1 Lab 1 Review of Mineral Properties Mineralogy A mineral is a naturally occurring, inorganic, homogenous solid with a definite (but sometimes slightly variable) chemical composition and a highly structured atomic arrangement. By analyzing the definition, naturally occurring will distinguish between those materials that are synthetically produced in a laboratory and those that are created by natural processes. All minerals are produced from various processes that are independent of living organisms and thus are inorganic. A majority of minerals are formed from some type of molten material derived from organic life. A homogenous solid refers to the mineral’s consistency throughout its mass and is neither liquid nor gas. Meaning that the mineral cannot be further subdivided into mineralogical components. The final two parts of the definition are in reference to its chemical composition and structure. Later in the course, it will become obvious that minerals can have alternative element constituents as part of its chemical formula. Generally, these variations are slight and the mineral is able to maintain its identity. The highly structured atomic arrangement refers to the atomic structure of the mineral. This indicates that there is an ordered framework of atoms building the mineral. And thus one comes to the subject of Mineralogy. Mineralogy is the study minerals and their various properties and uses. These aforementioned properties are the focus of this lab. Physical Properties In the beginning, when people of old began to study the importance of minerals from the earth, they had to examine what could be observed and tested by using various physical properties. Mechanical Properties Hardness The hardness of a mineral is its resistance to scratching or abrasion. Every mineral can be tested for its hardness by either Moh’s Hardness Scale or by a relative hardness scale. Moh’s Hardness Scale Hardness Mineral Hardness Mineral 1 talc 6 felspar 2 gypsum 7 quartz 3 calcite 8 topaz 4 flourite 9 corundum 5 apatite 10 diamond2 There is also a relative hardness scale that can be used in conjunction with the above scale. The items on this scale are common objects that can be used in testing the minerals. fingernail 2.2 glass plate 5.5 new penny 3.2 streak plate 7 steel nail 5 Cleavage The cleavage of a mineral is its tendency to break along weakened planes within the mineral. These areas of weakness are a result of weak chemical bonding within the atomic arrangement of the crystal. It is important to remember that not all minerals demonstrate cleavage. And in some minerals that are supposed to demonstrate a cleavage type, it is difficult to see these cleavages. There are several types listed below in the table. Cleavage Directions Angle Mineral Examples basal 1 - micas, graphite prismatic 2 ~60 or ~90 pyroxene, amphibole rhombohedral 3 not 90 calcite, dolomite cubic 3 90 halite, galena octahedral 4 ~70 flourite dodecahedral 6 60 sphalerite Fracture If a mineral does not exhibit cleavage of any type, then the mineral is said to fracture. The fracture of a mineral refers to its breakage in numerous random directions without any3 repetition of a set pattern. Fracture will occur in a mineral due to the internal chemical bonding being fairly uniform throughout the mineral’s mass. There are several terms for fracture which include conchoidal, hackly, and irregular. Tenacity Tenacity is defined as a mineral’s resistance to breaking. In other words, how difficult is it to break? Tenacity and hardness are two separate physical properties. Diamond, althought he hardest mineral, is very easy to break and shatter. Optical Properties These properties are defined by the way light interacts with the surface of the mineral. These interactions include absorption, refraction, reflection, and transmission of light. Color Color is a mineral’s most readily recognizable feature upon first observation. The absorption of certain wavelengths of light will produce a mineral’s color as only certain wavelengths will reach the observer’s eyes. However, the color of a mineral is not its most reliable characteristic. In many instances, the color of a mineral may vary greatly. Impurities of trace amounts of other elements present in the mineral may cause the differential coloration. For example, amethyst is merely quartz with trace amounts of ferric iron. Streak The streak of a mineral is a much more reliable test than the color of a mineral. The streak of a mineral is the representation of that mineral in its powdered form. This test allows several of the chemical bonds to be broken. The test is performed on a ceramic streak plate by taking the mineral and rubbing it on the streak plate in small circles to produce a pile of powder. In the identification of minerals, the streak test is more important for identifying those minerals with a metallic luster. Luster Luster is the way in which a mineral reflects light. The two major divisions of luster are metallic and nonmetallic. If a mineral has a metallic luster, then most of the light that “hits” the mineral is reflected back and is considered to be opaque. Nonmetallic minerals interact with the light differently by displacing a range of reflectivities and are considered to be translucent to transparent.4 There are several terms used to describe nonmetallic luster. They pertain to the amount of reflectivity that is displayed by the sample. Some of the terms include adamantine, resinous, vitreous, pearly, silky, greasy, and earthy. Diaphaneity Diaphaneity refers to the way in which light is transmitted through a mineral. If a mineral does not allow light to pass through it, then the mineral is opaque. A translucent mineral will only partially allow the passage of light through it to the point that an image cannot be seen clearly on the other side. A transparent mineral allows an image to be seen through it as the light passes through with little distortion. Luminescence The ability of a mineral to emit light when ground or struck is triboluminescence, when the mineral is heated is thermoluminescence, and when the mineral is excited by ultraviolet light is fluorescence. Other Properties Specific Gravity Specific gravity is a ratio between a mineral’s mass and an equal volume of water. Thus a specific gravity of 3 means the mineral weighs three