Earth Materials Unit: Minerals

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Purpose: To classify minerals and mineral groups according to their chemical composition.

-By the end of this unit, students are expected to be able to:

1. Outline the importance and abundance of various elements in the Earth’s crust. (Text: 69, 76 to 81)

2. Differentiate between rocks and minerals. (Text: 66 to 68, 76; Lab: 57 to 58).

3. Demonstrate an ability to use the following properties in identifying minerals. (Text: 74 to 76; Lab: 59 to 69)

a) Simple crystal shape b) Cleavage c) Fracture

d) Hardness (Mohs’ hardness scale) e) Specific gravity (relative density)

f) Colour g) Streak h) Lustre

i) Special properties (e.g. magnetism, reaction to dilute HCL)

4. Describe, identify, and classify the following minerals using references or tests as appropriate. (Text: 68 to 71)

a) Silicates (e.g. quartz) b) Oxides (e.g. hematite) c) Sulphides (e.g. pyrite)

d) Carbonates (e.g. calcite) e) Sulphates (e.g. gypsum) f) Native elements (e.g. gold)

g) Halides (e.g. halite)

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Part A: What is a Mineral?

-A mineral is any matter that possesses all of the following characteristics:

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*Can you tell the difference between the diamond (on the left) and the cubic zirconia?

- While being extremely useful in their own right (the mineral copper is an important component of all electrical circuitry), they are also important as the building blocks of rocks.

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-The chemical formulas for some minerals can be very complicated, but we are still able to group them according to their composition.

Part B: Mineral Identification

-Minerals can be identified based on their properties:

1. Physical Properties

a) Colour:

-This is the outer visible overall colour of the mineral. Although colour is the most obvious property, it can often be confusing because many minerals come in a variety of colours (ex. quartz – colourless, purple, white, pink, red, brown, black, grey).

b) Streak

- The streak is the line of powder obtained by rubbing it upon a hard, rough white surface (porcelain – a streak plate). The streak often differs in color from the specimen, and serves as an important distinguishing character.

-There are a few minerals that are harder than a streak plate. If this is the case the mineral will leave a gouge in the streak plate and the powder that appears will be from the streak plate, not the mineral. Quartz is one common mineral that is hard enough to scratch a streak plate.

c) Lustre:

- The luster is the nature of a mineral surface with respect to its reflective qualities (How the mineral reflects light). Luster can be either: metallic (shiny) or non-metallic (earthy, waxy-like a candle, vitreous, greasy).

d) Hardness

- The hardness of a mineral is the comparative ability of a substance to scratch or be scratched by another. This is measured on a scale of 10 called Mohs Scale of Hardness. On this scale talc, the softest, has a hardness of 1 and a diamond, the hardest, has a hardness of 10.

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e) Cleavage:

-The cleavage is the tendency of crystals, certain minerals, rocks, etc., to break in preferred directions so as to yield more or less smooth surfaces (cleavage planes). This breakage reflects underlying structure in the mineral. For example, mica cleaves to form very thin sheets. Mica is a silicate mineral with a strong two dimensional silicate structure but weak bonds in the third dimension; this is why it cleaves to produce two dimensional sheets.

-Some minerals exhibit tiny grooves called striations on the face of a cleavage plane and these can be used to positively identify some minerals. For example, plagioclase feldspars have striations whereas potassium feldspars do not.

f) Tenacity:

- Tenacity refers to the manner in which a mineral resists breakage. Terms that are used for tenacity are brittle (shatters like glass), malleable (like molding clay or gold, can be hammered or bent into new permanent shapes), flexible (like a plastic comb, bends, but returns to its original shape), or splintery (breaks into splinters), for example.

g) Density:

An object’s density is its mass per unit volume. Density compared to that of water is called the specific gravity. The average density of rocks is about 3g/mL, therefore the common minerals that make up these rocks also have densities of about 3 g/mL.

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2. Chemical Properties

-For example: Reaction to dilute hydrochloric acid is a chemical property that is useful for the identification of carbonate minerals, which have calcium carbonate in their structural that is completely dissolved by HCL acid.

3. Unique Properties

-Some properties are unique to just a few minerals:

a) Magnetism: Works well with minerals with a high iron content (e.g. Hematite).

b) Phosphorescence and fluorescence: Some minerals will glow under UV light.

c) Optical Properties: Which of the samples below is Calcite? Which is Halite?

Here’s how to tell: Double Refraction!

d) Radioactivity:

-Some minerals contain radioactive elements like Uranium-235 that decay over time.

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-Uranium was a popular addition to glassware between the 1880s and 1920s in order to produce different transparencies and colours (yellow-green). These dishes glow under UV light.

Part D: Mineral List

-The following are minerals that you will need to be able to describe, identify, and classify using references or tests (streaks, hardness, etc) as appropriate.

I. Silicates: quartz and varieties of quartz, talc, augite/pyroxene, hornblende/amphibole, garnet, asbestos, olivine, feldspar: potassium feldspar, plagioclase feldspar, mica (muscovite, biotite)

II. Oxides: hematite, limonite, magnetite

III. Sulphides: pyrite, chalcopyrite, galena, sphalerite, molybdenite, bornite

IV. Carbonates: calcite, malachite, azurite

V. Sulphates: gypsum

VI. Phosphates: apatite

VII. Native elements: graphite, gold

VIII. Halides: halite, fluorite

Part D: Classification of Minerals

-Only about 24 minerals are common despite there being over 3500. There are not more because:

a) Some combinations of elements do not occur.

b) Bulk of Earth’s crust is made up of only 8 chemical elements, with silicon and oxygen being by far the most common.

1. Silicates: SiO42-

-Silica= silicon and oxygen. Minerals containing these are silicates.

-95% of Earth’s crust.

Most common type of minerals, with 95% of Earth’s crust being made of silicates. Every silicon atom surrounds itself with four oxygens. The bonds between the silicon and its four

oxygens are very strong. The silicon atom and its four oxygens form a pyramid called the silicate tetrahedron, with

silicon in the center and oxygens at the four corners. The silicate tetrahedron is the fundamental building block of all silicate minerals. The silicate tetrahedron has a negative charge, forming a complex anion (ion with an overall negative charge of 4. Because of this it will combine with positively charged ions or share its oxygen atoms with other silicate tetrahedrons).

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Silicate tetrahedra link together by sharing oxygens. Thus, two tetrahedra share a single oxygen, bonding the two tetrahedra together. *Show Text Figure 2.23 on page 49.

A) Single or isolated tetrahedron (ex. olivine – dark, dense mineral)

Olivine: Olivine occurs in small quantities in both continental and oceanic rocks. However, olivine and pyroxene make up most of the mantle.

B) Single chain (ex. pyroxene, is used as an ore of lithium and for making steel)

Pyroxene: It is a major component of oceanic crust and the mantle and is abundant in some rocks of the continents. Amphibole (below) is also a group of minerals with similar properties. It is common in many rocks of the continents. Pyroxene and amphibole can resemble each other so closely that they are difficult to tell apart.

C) Double chains (ex. amphibole/hornblende, fibruous varieties used for fire-resistant clothing, tiles and brakes)

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D) Sheet chains (mica – biotite { fire resistant tiles, paint, rubber} or muscovite {computer chip substrates, electrical insulation, roof shingles, make-up})

Mica: Has a platy habit and perfect cleavage. Both result from the sheet linkages of silicate tetrahedra. Mica is common in continental rocks. The clay minerals are similar to mica in structure, composition, and platy habit. Individual clay crystals are so small that they can barely be seen with a good optical microscope. Most clay forms when other minerals weather at the Earth's surface. Thus, clay is abundant at and near the Earth's surface and is an important component of soil.

E) Three dimensional (ex. Quartz: used as an abrasive, to make glass, gemstones, also in watches “quartz movement” where crystal is integrated into electronics. When you run an electrical current through quart it oscillates at an almost perfect frequency. This is used by the time keeping device as a constant to measure time against. Good quartz watches are accurate to a few seconds a year!)

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Quartz: Pure Si02. It is the only silicate mineral that contains no cations other than silicon. It is widespread and abundant, in continental rocks but rare in oceanic crust and the mantle.

*Silicates come in 2 forms:

a) Ferromagnesian silicates (FeMg silicates)

- contain iron (Fe), magnesium (Mg), or both

- dark colour and dense minerals (ex. olivine, pyroxene, amphiboles, biotite).

b) Non-ferromagnesian silicates

- lack iron and magnesium

- light coloured, less dense (ex. plagioclase feldspar, potassium feldspar, quartz, muscovite)

2. Oxides: contain an element and oxygen

-Oxides: An element combines with oxygen. Rocks with high concentration of are iron ores.

ex. hematite (Fe2O3) limonite

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3. Sulphides: contain S2- (sulphide ion)

-Sulfides: Contain a sulfur ion.

ex. pyrite (FeS2) fool’s gold chalcopyrite

galena sphalerite

4. Carbonates: contain CO32- (carbonate ion) and fizz when exposed to HCL.

-Contain negatively charged carbonate radical and include calcium carbonate as minerals aragonite or calcite.

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ex. calcite (CaCO3) malachite azurite

5. Sulphates: contain SO42- (sulphate ion) VI. Phosphates: contain PO4

3- (phosphate ion)

ex. gypsum (CaSO4∙H20) ex. apatite - Ca5(PO4)3(F,Cl,OH)

-Sulphates: Contain a complex radical: Sulphate ion

-Phosphates: Contain phosphate ion

6. Native elements: pure metals or materials that occur naturally

-Native elements: only one element involved

ex. gold (Au) graphite (Pb)

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7. Halides: contain Cl-, F-, (Group VII of the periodic table) etc…

-Halides: Contain halogen elements fluorine and chlorine.

ex. halite (NaCl – salt) fluorite (occurs in many colours)

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