hardest mineral known, diamond, was assigned the number 10

6. Moh’s Hardness Scale

7. How does Moh’s Scale of Hardness Work? • The Mohs Hardness Scale ranks the order of hardness of minerals and some common objects. For example, your fingernail can scratch the minerals talc and gypsum, with a hardness of 2 or lower. A copper penny can scratch calcite, gypsum, and talc. • A common misunderstanding of how to identify a diamond is that it will scratch glass. While this is true, other minerals can scratch glass, too, as long as they have a hardness >6.

8. Cleavage • Cleavage is the ability of a mineral to break along preferred planes

9. What Causes Cleavage? • Certain planes of weakness exist in some minerals because of their particular atomic structure. Atomic bonds may be weaker in some directions than in others, so the mineral will tend to break, or cleave, in that direction. Minerals may have cleavage in only one direction, in only two directions, or in three or more directions. The cleavage angles at which these planes intersect may be distinctive.

10. How is cleavage described? • Minerals that have "perfect" cleavage almost always break in a preferred direction. Minerals that have "good" cleavage sometimes will break in a particular direction, and other times they may not.

11. Types of Cleavage • Examples of Cleavage

12. Luster • Luster refers to how light is reflected from the surface of a mineral. The two main types of luster are metallic and nonmetallic

13. Fracture • Fracture is a description of the way a mineral tends to break. It is different from cleavage and parting which are generally clean flat breaks along specific directions. Fracture occurs in all minerals even ones with cleavage, although a lot of cleavage directions can diminish the appearance of fracture surfaces. • Different minerals will break in different ways and leave a surface that can be described in a recognizable way. Is the broken area smooth? Irregular? Jagged? Splintery? These are some of the ways of describing fracture.

14. What does Fracture look like? • The most common fracture type is conchoidal. This is a smoothly curved fracture that is familiar to people who have examined broken glass. Sometimes described as a clam-shell fracture. Quartz has this fracture type and almost all specimens that have been broken, demonstrate this fracture type very well.

15. Hematite: is rather variable in its appearance - it can be in reddish brown, dark silvery-grey scaled masses & silvery-grey crystals. What they all have in common is a rust-red streak. Olivine: has an olive green appearance. It does not streak but because its hardness is 6.5 expect it to scratch a glass plate. Minerals that show fracture

16. What is Metallic Luster? • Minerals exhibiting metallic luster look like metal, such as a silvery appearance or that of a flat piece of steel. • Metallic luster • Nonmetallic luster

17. Streak • The streak of a mineral is the color of the powder left on a streak plate (piece of unglazed porcelain) when the mineral is scraped across it. The streak plate has a hardness of glass, so minerals with a Mohs Hardness >7 will scratch the streak plate and won't powder the mineral.

18. Where do you observe streak? • A minerals streak is determined by rubbing it on a streak plate, which is a piece of unglazed porcelian. The streak plate is essentially a type of glass so it isn't used on minerals with a hardness greater than 7.

19. Why perform a streak test? • Streak can be useful for identifying metallic and earthy minerals. Nonmetallic minerals usually give a white streak because they are very light-colored. Other minerals may have very distinctive streaks; hematite, for example, always gives a reddish brown streak no matter what type of luster it displays.

20. Streak of Hematite • Hematite could show metallic or non metallic luster. However, when a streak test is performed the same color is always shown.

21. Color • Minerals are colored because certain wave lengths of light are absorbed, and the color results from a combination of those wave lengths that reach the eye. • Some minerals show different colors along different crystallographic axes. This is known as pleochroism.

22. How is color used in identifying minerals? • For some minerals, color is directly realted to one of the major elements and can be characteristic and serve as a means of identification. Malichite is always green, azurite is always blue, and rhondonite is always red or pink. • Most metallic minerals color is constant such as the brass-yellow of chalcopyrite and the copper-red or niccolite. These minerals may also tarnish which is espeically true of the mineral bornite. It is called the "peacock ore" because of the blue-velvet tarnish that develops on the surface.

23. How useful is color in identifying minerals? • The color of a mineral is the first thing most people notice. But it can also be the least useful in identifying a mineral. Most minerals occur in more than one color. Fluorite can be clear, white, yellow, blue, purple, or green. The other properties, such as hardness, cleavage, and luster, must be used instead.

24. Color should be the last characteristic used when identifying minerals. • Both of these samples are the mineral fluorite. Note that the color is different but the crystal shape is the same.

25. The Five Steps in Mineral Identification • Step 1Determine the luster (metallic or nonmetallic) of your mineral. • Step 2Determine the hardness of your mineral. Using a glass plate, see if the mineral scratches it. Be careful, make sure the glass is on a table, do not hold in your hand. Firmly grasp your mineral and draw it over the glass. If the mineral powders, then use your fingernail to feel if the glass is scratched.

26. The Five Steps in Mineral Identification (continued) • Step 3Determine whether your mineral is light-colored or dark-colored (nonmetallics only). • Step 4Determine whether your sample has cleavage. • Step 5Your choices have been narrowed down, using the chart (which you will be taken to) see which physical properties match up with your mineral.