3.5: Mineral Properties: Testable Properties

Mineral Properties: Testable Properties

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Heft & Specific Gravity

Heft is related to specific gravity Links to an external site., or density, of the mineral. When geologists are in the field they are unable to take with them the equipment necessary to calculate specific gravity. Instead the heft test is preferred. The mineral is held in the palm of the hand, and the overall heaviness of the mineral is assessed. Does the mineral feel very heavy for its size? Very light? The chemistry of certain minerals, for example the metal bearing minerals, will often result in a high heft and feel heavy in your palm. Other minerals, like kaolinite or sulfur will feel relatively lighter in heft for their size.

Specific gravity Links to an external site. is the ratio of a mineral’s weight to the weight of an equal volume of water. A mineral with a specific gravity of 2 would weigh twice as much as water. Most minerals are heavier than water, and the average specific gravity for all minerals is approximately 2.7. Some minerals are quite heavy, such as pyrite with a specific gravity of 4.9-5.2, native copper, with a specific gravity of 8.8-9.0, and native gold at 19.3, which makes panning useful for gold, as the heavy mineral stays behind as you wash material out of the pan.

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Feel

This is how the mineral feels beneath your fingertips. We want to move away from the term “texture”, as this term will have a distinct meaning as we move into our rock units. Minerals may have a distinct feel: for example, olivine typically has a gritty or granular feel, whereas talc often feels greasy or soapy.

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Streak

Streak Links to an external site. is the color of the mineral left behind on an unglazed porcelain plate, or a streak plate (Figure 3.6). A mineral’s streak color may be different from its outward color. Many minerals have a white streak, which is not particularly diagnostic. However, a mineral with a colored streak – black, blue, gray, green, or red – is often diagnostic. If you are ever unsure of the streak color, wipe your finger along the streak plate; a mineral with a white streak will leave a white powder behind that will rub on your finger. If your instructor provides both a white and black streak plate, try streaking the mineral on both plates. Recall, if a mineral is harder than the ceramic streak plate, you will see a porcelain powder from the plate and not from the mineral.

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Tenacity

Tenacity Links to an external site. refers to the way a mineral resists breakage and represents the brittleness, flexibility or malleability of a mineral. It is different from a mineral's hardness. If a mineral shatters like glass, it is said to be brittle, like quartz, while minerals that can be hammered are malleable, like copper or gold. Minerals may be elastic, in which they are flexible and bend like a plastic comb but return to their original shape, like the micas. Sectile Links to an external site. minerals are soft like wax, and can be separated with a knife, like gypsum.

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Hardness

Hardness refers to the resistance of a mineral to being scratched by a different mineral or other material and is a product of the strength of the bonds between the atoms of a mineral. Whatever substance does the scratching is harder; the item scratched is softer. Hardness is based on a scale of 1 to 10, the Mohs Hardness Scale Links to an external site., created by a mineralogist named Friedrich Mohs (Table 3.1).

Table 3.1: Mohs Hardness Scale, a relative hardness scale, 1 represents a soft mineral, like talc, and 10 represents a hard mineral, like diamond. ​

Hardness Number	Mineral	Test Kit Item
10 (hardest)	Diamond	--
9	Corundum	--
8	Topaz	8.5 - Masonry drill bit
7	Quartz	--
6	Feldspar	6.5 - steel nail
5	Apatite	5.5 - Glass plate or knife blade
4	Fluorite	--
3	Calcite	3.5 - Copper
2	Gypsum	2.5 - Fingernail
1 (softest)	Talc	--

 

 

 

The Mohs’ scale lists ten minerals in order of relative hardness. Each mineral on the scale can scratch a mineral of a lower number. Your mineral kits will likely contain several items of a known hardness. The glass plate and steel nail have a hardness of 5.5, a copper penny, plate or sheet has a hardness of 3, and your fingernail has a hardness of 2.5. A mineral scratched by your fingernail is relatively soft, and its assessed hardness would be <2.5. When trying to scratch a surface, use force, but be cautious with the glass plate. ALWAYS lay the glass plate on a flat surface rather than holding it in your hand.

Occasionally mineral powder is left behind when a soft mineral scratches a harder surface, so feel for the groove or scratch. Materials of similar hardness may have difficulty scratching each other. For example, hornblende (amphibole) typically has a hardness right around glass, so it may or may not leave a scratch on the glass surface.

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Breakage

Minerals, even those with high hardness, can break. Typically, minerals break in one of two ways. A mineral can break with fracture or with cleavage. The type of breakage is dependent on the chemical bonds that are present at the atomic level of the mineral. There are many types of chemical bonds and forces that bind molecules together. The two most basic types of bonds are characterized as either ionic or covalent. In ionic bonding Links to an external site., electrons are being transferred from one atom to another in an ionic bond (Figure 3.7, left). These bonds are important to the formation of many minerals and are very strong. In contrast, covalent bonds Links to an external site. share electrons and are also often strong bonds (Figure 3.7, right). Most minerals are characterized by ionic bonds, covalent bonds, or a combination of the two, but there are other types of bonds that are important in minerals, including metallic bonds Links to an external site. and weaker electrostatic forces (hydrogen or Van der Waals bonds Links to an external site.). Metallic elements have outer electrons that are relatively loosely held. When bonds between such atoms are formed, these electrons can move freely from one atom to another. A metal can thus be thought of as an array of positively charged atomic nuclei immersed in a sea of mobile electrons. This feature accounts for two very important properties of metals: their electrical conductivity and their malleability. Molecules that are bonded ionically or covalently can also have other weaker electrostatic forces holding them together. Examples of this are the force holding graphite sheets together and the attraction between water molecules.

Recall, all minerals are characterized by a specific three-dimensional pattern known as a lattice or crystal structure. These structures range from the simple cubic pattern of halite (NaCl), to the very complex patterns of some silicate minerals. Two minerals may have the same composition, but very different crystal structures and properties are known as polymorphs Links to an external site.. Graphite and diamond, for example, are both composed only of carbon, but while diamond is the hardest substance known, graphite is softer than paper. Mineral lattices Links to an external site. have important implications for mineral properties. Lattices also determine the shape that mineral crystals grow in and how they break. For example, the right angles in the lattice of the mineral halite influence both the shape of its crystals (typically cubic), and the way those crystals break.

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Fracture

A mineral which contains chemical bonds that are equal, and the same throughout, will not have a pattern of breakage and is said to exhibit fracture Links to an external site.. This results in breakage that looks like when you break a glass or plate at home. Fracture surfaces are commonly uneven or conchoidal Links to an external site., a ribbed, smoothly curved surface similar to broken glass.

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Cleavage

A mineral which contains chemical bonds that are unequal, and are different throughout, will result in a pattern of breakage called cleavage. This results in breakage with a shape that looks like itself, every time the mineral is broken. As minerals are broken (such as with a rock hammer), any cleavage planes will result in flat surfaces parallel to the directions of weakness within the crystal, these are the zones of weakness.

A mineral may have one or more cleavage planes (Figure 3.8). Planes that are parallel are considered the same direction of cleavage and should only count as one.

• One direction of cleavage is termed basal or sheet cleavage. Minerals that display this cleavage will break off in flat sheets.
• Two directions of cleavage is termed prismatic.
• Three directions of cleavage at 90° is termed cubic.
• Three directions of cleavage NOT at 90° is termed rhombic.
• Four directions of cleavage is termed octahedral.

When 2 or more cleavage planes are present, it is important to pay attention to the angle of the cleavage planes. To determine the angle of cleavage, look at the intersection of cleavage planes. Commonly, cleavage planes will intersect at 90° (right angles) or 60°/120°. It can be challenging to distinguish cleavage from crystal form, particularly in imperfect samples. However, crystal form occurs as a mineral grows, while cleavage develops as a mineral breaks.