GEOL 1110, Lab #2, Cleavage and Fracture PDF

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GEOL 1110, Lab #2

LABORATORY 2 CLEAVAGE AND FRACTURE CLEAVAGE Cleavage, as it pertains to minerals, is a particular way that a mineral breaks or cleaves. Depending upon the internal atomic structure of a mineral it may break or cleave along a plane or planes of weakness. The result is that, as a mineral is broken, relatively smooth flat surfaces appear. For example, a raw diamond (Figure 2 - 1) may have a distinctive, prismatic shape or be an irregular mass with no smooth flat surfaces. However, when a diamond is cut or cleaved it will often have a very distinct, aesthetically pleasing design often seen in jewelry.

Figure 2 - 1. Raw diamonds.

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GEOL 1110, Lab #2

IMPORTANT NOTE Cleavage is defined by the smooth flat surfaces that appear when a mineral breaks. Do not confuse this with crystal habit, which also is defined by smooth flat surfaces, but these are formed when a mineral crystal grows. However, the cleavage of a mineral is often the same or very similar to its crystal habit. The cleavage of a mineral is defined by the number of flat surfaces that appear when a mineral breaks, the quality of the cleavage surfaces and the angle between them. The quality of cleavage refers to how smooth and continuous surfaces are and therefore how easily they are observed. Smooth, continuous surfaces describe good or even perfect cleavage. A rough, discontinuous surface is poor cleavage. The mineral biotite, for example, when broken separates into thin, flat plates, the surface of which is a cleavage surface (Figure 2 - 2 A). While there are two surfaces, the top and bottom of the mineral that are parallel, biotite has only one type of surface. Biotite therefore has one cleavage because there is only one type of surface. Also because biotite only has one type of cleavage surface there is no way to measure an angle between cleavage surfaces. The common minerals pyroxene and hornblende both have two poor to good cleavage surfaces (Figure 2 - 2 B). This means that when they break it is possible to see two types of smooth, flat surface. Key to distinguishing pyroxene from hornblende is the cleavage angle, the angle between adjacent cleavage surfaces. The cleavage angle for pyroxene is 93o or 87o, which for all intents and purposes is 90o or a right angle. The cleavage angle for hornblende is 56o or 124o. Sometimes it’s described more simply as ‘not 90 o’. Cleavage becomes increasingly more complex as the number of cleavage surfaces increases. Cleavage in three directions is characteristic of the common minerals galena and calcite (Figure 2 - 2 C). However, galena and calcite have different cleavage angles. The angle between adjacent cleavage surfaces in galena is 90o which imparts a cubic shape to a cleaved sample. The cleavage for galena is sometimes simplified by calling it cubic. The cleavage angle for calcite is not 90o, or more accurately measured as 60o or 120o. The cleavage of calcite is sometimes referred to a rhombic or rhombohedral.

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GEOL 1110, Lab #2

Figure 2 -2. Types of cleavage: A Single, platey biotite sample showing perfect or good cleavage in 1 direction.; B Pyroxene and hornblende cleavage, in 2 directions at approximately 90o and at approximately 60o or 120o. C Galena and calcite cleavage in 3 directions at 90o and not at 90o. D Fluorite and diamond cleavage in 4 directions.

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GEOL 1110, Lab #2

The Quality of Cleavage - Perfect, Good and Poor. Minerals with cleavage may display a pattern of flat, shiny surfaces with varying degrees of quality. Perfect or good cleavage, for example, results when the mineral breaks along a more continuous, smooth plane or surface (Figure 2 3 A). What is often more common is “poor” cleavage whereby the mineral breaks or cleaves along a series of small planes or surfaces that are not continuous and therefore do not form one large smooth surface (Figure 2 - 3 B). When cleavage is poor it is still possible to observe the number and orientation of cleavage surfaces, it just takes closer examination. You may have to use a microscope or enlarge an image. Another way is to move the mineral at different angles to a light source to see how all these small cleavage surfaces reflect light the same way.

A

B

Figure 2 - 3. A Good cleavage along a smooth, continuous surface. B Poor cleavage along a more irregular, discontinuous surface formed by a series of smaller cleavage surfaces.

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GEOL 1110, Lab #2

FRACTURE Fracture is different from cleavage. Fracture is when a mineral breaks and there is no flat surface or cleavage plane. A mineral with cleavage may break but perhaps not along one of its cleavage planes. In addition, not all minerals have cleavage. In this case when the mineral breaks it “fractures” along irregular surfaces which are not flat and there is no consistent angle between adjacent surfaces. The mineral quartz for example has no cleavage. Therefore, when quartz breaks it does not cleave or separate along smooth, continuous surfaces. Instead it fractures along rough irregular surfaces. There are two common forms of fracture: 1. uneven; and 2. conchoidal. 1. Uneven fracture refers to any mineral that has no cleavage and instead breaks along rough, irregular surfaces. The mineral quartz (Figure 2 - 4) has an uneven fracture.

Scale is in centimeters Figure 2 - 4. The uneven fracture of quartz.

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GEOL 1110, Lab #2

2. Conchoidal fracture refers to a smoothly curving fracture. Smooth continuous convex or concave surfaces appear when minerals such as microcrystalline quartz break (Figure 2 - 5). When more than one conchoidal surface is present on a single mineral they frequently intersect at sharp angles thereby imparting a “knife-edge” appearance to the mineral.

Scale is in centimeters Figure 2 - 5. The conchoidal fracture of microcrystalline quartz with its smooth curved surfaces.

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GEOL 1110, Lab #2

Here is some useful information that can help with cleavage as a property for mineral identification. 1. With a hand sample it is possible to observe cleavage when a mineral is moved or turned around until a flash of reflected light is seen from a particular location on the sample. This may be a reflection from a cleavage surface. The reflection may be from a single surface or it may come from many small parallel surfaces on the sample. Cleavage may also be seen because of thin parallel cracks on a surface of the mineral. Each set of parallel cracks and the surfaces they are on represent a cleavage. To look for additional cleavage, turn the sample again and look for more reflections. Estimate the angle between cleavages by rotating back and forth between reflections. 2. Cleavage angles need not be measured precisely. Estimate them carefully with the naked eye and determine whether the angle is larger, smaller or equal to 900. 3. Sometimes the use of a geometric term that describes the shape of a minerals cleavage is the best method of giving both number and angles of cleavage. For example, cubic cleavage implies 3 types of cleavage surfaces, in three different directions, all at 900 to one another (Figure 2 - 2 C). Rhombic cleavage has also has 3 types of flat surface, in three different directions, but the angle is not 900 (Figure 2 - 2 C), and octahedral implies 4 cleavages resulting in a regular 8 sided form with more than one angle (Figure 2 - 2 D). Either way can be used to describe the cleavage of a mineral, listing the number, orientation and angles between surfaces, or by using a geometric term.

EXERCISE 7: DETERMINING CLEAVAGE The property of cleavage is essential for recognizing and distinguishing between many different kinds of minerals. What follows are images of 9 different common minerals, all of which may or may not have some sort of cleavage. You can assume that all flat surfaces on these minerals is a cleavage surface and not part of crystal habit. Determine the cleavage for these 9 common minerals. Be sure to check and discuss your results with your instructor. 7

GEOL 1110, Lab #2

1. Galena

Number of cleavages? Cleavage angle?

2. Quartz

Number of cleavages? Cleavage angle?

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GEOL 1110, Lab #2

3. Calcite

Number of cleavages? Cleavage angle? 4. Biotite

Flat, platey biotite mineral

Flat, platey biotite mineral

This image shows two biotite minerals, one propped up against the other. Number of cleavages? Cleavage angle?

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GEOL 1110, Lab #2

5. Potassium Feldspar

Number of cleavages? Cleavage angle? 6. Pyroxene

Number of cleavages? Cleavage angle?

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GEOL 1110, Lab #2

7. Halite

Number of cleavages? Cleavage angle? 8. Fluorite

Scale is in centimeters Number of cleavages? Cleavage angle? 11

GEOL 1110, Lab #2

9. Pyrite

Number of cleavages? Cleavage angle? 10. Unknown Mineral Cleavage Angle Cleavage face #1

Cleavage face #2

Scale is in centimeters

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Time to try mineral identification. Although mineral identification is taught in lab 4, there’s no harm in starting to think about it now. We know something about unknown mineral #10. The mineral in this image has two cleavages. Cleavage face #1 is visible by the way light reflects off the many small surfaces that are parallel or aligned all the same way. Cleavage surface #2 is behind the mineral. The angle between these two cleavage surfaces is not 90o as indicated by the red angle on the image. Given that this mineral has two good cleavages not at 90 o, is black in colour with a glassy, non-metallic luster, what mineral do you think it is?

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