Hardness is a measure of whether a mineral will scratch or be scratched. Mohs Hardness Scale, shown in Table Hardness 1 2 3 4 5 6 7 8 Mineral Talc Gypsum Calcite Fluorite Apatite Feldspar Quartz Topaz Hardness 9 10 Mineral Corundum Diamond With a Mohs scale, anyone can test an unknown mineral for its hardness. Imagine you have an unknown mineral. You find that it can scratch fluorite or even apatite, but feldspar scratches it. You know then that the minerals hardness is between 5 and 6. Note that no other mineral can scratch diamond.
Luster describes the reflection of light off a minerals surface. Mineralogists have special terms to describe luster. One simple way to classify luster is based on whether the mineral is metallic or non-metallic. Minerals that are opaque and shiny, such as pyrite, have a metallic luster. Minerals such as quartz have a non-metallic luster. Different types of non-metallic luster are described in Table 1.1. Luster Adamantine Earthy Pearly Resinous Silky Vitreous Appearance Sparkly Dull, clay-like Pearl-like Like resins, such as tree sap Soft-looking with long fibers Glassy The streak of hematite across an unglazed porcelain plate is red-brown.
Density describes how much matter is in a certain amount of space: density = mass/volume. Mass is a measure of the amount of matter in an object. The amount of space an object takes up is described by its volume. The density of an object depends on its mass and its volume. For example, the water in a drinking glass has the same density as the water in the same volume of a swimming pool. Gold has a density of about 19 g/cm3 ; pyrite has a density of about 5 g/cm3 - thats another way to tell pyrite from gold. Quartz is even less dense than pyrite and has a density of 2.7 g/cm3 . The specific gravity of a substance compares its density to that of water. Substances that are more dense have higher specific gravity.
color streak and luster
how are minerals identified
There are a multitude of laboratory and field techniques for identifying minerals. While a mineralogist might use a high-powered microscope to identify some minerals, or even techniques like x-ray diffraction, most are recognizable using physical properties. The most common field techniques put the observer in the shoes of a detective, whose goal it is to determine, by process of elimination, what the mineral in question is. The process of elimination usually includes observing things like color, hardness, smell, solubility in acid, streak, striations and/or cleavage. Check out the mineral in the opening image. What is the minerals color? What is its shape? Are the individual crystals shiny or dull? Are there lines (striations) running across the minerals? In this concept, the properties used to identify minerals are described in more detail.
other identifying characteristics
Some minerals have other unique properties, some of which are listed in Table 1.3. Can you name a unique property that would allow you to instantly identify a mineral thats been described quite a bit in this concept? (Hint: It is most likely found on your dinner table.) Chrysotile has splintery fracture. Property Fluorescence Magnetism Radioactivity Reactivity Smell Taste Description Mineral glows under ultraviolet light Mineral is attracted to a magnet Mineral gives off radiation that can be measured with Geiger counter Bubbles form when mineral is ex- posed to a weak acid Some minerals have a distinctive smell Some minerals taste salty Example of Mineral Fluorite Magnetite Uraninite Calcite Sulfur (smells like rotten eggs) Halite
Streak is the color of a minerals powder, which often is not the same color as the mineral itself. Many minerals, such as the quartz in the Figure 1.1, do not have streak. Hematite is an example of a mineral that displays a certain color in hand sample (typically black to steel gray, sometimes reddish), and a different streak color (red/brown).
Color may be the first feature you notice about a mineral, but color is not often important for mineral identification. For example, quartz can be colorless, purple (amethyst), or a variety of other colors depending on chemical impurities Figure 1.1.
cleavage and fracture
Breaking a mineral breaks its chemical bonds. Since some bonds are weaker than other bonds, each type of mineral is likely to break where the bonds between the atoms are weaker. For that reason, minerals break apart in characteristic ways. Cleavage is the tendency of a mineral to break along certain planes to make smooth surfaces. Halite (Figure 1.3) breaks between layers of sodium and chlorine to form cubes with smooth surfaces. Mica has cleavage in one direction and forms sheets (Figure 1.4). Minerals can cleave into polygons. Magnetite forms octahedrons (Figure 1.5). One reason gemstones are beautiful is that the cleavage planes make an attractive crystal shape with smooth faces. Fracture is a break in a mineral that is not along a cleavage plane. Fracture is not always the same in the same mineral because fracture is not determined by the structure of the mineral. Minerals may have characteristic fractures (Figure 1.6). Metals usually fracture into jagged edges. If a mineral splinters like wood, it may be fibrous. Some minerals, such as quartz, form smooth curved surfaces when they fracture. Sheets of mica.
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halite breaks into cubes because
a) it fractures along perpendicular planes. --> b) this is the way the weaker chemical bonds break. c) its fibers are perpendicular to each other and that is how it breaks. d) none of these.
a measure of whether a mineral can scratch another mineral or be scratched by another mineral.
a) appearance b) luster c) cleavage --> d) mohs hardness scale
mica has fractures in one direction and form sheets.
a) true --> b) false
apatite has a hardness of 5 and topaz has a hardness of 8. if a mineral can scratch apatite but is scratched by topaz, it has a hardness of
a) less than 5 or more than 8. b) 5 or 8 --> c) between 5 and 8. d) 7
radioactivity describes how much matter is in a certain amount of space.
a) true --> b) false
the way light reflects off of a minerals surface is its
a) cleavage b) streak --> c) luster d) color
--> a) glassy b) soft-looking with long fibers c) sparkly d) pearl-like
the main mineral that bubbles when a weak acid is put on it is
a) magnetite --> b) calcite c) quartz d) gold
diamond is the hardest mineral.
--> a) true b) false
which of these descriptions does not identify gold?
a) high density b) shiny surfaces --> c) reacts with acid d) distinctive color
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