Rocks

Lesson 6: Rocks

Content Standard

The learners demonstrate an understanding of the three main categories of rocks, and the origin and environment of formation of common minerals and rocks.

Learning Competency

The learners shall be able to make a plan that the community may use to conserve and protect its resources for future generations. The learners will be able to classify rocks into igneous, sedimentary and metamorphic (S11/12ES- Ib-10).

Specific Learning Outcomes

At the end of the lesson, the learners will be able to

1.  Classify and describe the three basic rock types;

2.  Establish relationships between rock types and the origin and environment of deposition/formation;

3.  Understand the different geologic processes involved in  rock formation

ROCKS

Rocks are aggregate of minerals. It can be composed of single mineral (e.g. Quartzite, a metamorphic rock composed predominantly of Quartz) or more commonly, as an aggregate of two or more minerals. A mineral name can be used as a rock name (e.g. Gypsum Rock which is composed predominantly of the mineral Gypsum (CaSO4)).

A.      Igneous rocks - rocks that are formed from the solidification of molten rock material (magma or lava).  Molten rock material can solidify below the surface of the earth (plutonic igneous rocks) or at the surface of the Earth (volcanic igneous rocks).  Minerals are formed during the crystallization of the magma. Note that the rate of cooling is one of the most important factors that control crystal size and the texture of the rock in general.

Magma is a molten rock material beneath the surface of the earth. Lava is molten rock material extruded to the surface of the earth through volcanic or fissure eruptions.

Plutonic or intrusive rocks

·         from solidified magma underneath the earth

·         gradual lowering of the temperature gradient at depth towards the surface would cause slow cooling/crystallization

·         Phaneritic texture

·         Examples: granite, diorite, gabbro

Volcanic or extrusive rocks

•        from solidified lava at or near the surface of the earth

•        fast rate of cooling/crystallization due to huge variance in the temperature between Earth’s surface and underneath

•        common textures: aphanitic, porphyritic and vesicular

•        examples: rhyolite, andesite, basalt

•        pyroclastic rocks: fragmental rocks usually associated with violent or explosive type of eruption. Examples tuff and  pyroclastic flow deposits (ignimbrite)

Igneous rocks are also classified according to silica content: felsic, intermediate, mafic and ultramafic.

•        felsic: also called granitic; >65% silica, generally light-colored

•        intermediate: also called andesitic; 55-65% silica; generally medium colored (medium gray)

•        mafic: also called basaltic; 45-55% silica; generally dark colored

•        ultramafic: <45% silica; generally very dark colored; composed mainly of olivine and pyroxene which are the major constituents of the upper mantle

B.      Sedimentary rocks- These are rocks that formed through the accumulation, compaction, and cementation of sediments.  They generally form at surface or near surface conditions.

•        Sedimentary processes at or near the surface of the Earth include: weathering of rocks, sediment transport and deposition, compaction and cementation

•        Factors in sedimentary processes: weathering and transport agents (water, wind ice)

•        Common sedimentary features: strata and fossils

•        Strata: >1cm is called bedding and anything less is called lamination; layering is the result of a change in grain size and composition; each layer represents a distinct period of deposition.

•        Fossils: remains and traces of plants and animals that are preserved in rocks

Non-clastic / Chemical/Biochemical – derived from sediments that precipitated from concentrated solutions (e.g. seawater) or from the accumulation of biologic or organic material (e.g. shells, plant material).  They are further classified on the basis of chemical composition.

Clastic/terrigenous - form from the accumulation and lithification of sediments derived from the breakdown of pre-existing rocks.  They are further classified according to dominant grain size.

C.      Metamorphic rocks - rocks that form from the transformation of pre-existing rocks (igneous, sedimentary, or metamorphic rocks) through the process of metamorphism.  Metamorphism can involve changes in the physical and chemical properties of rocks in response to heat, pressure, and chemically active fluids. They are commonly formed underneath the earth through metamorphism.

Contact metamorphism

•        Heat as the main factor: occurs when a pre-existing rocks get in contact with a heat source (magma)

•        Occurs on a relatively small scale: around the vicinity of intruding magma

•        Creates non-foliated metamorphic rocks (e.g. hornfels)

Regional metamorphism

•        Pressure as main factor: occurs in areas that have undergone deformation during orogenic event resulting in mountain belts

•        Occurs in a regional/large scale

•        Creates foliated metamorphic rocks such as schist and gneiss

•        Non-foliated rocks like marble also form thru regional metamorphism, where pressure is not intense, far from the main geologic event.

THE ROCK CYCLE

The rock cycle illustrates how geologic processes occurring both at the surface and underneath the Earth’s surface can change a rock from one type to another.

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ALTERNATIVE LEARNING RESOURCE

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Minerals

LESSON 5

Minerals

Content Standard

The learners demonstrate an understanding of the three main categories of rocks.

Learning Competency

The learners shall be able to make a plan that the community may use to conserve and protect its resources for future generations. The learners shall be able to identify common rock-forming minerals using their physical and chemical properties (S11/12ES-Ia-9). 

Specific Learning Outcomes

At the end of the lesson, the learners will be able to

1.  Demonstrate understanding about physical and chemical properties of minerals

2.  Identify some common rock-forming minerals

3.  Classify minerals based on chemical affinity

MINERALS

Minerals are:

1.       Naturally occurring

2.       Crystalline solid

3.       Inorganic

4.       Definite composition

5.       Ordered internal structure

Composition of Minerals

1.       Silicates – minerals containing the two most abundant elements in the Earth’s crust, namely, silicon and oxygen. 

a.       When linked together, these two elements form the silicon oxygen tetrahedron - the fundamental building block of silicate minerals.

b.      Over 90% of rock-forming minerals belong to this group.

2.       Oxides – minerals composed of oxygen anion (O2-) combined with one or more metal ions

3.       Sulfates – minerals containing sulfur and oxygen in the form of the (SO4)- anion

4.       Sulfides – minerals containing sulfur and a metal; some sulfides are sources of economically important metals such as copper, lead, and zinc.

5.       Carbonates – minerals containing the carbonate (CO3)2- anion combined with other elements

6.       Halides – minerals containing halogen elements combined with one or more metals

7.       Native Elements – minerals that form as individual elements

a.       Metals and Intermetals – minerals with high thermal and electrical conductivity, typically with metallic luster, low hardness (gold, lead)

b.      Semi-metals – minerals that are more fragile than metals and have lower conductivity

(arsenic, bismuth)

c.       Nonmetals – nonconductive (sulfur, diamond)

PROPERTIES OF MINERALS

1.       Color

The most obvious property of a mineral, its color, is unfortunately also the least diagnostic. In the same way that a headache is a symptom for a whole host of problems from the flu to a head injury, many minerals share the same color. For example, several minerals are green in color – olivine, epidote, and actinolite, just to name a few. On the other extreme, one mineral can take on several different colors if there are impurities in the chemical composition, such as quartz, which can be clear, smoky, pink, purple, or yellow.

Part of the reason that the color of minerals is not uniquely diagnostic is that there are several components of the crystal compositions and structure that can produce color. The presence of some elements, such as iron, always results in a colored mineral, but iron can produce a wide variety of colors depending on its state of oxidation – black, red, or green, most commonly. Some minerals have color-producing elements in their crystal structure, like olivine (Fe₂SiO₄), while others incorporate them as impurities, like quartz (SiO₂). All of this variability makes it difficult to solely use color to identify a mineral. However, in combination with other properties such as crystal form, color can help narrow the possibilities. As an example, hornblende, biotite, and muscovite are all very commonly found in rocks such as granite. Hornblende and biotite are both black, but they can be easily distinguished by their crystal form because biotite occurs in sheets, while hornblende forms stout prisms (Figure 1). Muscovite and biotite both form in sheets, but they are different colors – muscovite is colorless, in fact.

Figure 1: These three minerals can be distinguished using both color and form. Hornblende (left) and biotite (middle) share the same color, but are different forms; muscovite (right) and biotite share form but not color.

2.       Crystal form/Habit

The external shape of a mineral crystal (or its crystal form) is determined largely by its internal atomic structure, which means that this property can be highly diagnostic. Specifically, the form of a crystal is defined by the angular relationships between crystal faces (recall Steno's Law of Interfacial Angles as discussed in our Minerals I module). Some minerals, like halite (NaCl, or salt) and pyrite (FeS) have a cubic form (see Figure 3, left); others like tourmaline (see Figure 2, middle) are prismatic. Some minerals, like azurite and malachite, which are both copper ores, don't form regular crystals, and are amorphous (Figure 2).

Figure 2: Examples of different types of crystal forms. On the left, pyrite has a cubic form; tourmaline (middle) is prismatic; azurite and malachite (on the right) are often amorphous.

Unfortunately, we don't always get to see the crystal form. We see perfect crystals only when they have had a chance to grow into a cavity, such as in a geode. When crystals grow in the context of cooling magma, however, they are competing for space with all of the other crystals that are trying to grow and they tend to fill in whatever space they can. The shape of the crystal can vary quite a bit depending on the amount of space available, but the angle between the crystal faces will always be the same.

3.       Hardness

The hardness of a mineral can be tested in several ways. Most commonly, minerals are compared to an object of known hardness using a scratch test – if a nail, for example, can scratch a crystal, than the nail is harder than that mineral. In the early 1800s, Friedrich Mohs, an Austrian mineralogist, developed a relative hardness scale based on the scratch test. He assigned integer numbers to each mineral, where 1 is the softest and 10 is the hardest. This scale is shown in Figure 3.

Figure 3: Mohs' scale of mineral hardness, where 1 is the softest and 10 is the hardest.

The scale is not linear (corundum is actually 4 times as hard as quartz), and other methods have now provided more rigorous measurements of hardness. Despite the lack of precision in the Mohs scale, it remains useful because it is simple, easy to remember, and easy to test. The steel of a pocketknife (a common tool for geologists to carry in the field) falls almost right in the middle, so it is easy to distinguish the upper half from the lower half. For example, quartz and calcite can look exactly the same – both are colorless and translucent, and occur in a wide variety of rocks. But a simple scratch test can tell them apart; calcite will be scratched by a pocketknife or rock hammer and quartz will not. Gypsum can also look a lot like calcite, but is so soft that it can be scratched by a fingernail.

Variations in hardness make minerals useful for different purposes. The softness of calcite makes it a popular material for sculpture (marble is made up entirely of calcite), whereas the hardness of diamond means that it is used as an abrasive to polish rock.

4.       Luster

The luster of a mineral is the way that it reflects light. This may seem like a difficult distinction to make, but picture the difference between the way light reflects off a glass window and the way it reflects off of a shiny chrome car bumper. A mineral that reflects light the way glass does has a vitreous (or glassy) luster; a mineral that reflects light like chrome has a metallic luster. There are a variety of additional possibilities for luster, including pearly, waxy, and resinous (see pictures in Figure 4). Minerals that are as brilliantly reflective as diamond have an adamantine luster. With a little practice, luster is as easily recognized as color and can be quite distinctive, particularly for minerals that occur in multiple colors like quartz.

Figure 4: Examples of only a few of the different lusters that can be seen in minerals. Galena (left) has a metallic luster, amber (middle) is resinous, and quartz (right) is glassy.

5.       Cleavage and fracture

Most minerals contain inherent weaknesses within their atomic structures, a plane along which the bond strength is lower than the surrounding bonds. When hit with a hammer or otherwise broken, a mineral will tend to break along that plane of pre-existing weakness. This type of breakage is called cleavage, and the quality of the cleavage varies with the strength of the bonds. Biotite, for example, has layers of extremely weak hydrogen bonds that break very easily, thus biotite breaks along flat planes and is considered to have perfect cleavage (see Figure 5). Other minerals cleave along planar surfaces of varying roughness – these are considered to have good to poor cleavage.

Figure 5: Several conchoidal fractures are visible in the mineral samples above. Note the concave surface and the curved ribs.

Some minerals don't have any planes of weakness in their atomic structure. These minerals don't have any cleavage, and instead they fracture. Quartz fractures in a distinctive fashion, called conchoidal, which produces a concave surface with a series of arcuate ribs similar to the way that glass fractures (see Figure 6). For quartz, in fact, this lack of cleavage is a distinguishing property.

6.       Density and Specific Gravity

The density of minerals varies widely from about 1.01 g/cm³ to about 17.5 g/cm³. The density of water is 1 g/cm³, pure iron has a density of 7.6 g/cm³, pure gold, 17.65 g/cm³. Minerals, therefore, occupy the range of densities between water and pure gold. Measuring the density of a specific mineral requires time-consuming techniques, and most geologists have developed a more intuitive sense for what is "normal" density, what is unusually heavy for its size, and what is unusually light. By "hefting" a rock, experienced geologists can usually guess if the rock is made up of minerals that contain iron or lead, for example, because it feels heavier than an average rock of the same size.

ALTERNATIVE LEARNING MATERIAL

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