Mineral Identification Basics How to identify your mineral

Mineral Identification Basics PHYSICAL PROPERTIES HARDNESS HARDNESS is defined as the resistance a mineral has to being scratched - its “scratchability”. Hardness tests are done by scratching one mineral against another. The mineral that is scratched is softer than the other. The hardness of a mineral is directly related to the type of chemical bond and it strength. Diamond, hardest of all naturally occurring minerals, is covalently bonded. Each carbon atom shares 4 electrons with neighboring carbon atoms. Pyrite Crystals Hardness of 6.5

Mineral Identification Basics PHYSICAL PROPERTIES HARDNESS In this photo, a quartz crystal was rubbed across a glass plate. The result is that the glass plate was scratched. The quartz is harder than the glass. HINT: In doing a hardness test try to pick a smooth or flat surface on the mineral to be scratched. Try to pick a point or a sharp edge on the mineral that you think will do the scratching. Glass is usually a good place to start because it is in the middle of the hardness table, it has a flat, smooth surface and it is easily obtained. Hardness is determined by the manner in which a mineral can scratch another mineral or substance like glass, OR be scratched by something. The illustration shows a piece of QUARTZ that has been rubbed over a glass plate and the resulting scratch. Quartz is harder than glass.

Mineral Identification Basics PHYSICAL PROPERTIES HARDNESS MOH’S SCALE OF MINERAL HARDNESS 1. TALC 2. GYPSUM 3. CALCITE 4. FLUORITE 5. APATITE (*) 6. FELDSPAR 7. QUARTZ 8. TOPAZ 9. CORUNDUM 10. DIAMOND MOH’S SCALE OF HARDNESS represent minerals of increasing hardness that are somewhat easily obtained. As an example, talc is scratched by gypsum and gypsum is scratched by calcite and calcite is scratched by fluorite and so on. Of course nothing in nature scratches a diamond - except another diamond. OTHER MATERIALS COMMONLY USED: 2.5 - FINGERNAIL 3 - COPPER PENNY 4 – Iron Nail 5.5 - GLASS 6-6.5 - STEEL FILE 7 – Ceramic Tile Moh’s scale is a list of minerals with increasing hardness.(*)

Mineral Identification Basics PHYSICAL PROPERTIES HARDNESS Remember that hardness is the resistance a mineral has to being scratched - NOT how easily it breaks apart. Also be sure to determine the hardness of a mineral on a fresh surface whenever possible. Some minerals have a tendency to oxidize or corrode. These surface deposits usually have a different hardness than the fresh mineral. There are many “micaceous” ( having the appearance of mica, i.e., consisting of many small flakes packed tightly together.) minerals that will fall apart when pressed against a glass plate or mineral specimen during the testing process. Be aware that the friable nature of some minerals.

What does cleave mean? It means to break apart or to break in two pieces

Mineral Identification Basics PHYSICAL PROPERTIES CLEAVAGE CLEAVAGE is the property of a mineral that allows it to break repeatedly along smooth, flat surfaces. These are FLUORITE cleavage fragments. Cleavage is best observed when the specimen is broken. Not all minerals have cleavage. Some minerals have cleavage and fracture. With experience in breaking minerals an appreciation for their cleavage can be obtained. Common, inexpensive minerals to test are calcite and halite. The FLUORITE cleavage fragments are indeed just that - cleavage fragments. These are, however, cleaved skillfully. Taking a large crystal of Fluorite and breaking it will most likely not produce these nice octahedral fragments. The angles will be the same, but they will not be equally displayed on the fragments. It takes a lot of practice and a lot of Fluorite to get these fragments. Note also that Diamond has this same type of cleavage. These GALENA cleavage fragments were produced when the crystal was hit with a hammer. Note the consistency of the 90o angles along the edges.

Mineral Identification Basics PHYSICAL PROPERTIES CLEAVAGE Within this crystalline pattern it is easy to see how atoms will separate to produce cleavage with cubic (90o) angles. (*) It is similar to tearing a piece of paper that has perforations in it. The paper has a tendency to tear along the perforations. They are zones of weakness. (*) Cleavage is guided by the atomic structure. Atoms in minerals are held together by electrical forces. If the mineral is held together by electrical forces (bonds) that have different strengths in different directions, the mineral typically breaks along the weaker bonded planes to produce cleavage. In this example the lines represent breaks between the atoms that make up the mineral. Cleavage is guided by the atomic structure. (*)

Mineral Identification Basics PHYSICAL PROPERTIES CLEAVAGE These pictures show different cleavage angles and the quality of cleavage. Fluorite has cleavage in four directions. A thin sheet of Muscovite seen on edge. Mica has perfect cleavage in ONE direction. Mica is also elastic - it tends to spring back when bent.

Mineral Identification Basics PHYSICAL PROPERTIES CLEAVAGE Blocky Cleavage 2 directions Orthoclase feldspar has good cleavage in 2 directions. The blocky appearance of this specimen is a hint that it has cleavage. The clue that the specimen has cleavage is the fact that numerous faces will reflect light at the same time. The best way to get a feeling for cleavage is to break minerals. Orthoclase Feldspar

Mineral Identification Basics PHYSICAL PROPERTIES CLEAVAGE Common salt (the mineral HALITE) has very good cleavage in 3 directions. These 3 directions of cleavage are mutually perpendicular resulting in cubic cleavage. Halite cleavage cubes are crystalline in that they have an orderly internal structure, but, the cleavage fragments shown above are the result of breaking larger crystals. These cleavage fragments can be further broken smaller and smaller and with higher and higher magnification you would be able to see this cubic cleavage. The cleavage would continue down to the size of the unit cell. Upon breaking this truly microscopic building block of halite, you would no longer have halite. Minerals are defined by their internal structure and size and shape of their unit cell.

Mineral Identification Basics PHYSICAL PROPERTIES CLEAVAGE Rhombohedral Cleavage - 3 directions CALCITE Note that in the close-up view (the last picture) of the calcite fragments, there are many different shapes - BUT the cleavage ANGLES are preserved. Even these tiny fragments have rhombohedral cleavage.

Mineral Identification Basics PHYSICAL PROPERTIES FRACTURE FRACTURE is defined as the way a mineral breaks other than cleavage. Remember that minerals can have cleavage and fracture. Conchoidal (curved like a conch shell) is produced in materials that have equal bond strengths in all directions. Glass (although not a mineral) is an excellent example. When the glass is struck, shock waves pass through the glass producing the curved fractures. This is a piece of volcanic glass called OBSIDIAN. Even though it is NOT a mineral, it is shown here because it has excellent conchoidal fracture.

Mineral Identification Basics PHYSICAL PROPERTIES FRACTURE This Quartz crystal will be struck with a hammer to show how that the external form of the crystal does not repeat when broken. (The flat crystal faces are not cleavage faces.) This is a good example of conchoidal fracture. There are various grades of fracture. This quartz crystal displays conchoidal fracture very nicely. But a lot of quartz breaks unevenly or with sub-conchoidal fracture. Note the smooth curved surfaces.

Mineral Identification Basics PHYSICAL PROPERTIES STREAK STREAK is defined as the color of the mineral in powder form. Hematite on Streak Plate Streak is normally obtained by rubbing a mineral across a “streak plate”. This is a piece of unglazed porcelain. The streak plate has a hardness of around 7 and rough texture that allows the minerals to be turned to a powder. This powder is the streak. The streak of a mineral may look very different than the color of the mineral. Streak is very useful for dark colored and metallic minerals. Hematite has a reddish brown streak.

Mineral Identification Basics PHYSICAL PROPERTIES STREAK Sphalerite is a dark mineral, however, it has a light colored streak. Next to the reddish brown streak of hematite is a light yellow streak. This is the streak of the sphalerite. Sphalerite powder also has a distinctive “rotten eggs” smell. Light colored streaks are often difficult to see against the white streak plate. It is often useful to rub your finger across the powder to see the streak color. Sphalerite has a light yellow streak.

Mineral Identification Basics PHYSICAL PROPERTIES LUSTER LUSTER is defined as the quality of reflected light. Minerals have been grossly separated into either METALLIC or NON-METALLIC lusters. Following are some examples: The determination of luster is an acquired skill. Although some lusters are obvious (like metallic or earthy), some lusters are more difficult to distinguish without practice. For example graphite has a “greasy” to “sub-metallic” luster. Some minerals “look hard” and have an adamantine luster. The best way to appreciate this physical property is to see actual specimens. It is difficult to capture the subtle variations in luster in an image. Native Silver has a Metallic Luster. (*)

Mineral Identification Basics PHYSICAL PROPERTIES LUSTER METALLIC Stibnite Galena Marcasite Pyrite These simply look like metals. The basic idea for Metallic Luster is that the minerals look like metals. (*)

Mineral Identification Basics NON-METALLIC LUSTER VITREOUS Wulfenite Olivine - Peridot Spinel Quartz Vitreous Luster means that the mineral has a “glassy” look. Normally we think of glass as being clear, but there are many different colors of glass and they are all very “glassy” looking. Even china plates and glazed porcelain are vitreous. Here are some examples: (*) The term VITREOUS means “glassy”, having the appearance of glazed porcelain. Glass is also vitreous. The important concept to remember is that minerals with vitreous lusters can come in a wide variety of colors. They can be black, clear, white or any combination of colors.

Mineral Identification Basics NON METALLIC LUSTER Miscellaneous Lusters Apophyllite – Pearly (*) Asbestos - Silky Graphite has a greasy or submetallic luster and easily marks paper. Limonite - Dull or Earthy Sphalerite - Resinous There are yet other types of lusters including waxy, adamantine and oily. The prefix sub- is also used, e.g., sub metallic, sub vitreous, etc.

Mineral Identification Basics PHYSICAL PROPERTIES LUSTER This is the same piece but the left side has been buffed with a steel brush. Note the bright metallic luster. The moral to this story is to look at a fresh surface whenever possible. Other good examples of minerals that look very different on a fresh surface are bornite and chalcopyrite - both copper sulfides. This piece of Native Copper is severely weathered. It does not look metallic.

Mineral Identification Basics PHYSICAL PROPERTIES COLOR The COLOR of a mineral is usually the first thing that a person notices when observing a mineral. However, it is normally NOT the best physical property to begin the mineral identification process. Also note the colors of FLUORITE on slide #15. Various colors of CALCITE.

Mineral Identification Basics PHYSICAL PROPERTIES COLOR Amethyst Ionic Iron Clear - Without Impurities Quartz comes in a wide range of colors. It is very easily colored by even trace amounts of impurities. Chlorite inclusions Hematite Inclusions Quartz comes in a wide range of colors. It is very easily colored by even trace amounts of impurities. Various colors of Quartz.

Mineral Identification Basics INDICATIVE COLOR Turquoise Rhodochrosite Some minerals do have a certain color associated with them. Here are some examples: (*) Malachite Azurite (*) Sulfur no notes

Mineral Identification Basics PHYSICAL PROPERTIES MAGNETISM MAGNETISM is the ability of a mineral to be attracted by a magnet. Much of the black sand found while gold panning is magnetite and can be removed by a magnet. Also, most meteorites are magnetic. NOTE: Being magnetic is NOT the same as being a magnet. This is a piece of MAGNETITE with a magnet adhering to it. Magnetite is a mineral that is strongly magnetic in that a magnet will easily be attracted to it.

Mineral Identification Basics PHYSICAL PROPERTIES MAGNETISM Obviously, not all magnetite will be a magnet and attract bits of iron. LODESTONE is a variety of Magnetite that is naturally a magnet.

Mineral Identification Basics CHEMICAL PROPERTIES REACTION TO HYDROCHLORIC ACID Some minerals, notably the carbonates, react to cold dilute HCl. In this illustration a piece of CALCITE is shown to react (fizz) after HCl is applied. This reaction neutralizes the acid. The bubbling is the release of carbon dioxide. The “H” of the HCl combines with oxygen to produce water and the “Cl” combines with the Ca of the calcite to form a calcium chloride salt. Calcite Reacts to HCl (*)