Lesson 5: Minerals and Rocks

“I adore wearing gems, but not because they are mine. You can’t possess radiance, you can only admire it.”

-Elizabeth Taylor

Good day everyone! Today we are going to learn about the three main categories of rocks.

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

• Define what a mineral is.
• Give emphasis to the five requirements for a material to be considered a mineral (i.e. naturally occurring is not man-made or machine-generated, inorganic is not a byproduct of living things, etc.).

A.) Mineral Properties

MINERAL	HALITE (TABLE SALT)
Chemical Composition	NaCl
Luster	non-metallic – vitreous; transparent to translucent
Hardness	soft (2-2.5)
Color	white
Streak	white
Crystal Form / Habit	cubic
Cleavage	perfect cubic
Specific Gravity	light (2.2)
Other Properties	salty taste; very soluble; produces reddish spark in flame

Definitions of Mineral Properties

1. Luster – it is the quality and intensity of reflected light exhibited by the mineral a. Metallic – generally opaque and exhibit a resplendent shine similar to a polished metal b. Non-metallic – vitreous (glassy), adamantine (brilliant/diamond-like), resinous, silky, pearly, dull (earthy), greasy, etc.
2. Hardness – it is a measure of the resistance of a mineral (not specifically surface) to abrasion.

• The use of a hardness scale designed by German geologist/mineralogist Friedrich Mohs in 1812 (Mohs Scale of Hardness). One of the most important tests for identifying mineral specimens is the Mohs Hardness Test. This test compares the resistance of a mineral to being scratched by ten reference minerals known as the Mohs Hardness Scale. The test is useful because most specimens of a given mineral are very close to the same hardness. This makes hardness a reliable diagnostic property for most minerals.
• Friedrich Mohs, a German mineralogist, developed the scale in 1812. He selected ten minerals of distinctly different hardness that ranged from a very soft mineral (talc) to a very hard mineral (diamond). With the exception of diamond, the minerals are all relatively common and easy or inexpensive to obtain.

Photo from http://www.instructables.com/id/How-to-identify-a-Mineral/step3/Hardness/ (8/30/2015)

• Differences between the users of Mohs Scale of hardness

PROS	CONS
Easy to do the test	The scale is qualitative and not quantitative
Can be done anywhere, anytime as long as the place is not dark	Cannot be used to test accurate hardness of industrial materials
Mohs scale is highly relevant for field geologists to roughly identify minerals using scratch kits
Can be done without or few kits – handy

 

3. Color and streak – Color maybe a unique identifying property of certain minerals (e.g. malachite – green, azurite – blue). There are also lots of minerals that share similar or the same color/s. In  addition, some minerals can exhibit a range of colors.   The mineral quartz for example, can be pink (rose quartz), purple (amethyst), orange (citrine), white (colorless quartz) etc.  Streak on the other hand is the color of a mineral in powdered form. Note that the color of a mineral could be different from the streak. For example, pyrite (FeS2) exhibits golden color (hence the other term of pyrite which is Fool’s Gold) but has a black or dark gray streak. Streak is a better diagnostic property as compared to color.  Streak is inherent to almost every mineral.  Color maybe unreliable for identification as impurities within the minerals may give the minerals a different color.

Color and streak. From https://www.instructables.com/id/How-to-identify-a-Mineral/

4. Crystal Form/Habit –The external shape of a crystal or groups of crystals is displayed / observed as these crystals grow in open spaces. The form reflects the supposedly internal structure (of atoms and ions) of the crystal (mineral). It is the natural shape of the mineral before the development of  any cleavage or fracture. Examples include prismatic, tabular, bladed, platy, reniform and equant. A  mineral that do not have a crystal structure is described as amorphous. The crystal form also define the relative growth of the crystal in 3 dimension which are its length, width and height.

Various crystal forms. Taken from: http://maggiesscienceconnection.weebly.com/crystals.html

5. Cleavage – It is the property of some minerals to break along parallel repetitive planes of weakness to form smooth, flat surfaces. These planes of weakness are inherent in the bonding of atoms that makes up the mineral. These planes of weakness are parallel to the atomic planes and appear to be repeating within the mineral. When minerals break evenly in more than one direction, cleavage is described by the number of cleavage directions and the angle(s) between planes (e.g. cleavage in 2 directions at 90 degrees to each other).

Examples of cleavage in rocks. Source: http://academic.brooklyn.cuny.edu/geology/grocha/mineral/cleavage.html

6. Fracture – Some minerals may not have cleavages but exhibit broken surfaces that are irregular and non-planar. Quartz for example has an inherent weakness in the crystal structure that is not planar. Examples of fracture are conchoidal, fibrous, hackly, and uneven among others.
7. Specific Gravity – It is the ratio of the weight of a mineral to the weight of an equal volume of water. A bucket of silver (SG 10) would weigh 10 times more than a bucket of water (SG 1). It is a measure to express the density (mass per unit volume) of a mineral.  The specific gravity of a mineral is numerically equal to density.
8. Others – There are certain unique properties of minerals that actually help in their identification (e.g. magnetism, odor, taste, tenacity, reaction to acid, etc.). Magnetite is strongly magnetic; sulfur has distinctive smell; halite is salty; calcite fizzes with acid as with dolomite but in powdered form.

B.) Chemical Distribution

1. Silicates – minerals containing 2 of the most abundant elements in the Earth’s crust, namely, silicon and oxygen. When linked together, these two elements form the silicon oxygen tetrahedron – the fundamental building block of silicate minerals. Over 90% of the rock-forming minerals belong to this group.  Aside from Si (46.6 % by wt.) and O (27.7%), the other most common elements that make the earth’s crust are Al (8.1), Fe (5.0), Ca (3.6), Mg (3.1), Na (2.8) and K 2.6).

2. Oxides – minerals containing Oxygen anion (O2-) combined with one or more metal ions

3. Sulfates – minerals containing Sulfur and Oxygen anion (SO4)- combined with other ions

4. Sulfides – minerals containing sulfur anion (S2)- combined with one or more ions. Some sulfides are sources of economically important metals such as copper, lead and zinc.

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

6. Native Elements – minerals that form as individual elements

• Metals and Inter-metals – minerals with high thermal and electrical conductivity, typically with metallic luster, low hardness (gold, lead)
• Semi-metals – minerals that are more fragile than metals and have lower conductivity (arsenic, bismuth)
• Nonmetals – nonconductive (sulfur, diamond)

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

That’s it for today’s blog! I hope you learned a lot from this lesson. We from Earth Science Made Easy wish that you continue to

Sources:

• Rocha, G. (n.d.). MINERALS: Cleavage and Fracture, Retrieved from http://academic.brooklyn.cuny.edu/geology/grocha/mineral/cleavage.html
• Licuanan et al.(2016). Teaching Guide for Senior High School EARTH SCIENCE. Quezon City, Philippines: Commission on Higher Education.

 ABOUT  THE
AUTHOR

The author of today’s blog is Yraisah Raula D. Pascua. A grade 11 student of Aurora National Science High School. In her class she is known to be one of the creative when it comes to art.