Chapter 1. Minerals

Physical Properties of Minerals

Identifying a mineral is a little like playing detective. Minerals are identified by their physical properties. How would you describe the mineral making up the sample in Figure 1.2? You might say that it is shiny, gold, and shaped like cubes. Each of these characteristics is a physical property.

 

A mass of metallic cubes with striated surfaces and a dull gold colour.
Figure 1.2 | A mass of pyrite crystals found in the Andes Mountains in Peru. Source: James St. John (2010) CC BY 2.0 View source

Physical properties can vary within the same minerals, so caution should be applied when identifying minerals based on any one property. Colour is an example of a property that is not a very helpful diagnostic tool in many cases, because some minerals, such as quartz, can come in a variety of colours (Figure 1.3).

 

Seven different coloured varieties of the mineral quartz, including rock crystal (colourless), amethyst (purple), citrine (yellow), rose quartz (pink), jasper (red and opaque), agate (concentric layers and opaque), and smoky quartz (dark grey to black).
Figure 1.3 | Seven varieties of quartz demonstrating the difficulty of identifying some minerals based on colour alone. Source: Karla Panchuk (2020) CC BY-SA 4.0. Photo credits

Nevertheless, colour can occasionally be helpful, as in the case of olivine, which comes in shades of olive green (Figure 1.4). This overview covers each of the key physical properties in detail to prepare you to identify the minerals in your kit.

 

Figure 1.4 | A granular mass of the mineral olivine, showing its characteristic olive green colour. Source: Joyce M. McBeth (2018) CC BY 4.0 view source

Streak

As you have seen in Figure 1.3, the colour of a mineral specimen is not necessarily a helpful diagnostic property. What may surprise you is that when minerals are powdered, the colour of the powder is consistent between different specimens of a particular mineral, even if the specimens themselves appear to be very different colours. This makes the powder colour very useful for identifying the mineral.

A mineral’s streak is the colour of the powder mark left behind when the mineral is rubbed on an unglazed piece of porcelain called a streak plate (Figure 1.5). Notice that the streak of pyrite in Figure 1.5 is dark grey, even though the pyrite is a dull gold colour.

 

A dark grey mark is present on a white porcelain tile. A sample of pyrite (a dull gold colour) is next to the tile, and was scraped against it to make the mark.
Figure 1.5 | An example of the dark grey streak left behind when pyrite is rubbed along a streak plate. Source: Randa Harris (2015) CC BY-SA 3.0 view source
Your lab kit includes white and black streak plates. If the streak of a mineral is light in colour it may not show up on the white streak plate. If you test the streak of a mineral on the white plate and can’t see a mark, try the black plate.

Lustre

Lustre refers to how a mineral’s surface reflects light. In general, lustre can be described as metallic or non-metallic. Minerals with a metallic lustre shine brightly, are opaque, and often have the colour of a metal, such as silver, gold, copper, or brass (Figure 1.6). If a mineral with a metallic lustre becomes tarnished, then a submetallic (or semi-metallic) lustre results.

 

Minerals with a bright and shiny lustre include chalcopyrite and pyrite, which both have a gold colour, and sphalerite, and hematite, which have a silver colour. Tarnished chalcopyrite has a submetallic lustre, and duller and darker colours. Submetallic hematite has a duller surface with a hint of red discolouration.
Figure 1.6 | Minerals with a metallic lustre are bright and shiny. If the metallic lustre tarnishes, a discoloured submetallic lustre can develop. Source: Karla Panchuk (2020) CC BY-SA 4.0. Photo credits

While minerals with a metallic lustre are often shiny, not all shiny minerals are metallic. Bright and shiny minerals can be adamantine or glassy (vitreous). Minerals might have a waxy, silky, or pearl lustre, and some are dull (earthy). Figure 1.7 shows examples of these lustres, but it doesn’t include all of the possibilities. Minerals could have a resinous lustre, or even a greasy lustre!

 

Six minerals that reflect light in different ways. Bauxite has a dull or earthy lustre. Chrysopase has a waxy lustre. Quartz has a glassy or vitreous lustre. Topaz has an adamantine lustre. Lepidolite has a pearl lustre. Tremolite has a silky lustre.
Figure 1.7 | Examples of mineral lustres. Source: Karla Panchuk (2020) CC BY-SA 4.0. Photo credits

Crystal Form & Mineral Habit

The geometric shape that a mineral naturally grows into is its crystal form. Crystal form reflects the orderly internal arrangement of atoms within the mineral. If minerals have space to grow when they are developing, they will display their crystal form (e.g., Figure 1.8). Ideal growth conditions do not always occur, however. If a mineral encounters another crystal, it will continue to grow according to the same internal atomic arrangement as before, but it will have to follow that pattern within the available space.   so many minerals do not display their ideal crystal form due to crowded conditions during growth. Examples of crystal form are shown in Figure 2.8.

 

Crystal form examples include a cube, a 6-sided (hexagonal) prism, a rhombodedron, an octahedron (8 faces), and a dodecahedron (12 faces).
Figure 1.8 | Some examples of crystal form. Source: Lyndsay Hauber & Joyce M. McBeth (2018) CC BY 4.0, after Randa Harris (2015) CC BY-SA 3.0 view source

Mineral habit addresses the fact that even though conditions often don’t permit minerals to develop as perfect examples of their crystal form, there are shapes and arrangements that may characterize a particular mineral, either as a single crystal, or when multiple crystals grow together.

Cleavage and Fracture

The chemical bonds between atoms within a mineral are not necessarily the same strength. When the mineral breaks, it will come apart along these zones of weakness. If the zones of weakness are aligned, then the mineral will break along a plane called a cleavage plane. If the mineral has perfect cleavage, the broken surface will be smooth and flat. If the cleavage is poor, then the break will be more irregular.

Minerals for which the zones of weakness are not aligned within a plane, and minerals with bonds that are the same strength in all directions, do not have cleavage. When they break, fracture occurs. Fractured surfaces may exhibit ribbed curved breaks, called choncoidal fracture (Figure 1.9), similar to the curved breaks that form when you get a chip in your windshield.

 

A black rock with a glassy surface. Pieces have been chipped away from the surface, leaving curved breaks behind.
Figure 1.9 | This piece of igneous rock called obsidian has been hit with a hammer and is displaying conchoidal fracture. Source: Joyce M. McBeth (2018) CC BY 4.0 view source

Cleavage or Crystal Form?

You have now seen that flat surfaces on a mineral can be the result of how that mineral grew (its crystal form or mineral habit), or because of how the mineral broke. One way to tell the difference when examining a sample is to see if the flat surface repeats where the sample is broken. Look for a damaged edge or corner and see whether the break is irregular, or whether it happens in steps. The steps might be very tiny, so look carefully to avoid mistaking them for fracture.

Test Your Understanding

Figure 1.10 shows a sample of the mineral pyrite with a mineral habit consisting of a pile of cubes. Are the faces of the cubes a result of cleavage, or is that just how the pyrite crystals grew? Hint: Look at the enlarged view.

Left: A mass made up of cube-shaped crystals. Right: An enlarged view of one of the crystals showing a damaged edge. The edge is bumpy with some curved breaks.
Figure 1.10 | A sample of pyrite. Source: Karla Panchuk (2020) CC BY-SA 4.0. Photo: Uoaei1 (2019) CC BY-SA 4.0 view source

 

Types of Cleavage

A mineral may have one or more cleavage planes (Figure 1.11). Planes that are parallel to each other are considered the same direction of cleavage, and only count as one. If there is only one direction of cleavage, it is called basal cleavage. Minerals with basal cleavage will break apart in flat sheets. Two directions of cleavage is termed prismatic, while three directions of cleavage at 90o is referred to as cubic. If there are three directions of cleavage not at 90o, the cleavage is rhombohedral. A mineral with four directions of cleavage has octahedral cleavage.

 

No cleavage: the sample has curved breaks but no flat surfaces. One direction: the sample breaks into sheets. Two planes near 90 degrees:two sets of surfaces are arranged at right anges to each other. Two planes not at 90: two sets of surfaces are arranged at ~60/120 degrees to each other. Cubic cleavage: Breaks happen in three directions at right angles to each other. Three planes not at 90: breaks create a rhombohedral shape. Four cleavage planes: pyramid shapes can result.
Figure 1.11 | The main types of cleavage, along with illustrations indicating the cleavage angles and directions.
Source: Lyndsay Hauber & Joyce M. McBeth (2018) CC BY 4.0, after Randa Harris (2015) CC BY-SA 3.0 view source

When 2 or more cleavage planes are present, it is important to pay attention to the angle between the cleavage planes. To determine the angle, find a spot where the cleavage planes intersect. This will be an edge or corner. Commonly, cleavage planes will intersect at 60o, 90o (right angles), or 120o.

Remember: Not every flat surface on a mineral is a cleavage plane. Crystal faces can be flat, but they form as a mineral grows. Flat cleavage surfaces form when a mineral breaks.

The angle between cleavage planes is an important characteristic to use when trying to distinguish between the minerals pyroxene and amphibole. Both minerals are black or greenish-black, with similar hardness, making them difficult to tell apart. Cleavage angles in pyroxene are near 90o, so expect it to look boxy and form right angles (Figure 1.12, left), while cleavage angles in amphibole are 60o and 120o, so expect a more bladed or pyramid like appearance (Figure 1.12, right).

 

Left: A dark mineral comes to a sharp point. The sides of the point have 56 degrees between them. Right: a dark mineral has squared-off sides.
Figure 1.12 | Comparison of cleavage angles between amphibole (left) and pyroxene (right). Amphibole has cleavage angles at or near 60o and 120o, and pyroxene has angle at or near 90o. Source: Joyce M. McBeth (2018) CC BY 4.0, after Randa Harris (2015) CC BY-SA 3.0 view source

Test Your Understanding

Figure 1.13 shows a mineral that is either amphibole or pyroxene. Your task is to determine which.

  1. Examine the photo to see if you can spot the cleavage. When you’re done, click the white arrow at the right of the image to see some of the cleavage marked in. Did you identify it correctly?
  2. Next, answer the questions below Figure 1.13 to indicate whether the mineral is amphibole or pyroxene. Hint: Consult Figure 1.12 if you’re stuck.

Hardness

Hardness refers to the resistance of a mineral to being scratched, and is determined by the strength of the chemical bonds between atoms within a mineral. Hardness is assessed by trying to scratch the mineral with another substance. The material that ends up with a scratch in it is softer; the material that made the scratch is harder.

Hardness is described on a scale of 1 to 10 created by a mineralogist named Friedrich Mohs (Table 1.1). The Mohs hardness scale lists ten minerals in order of relative hardness. Each mineral on the scale can scratch minerals with lower hardness numbers. Your lab kit comes with several items of a known hardness, also listed in Table 1.1 under Lab Kit Items. To test the hardness of a sample, you will scrape it on the testing item, or else try to scratch the testing item against it.

Source: Karla Panchuk (2020) CC BY-SA 4.0, modified after Randa Harris (2015) CC BY-SA 3.0 view source
Table 1.1 Mohs Hardness of Scale
Mohs Hardness Mineral Lab Kit Items
1 Talc (softest)
2 Gypsum 2.5 – Natural fingernail (supplied by you)
3 Calcite 3 – Copper penny; 3.5 – Brass screw
4 Fluorite 4 – Nail (steel)
5 Apatite 5.5 – Glass plate
6 Orthoclase Feldspar 6.5 – Streak plates
7 Quartz
8 Topaz
9 Corundum
10 Diamond (hardest)

Things to keep in mind when testing hardness:

  • When using the glass plate or streak plates, always place them on a flat surface rather than holding them in the palm of your hand. If the plate breaks in your hand, you may be injured.
  • Don’t confuse mineral powder with a scratch. For example, if there is powder on the glass plate after you scrape a sample across it, brush the powder away with your finger to see whether the the plate is actually scratched. It could be that the powder is from the mineral, in which case the mineral is softer than the plate.
  • If you’re not sure about the outcome of a hardness test, try the test on different parts of the sample. On rare occasions, parts of another mineral might be included with the sample you’re testing, giving confusing results.
  • Materials of similar hardness have difficulty scratching each other. If you can’t tell whether your sample or the testing item was scratched, that might be why. Try testing the hardness with other materials to check.

 

Test Your Understanding

Figures 1.14 and 1.15 show the result of hardness tests for two mystery minerals. Refer to the hardness of the lab kit items in Table 1.1 to answer the questions.

Mystery Mineral A

A colourless translucent mineral displays a scratch mark made by a fingernail.
Figure 1.14 | Mystery mineral A displays a scratch mark left by a fingernail. Source: Karla Panchuk (2020) CC BY-NC-SA 4.0

Mystery Mineral B

A green mineral has left a scratch on a strip of copper, but not on a glass plate.
Figure 1.15 | Mystery mineral B scratches one of the test items, but not the other. Source: Karla Panchuk (2020) CC BY-NC-SA 4.0

 

Other Physical Properties for Mineral Identification

There are a variety of other physical properties that can be used for mineral identification. Some are very basic, like taste, smell, or texture. For example:

  • Halite will taste salty.
  • Sphalerite will release a sulphurous odour when streaked or crushed.
  • Talc will feel like a bar of soap.
For safety reasons, please do not taste your samples. While this has been a fairly common thing to do in the past, it is no longer a good practice with mineral kits that must be shared amongst students. For your own safety and that of others, also remember to wash your hands before and after handling your lab kit.

Tenacity

The way a mineral resists breakage is its tenacity. If a mineral shatters like glass, it is said to be brittle (like quartz). Minerals like copper that can be hammered into sheets are malleable (Figure  1.16, left). Minerals that can be deformed, but return to their original shape after the force is removed, are elastic (Figure 1.1.6, right). Sectile minerals are soft like wax, and can be separated into layers with a knife (e.g., gypsum).
Figure 1.16 | Left: Copper, which can be hammered into thin sheets, is malleable. Right: Muscovite mica, which bends but returns to its original shape, is elastic. Source: Randa Harris (2015) CC BY-SA 3.0 view source

Reaction with Acid

Some minerals react when dilute hydrochloric acid is placed on them. In particular, carbonate minerals—minerals that include the carbonate anion, CO3-2, in their chemical formula—will effervesce or fizz when acid is applied to them because the chemical reaction releases carbon dioxide gas (Figure 1.17). When you test a mineral with acid, be cautious and use just a drop of the acid. Use your magnifying glass to look closely for bubbles. The acid that is typically used for this test is very dilute and will not burn your skin or clothing, but you should still wash your hands after use. Also make sure that you rinse with water and wipe off the acid from the minerals that you test.

 

Figure 1.17 | A piece of limestone (consisting mostly of the mineral calcite) showing the formation of bubbles produced by a chemical reaction with acid. Source: Karla Panchuk (2020) CC BY-NC-SA 4.0.

Magnetism

Some minerals, such as the aptly named magnetite, are magnetic. You can test for this by seeing if the mineral attracts the steel nail in your kit, or else by seeing if it will attract another magnet. Note that floppy-style fridge magnets won’t work for this test.

Density or Specific Gravity

Density is an object’s mass per unit volume. In other words, density tells you how heavy something is for its size. For minerals, this property is frequently referred to as specific gravity. Specific gravity is the ratio of a mineral’s weight to the weight of an equal volume of water. A mineral with a specific gravity of 2 would weigh twice as much as water.

Density and specific gravity describe the same characteristic of a material, and will have the same numerical value. The difference is that specific gravity is unitless (a number only) whereas density can have units of g/cm3 or kg/m3. The reason for using two systems that amount to the same thing likely has to do with the ease of the method for measuring specific gravity. This can be accomplished with a scale only, and does not require volume measurements to be made.

Most minerals are heavier than water, and the average specific gravity for all minerals is approximately 2.7 (a density of 2.7 g/cm3). Some minerals are quite heavy, such as pyrite with a specific gravity of 4.9-5.2, native copper, with a specific gravity of 8.8-9.0, and native gold at 19.3. The high specific gravity of gold is a handy characteristic because it makes panning for gold possible—the heavy mineral will stay on the bottom of the pan when you wash out the lighter materials.

Surface Patterns

Some minerals have distinctive surface patterns. Potassium feldspar may display thin light-coloured wavy lines called exsolution layers (or exsolution lamellae). These form because under some conditions, the feldspar crystal unmixes itself into two separate types of feldspar. The majority remains potassium feldspar, but the thin layers are more sodium rich.

Left: Hand sample of potassium felsdpar displaying thin wavy lines, and a magnified view of the thin wavy lines. The lines are labelled "exsolution layers). Right: A microscope view of exsolution layers, wherhe the layers appear as irregular bands of material cutting through the main part of the crystal.
Figure 1.18 | Exsolution layers. Left: Exsolution layers in a hand sample of potassium feldpar. Right: A microscopic view of exsolution layers. The black and white plaid pattern is characteristic of potassium feldspar when viewed under polarized light. Source: Left- Karla Panchuk (2020) CC BY-NC-SA 4.0. Right- Karla Panchuk (2020) CC BY-SA 4.0. Photo credits
Striations are thin parallel lines that appear to be etched into a mineral’s surface. Striations are a common feature of pyrite (Figures 1.2 and 1.10), as well as of plagioclase feldspar (Figure 1.19). The presence of striations can be a handy way to tell potassium feldspar from plagioclase feldspar. Note that you may need to look very carefully to spot them. The blue and purple glow coming from the plagioclase feldspar in Figure 1.19 is only visible when light hits the mineral from a particular direction. Normally the mineral would be grey in colour. This property is called colour play, or pleochroism. It is another characteristic that can distinguish plagioclase feldspar from potassium feldspar.
Triangular fragment of a mineral with a surface covered in thin parallel lines. The mineral appears to glow blue and purple.
Figure 1.19 | Plagioclase feldspar (variety labradorite) exhibiting striations (thin lines cutting upper right to lower left) and a blue glow from pleochroism. Source: Mike Beauregard (2011) CC BY 2.0. view source

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Laboratory Manual for Earth Science Copyright © 2020 by Karla Panchuk is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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