Chapter 5

Minerals

Minerals• Minerals are the building blocks of rocks. They are non-

living, solid, and, like all matter, are made of atoms of elements. There are many different types of minerals and each type is made of particular groups of atoms. The atoms are arranged in a network called a crystal lattice. The lattice of atoms is what gives a mineral its crystal shape.

• Different types of minerals have different crystal shapes. Most minerals can grow into crystal shapes if they have enough space as they grow. But there are often so many different crystals growing in the same little area that they all compete for space and none of the crystals is able to grow very large.

WHAT IS A MINERAL?

• Minerals are homogeneous, naturally occurring, inorganic solids. Each mineral has a definite chemical composition and a characteristic crystalline structure. A mineral may be a single element such as copper (Cu) or gold (Au), or it may be a compound made up of a number of elements. About 2,500 different minerals have been described.

A. Formation of Minerals

• There are two main ways that new crystals of minerals grow. Some minerals form when molten rock, called magma below a planet’s surface and lava above, cools and atoms bond together into mineral crystals.

• Other minerals form when water that has atoms of dissolved elements in it, evaporates away. The atoms get very close to each other and may bond together to form solid minerals.

Composition and Structure• Silicate minerals

The most common mineral group on Earth is the silicate minerals, which all have the elements silica and oxygen as their main ingredients. Most silicate minerals form when molten rock cools, either at or near the Earth's surface or deep underground.

• Examples: Quartz, feldspar, mica, amphibole

• Non-silicate mineralsThere are many different groups of other minerals that are known as non-silicate minerals. Some of these groups form when magma cools, while others form when water evaporates away leaving mineral crystals behind, or when other minerals decompose.

• Examples: Gypsum, Halite, Graphite, Gold, Silver, Calcite, Pyrite and Corundum

Silicate Minerals

Quartz Micas

Feldspars Amphibole group

Non-Silicate Minerals

Hematite (Fe2O3) Gypsum (CaSO4 (+2H2O))

Halite (NaCl) Calcite (CaCO3) Halite (NaCl)

Sulfur (S) Pyrite (FeS2)

Non-Silicate Minerals

PROPERTIES OF MINERALS• Characteristics used in the identification & study of minerals.

These are the most common characteristics used when describing minerals.

• Color – this varies depending on the chemicals present and is the least informative in identifying a mineral variety

• Streak – is the color of the fine powder of mineral made against a streak plate.

• Luster– what the surface looks like in the light • Specific Gravity – how heavy it feels, heft• Crystal Form – shape of crystal, shape the mineral would

take if it had room to grow in a cavity, not massive – some minerals have a number of different crystal shapes

• Cleavage – pattern when mineral is broken – in planes or conchoidal

• Fracture - pertains to uneven, irregular and non-planar break• Hardness – what it can scratch & what scratches it

• LusterLuster is the way light is reflected by the surface of a mineral. The shiny surface of metals is called metallic luster. Other minerals have a glassy, pearly, or dull (earthy) luster.

• ColorMinerals can be very beautiful colors. However, if you are trying to identify a mineral, remember that the same type of minerals can have different colors. For instance, the minerals quartz can be found in many colors including pink, purple, white, or black.

• StreakStreak is the color of a mineral when it is powdered and it is often different from the color of the whole mineral. Minerals that come in different colors usually have the same color streak. To powder a little bit of a mineral, rub it against a small white piece of porcelain called a streak plate. You can’t measure streak with every mineral because some are too hard to powder against the streak plate.

• CleavageMinerals that have cleavage will break in a certain direction where the bonds between atoms are not strong.

• FractureMinerals that do not have cleavage will fracture when they are broken. If the fracture has a smooth curved surface it is called a conchoidal fracture, otherwise most minerals fracture irregularly.

Hardness• The harder a mineral is, the

less likely it is to be scratched. • Mohs scale number (mineral

example)1 (Talc)2 (Gypsum)3 (Calcite)4 (Fluorite)5 (Apatite)6 (Orthoclase Feldspar)7 (Quartz)8 (Topaz)9 (Corundum)10 (Diamond)

• Hardness of other common objects

• Fingernail: 2.5• Copper penny: 3.5 • pocket knife or common nail

(5.2) • piece of glass (5.5) • steel file or concrete nail (7.5)

• MOHS SCALE OF HARDNESS - a scale devised by Friedrich Mohs

Shapes of Mineral Crystals

Cubic crystals are not always cube shaped! There are many that are shaped like octahedrons (eight faces), and some that are shaped like dodecahedrons (10 faces).

Tetragonal crystals are shaped like cubic crystals but are longer in one direction making shapes like double pyramids and prisms.

Orthorhombic crystals are often shaped like rhombic prisms or dipyramids (two pyramids stuck together). They often look a bit like tetragonal crystals except that they are not square in cross section (when you look at the crystal on end).

Triclinic crystals are sometimes very strange shapes! They are usually not symmetrical from one side to the other.

Hexagonal crystals often look like six-sided prisms. When you look at the crystal on end, it looks like a hexagon shape in cross section.

Monoclinic crystals look like tetragonal crystals that have been skewed. They often form prism shapes and double pyramids.

CLEAVAGE• Cleavage is when a mineral breaks with smooth

flat surfaces. • Cleavage can be described as perfect, good,

imperfect, poor. It can also be described as: • Perfect 1 way ~ breaks on one perfect cleavage

plane, crystals break into slices, sheets peel off • Perfect 2 ways ~ breaks into elongated boxy

shapes, 90 degree angles • Perfect 3 ways ~ breaks into perfect rhombs,

pieces look like squished boxes • No cleavage ~ does not break regularly

FRACTURE• Fracture is when a mineral breaks, but the

surface is not regular, does not show cleavage.Words that describe what a break in a rock or mineral looks like:

• Conchoidal ~ curved break like what happens with thick glass or bottle bottom, shell shaped, can be rough or smooth

• Jagged ~ metals, sharp point that scratches or snags fingertips, hackly

• Splintery ~ fibrous • Uneven ~ rough surface, not smooth

Special Properties

• Transparency - The ability to transmit light. Depending on a number of things, rocks & minerals can also transmit light. Many rocks that are opaque when in a chunk, are translucent when cut into very thin slices. Gems stones are often valued on how clear, or transparent they are.

• Special Properties– magnetism, chatoyancy, fluorescence, odor,, burn test