Mineral Properties Lab

Learning Objectives Classify and visually identify common minerals based on their physical properties.

Materials Vinegar or lime juice

The following items from the Rocks and Minerals Kit: o Hand Lens o Magnet o Nail o Copper stripe o Glass streak plate o Ceramic streak plate o 30 mL plastic bottle (for vinegar or juice) o Bag of minerals

Introduction Almost all rocks are made up of smaller components called minerals. Minerals are composed of chemical elements (like carbon and hydrogen). There are 92 naturally occurring elements, each with unique characteristics. The smallest bit of an element that still retains its identifying characteristic is called an atom. The atoms of most elements on Earth cannot exist as single particles. Instead, they usually combine with other elements to form a solid chemical compound. This is what the geologist calls a mineral. A standard definition states that a mineral is (1) a naturally occurring chemical compound; (2) a crystalline solid (3) an orderly arrangement of atoms and molecules forming a crystal; (4) a specific chemical composition that can be closely described with a chemical formula; (5) formed inorganically. I typed that like I said it all in one breath. Salt is a mineral: halite (from the Greek halos, meaning "salt"). It (1) occurs in nature; not man-made; (2) it’s a solid; not a liquid or gas; (3) salt crystals form into cubes (some minerals form hexagons or pyramids); (4) the chemical formula is NaCl, sodium chloride, water is H2O, carbon dioxide is CO2;(5) no creatures or plants made the salt. For instance, a seashell is mostly calcium carbonate, CaCO3, formed by a mollusk. Once formed, a mineral can be changed by breaking the chemical bonds between the atoms. This can be done in nature by the same processes that the chemist uses in the laboratory. These processes include, heat, pressure, acids, water, and gases, all of which are present in all natural environments. These same environments account for the difference rock types that characterize our Earth. Some Earth minerals you already know. These include such minerals as diamonds – the hardest mineral on Earth, made of carbon; graphite, the material in your pencil, also made of carbon; amethyst, quartz, garnet, and tourmaline are semiprecious gems (complex silicates). The most common mineral on Earth is the one you may now know: feldspar The Introduction is a derivative of Judson, Sheldon, et al. Problem Solving in Geology. Upper Saddle River: Prentice Hall, 2000. Print. Used under Fair Use.

Physical Traits of Minerals Color Color is one of the most obvious physical traits. It is not a reliable distinguishing trait for many minerals because their range of composition and impurities produce a range of colors. Some minerals are always the same color. However, for some minerals it is a diagnostic property.

1. Green Amethyst, 2. Green Amethyst, 3. Gold Green Quartz, 4. Rose Quartz, 5. Amethyst Maraba, 6. Lemon Quartz, 7.Champagne Quartz, 8. Honey Quartz, 9. Cognac Quartz Figure 1. Quartz minerals are found in a variety of colors, making it an unreliable diagnostic property. Photo by MineRough. 2014. <http://www.minerough.com>. Digital. Used under Fair Use.

Luster A mineral has a metallic luster if it reflects light like a shiny piece of metal. Minerals with nonmetallic luster have a wide variety of appearances. Nonmetallic luster is subdivided into terms: vitreous (glassy), greasy, dull or earthy, pearly, silky, waxy, and others.

Pyrite Olivine Muscovite

Metallic Luster Nonmetallic Luster: Vitreous Nonmetallic Luster: Pearly

Figure 2. Minerals exhibit a variety of lusters. Photos by Alden, Andrew. 2014. <http://geology.about.com/>. Digital. Used under Fair Use.

Hardness This is a very useful trait that is related to the strength of the atomic bonds in a mineral. To test the hardness of a mineral, you attempt to scratch the mineral with something of known hardness (another mineral or object). Mineralogists use a scale of relative hardness called Mohs’ scale (Fig 2) which ranks ten minerals from 1 (softest) to 10 (hardest). For example, if a mineral scratches a penny, the mineral has hardness greater than a penny’s hardness. If the mineral will not scratch glass, then the mineral is softer than glass. To test hardness, place the copper plate, glass plate, or finger on a sturdy flat surface. Firmly grip one of your mineral specimens and attempt to scratch the material. If the mineral leaves a scratch, the mineral is harder than the object. Use the nail to scratch the mineral specimens. If the nail leaves a scratch, the mineral is softer than the nail. Watch the instructional video on YouTube (http://www.youtube.com/watch?v=R-bw7_u3gSQ)

WARNING: Do not hold the glass plate in your hand while performing hardness tests. It could easily break and injure you.

Figure 3. Mohs’ Hardness Scale courtesy of the Trustees of Indiana University, Indiana Geological Survey. 2014 < http://igs.indiana.edu>. Digital. Public Domain.

Cleavage Cleavage is the ability of a mineral to break along a specific direction or plane. It is a break between planes of tiny particles in a mineral's internal structure. Some minerals have no cleavage; some have only one direction of cleavage; others have two or more directions. Each cleavage direction may produce many parallel surfaces, like pages in a book, or cleavage may appear as a series of parallel steps which all reflect light at the same angle. When there are two or three directions, it is important to note whether they are perpendicular or oblique to each other. When describing cleavage always include:

1. How many directions of cleavage are present. 2. Angular relationships between more than one directions (90° or oblique) 3. Quality of cleavage (perfect, good, or poor)

Perfect cleavage is easily recognized because it characteristically develops a smooth, even surface which will reflect light like a mirror. However, cleavage planes may occur in a step-like manner and appear at first to be an irregular fracture. If the specimen is rotated in front of a light, the small parallel cleavage planes will reflect light in the same manner as a large smooth cleavage surface. An uneven fracture will not concentrate light in any particular direction In some minerals the crystalline structure is so well bonded together that there is no tendency to break along one plane in preference to any other. Such minerals do not possess cleavage, but break (or fracture) in an irregular manner.

Figure 4. Some minerals break along one or multiple smooth planes. Photo courtesy of Rocha, Guillermo. 2014. <http://academic.brooklyn.cuny.edu> Digital. Used under Fair Use.

Fracture A fracture is any break in the mineral which is not a cleavage plane. Metals do not exhibit fracture. Some minerals have a conchoidal fracture, which produces a smooth curving break, like broken glass.

Streak The color of powder from some minerals is more distinctive than the color of the sample itself. Samples are powdered by rubbing them (like a scratch-off ticket) against an unglazed piece of porcelain called a streak plate. Your kit may have either a white or black streak plate. This test only works on minerals that are softer than the steak plate. Why? The streak plate has a hardness of about 6.5. If a diamond, hardness of 10, was rubbed on the streak plate, the diamond would scratch the plate. Many minerals have a white streak, so it’s not always useful. Watch the instructional video on YouTube (https://www.youtube.com/watch?v=lVx5O-_Y05Q)

Figure 5. The brass colored mineral on the left leaves a dark gray streak. The dark pink mineral in the upper right leaves a white streak. Image courtesy of Ra’ike. 2013. http://en.wikipedia.org. Digital. Used under Attribution-Share Alike 3.0.

Heft (Density) You can estimate density by lifting a mineral specimen in your hand and making an educated guess. A few mineral in your kit are a lot heavier than the others. The heaviest mineral in your kit has a density of 7.5. Water has a density of 1.0, so that heavy mineral is 7.5 times denser. The average heft of many minerals is between 2.5-2.8. We describe heft as light, medium, or heavy in comparison with other minerals specimens of the same size.

Other Diagnostic Properties Magnetism (magnetic), taste (ex. salty), smell (ex. rotten eggs), feel (ex. soapy), light transmission (ex. translucent), tenacity (ex. flexible), chemical reaction (bubble in mild acid), fluoresces in UV light, and others.

WARNING: Do not taste your minerals while performing other diagnostic tests. The minerals don’t know where your mouth has been and you don’t know where the mineral has been.

Igneous Rock Identification How to Test Physical Traits Determining the physical traits of minerals can be a lot of fun (so say the Geologist). It’s detective work to find all the possible information about unknown mineral. Separate your minerals on a large piece of paper and number them 1 through 20. Make sure you keep track what number you assign your minerals. You will use the minerals and their given numbers for Mineralogy Labs Part 1 and Part 2. For example, if #11 is your yellow mineral, it will still be #11 for the Mineralogy - Part 2 assignment. I like to take a stack of index cards, cut them in half, number them 1 through 20, and place a mineral on each card. Follow these steps for each of your mineral specimens:

1 •Describe the color of the mineral.

•Examples: “Bright brassy yellow”, “colorless (clear)”, “transparent faint tan.”

2 •Identify the luster. Is it metallic or non-metallic? If it is non-metallic, what subcategory is it?

•Examples: Non-metallic: vitreous luster, metallic luster, etc.

3

•Determine the hardness of the mineral, try to get as close as you can. This number will be a range.

•Example: A mineral scratches copper (H=2.5), but not my fingernail (H=3.5). So the mineral has hardness greater than 2.5 and less than 3.5 or 2.5 - 3.5.

4

•Cleavage may not be evident on every sample. Sometimes cleavage is easy to see, other times it’s difficult to distinguish.

•Examples: “fracture”, “one direction”, “three directions”, etc.

5 •What color streak does the sample leave behind?

•Examples: “white”, “dark gray”, “no streak” (for minerals harder than the streak plate).

6

•Deduce the heft of the specimen. Is the sample significantly heavier or lighter than the other specimens?

•Examples: “light”, “modest”, “heavy”, etc.

7

•Identify any diagnostic properties. Not all minerals have obvious revealing traits.

•Does it fizz with applied acid? Is it magnetic? What does it smell like? Is it particully dense? What type of transparency does this specimen exhibit?

Test Physical Traits Print Table 1.Mineral Properties Worksheet, on the next page. You may identify the minerals in any order. For each unknown mineral, follow the seven steps listed above and complete the Mineral Properties Worksheet (Table 1). To help you get started, I’ve provided an example of how to test physical traits of a mineral. An unknown specimen (Fig. 6) can be described as white to colorless, semi-opaque. It has a non-metallic, vitreous luster. Using the hardness kit, I discovered the mineral is hard than the copper plate and softer than the glass (won’t scratch it). The hardness is somewhere between 3.5 and 5.5. There are eight planes of cleavage; there are 8 non-parallel sides. For streak, I rubbed the mineral across the porcelain plate; it left a white streak. Holding the mineral in my hand, it felt pretty light for its size. I tried a lot of diagnostic properties, it wasn’t magnetic, nor fizzes, nor did it smell. However, I discovered it glowed under ultraviolet light (a black light).

Figure 6. This unknown mineral is approximately the size of a marble. You can a purple version of this type of mineral on YouTube (http://www.youtube.com/watch?v=IjvA9toyfyY) Photo by Alden, Andrew. 2007. <http://geology.about.com/>. Digital. Used under Fair Use.

1. White to colorless, semi-opaque 2. Non-metallic, vitreous luster 3. Hardness between 3.5 and 5.5 4. Octahedral cleavage 5. White streak 6. Low moderate heft 7. Purplish blue glow under a UV light

Table 1. Igneous Rock Worksheet

Sample Color Luster Hardness Cleavage /

Fracture Streak Heft

Diagnostic Properties

Example White to colorless

Non-metallic, vitreous luster

3.5 to 5.5 Octahedral White Moderately low Glows under

UV light

1

2

3

4

5

6

7

8

9

10

Table 1. Igneous Rock Worksheet (continued)

Sample Color Luster Hardness Cleavage / Fracture

Streak Heft Diagnostic Properties

11

12

13

14

15

16

17

18

19

20