Mineral Properties

OBJECTIVES

• Understand the different physical properties used to identify minerals.  
• Understand the classifications of minerals.  
• Connect minerals with their elements and the economic uses of many of the important minerals.
• Use the internet to explore the many minerals that we depend upon and deepen your understanding of the common minerals used in everyday life.

MATERIALS

• A box of approximately 20-24 minerals provided by your Instructor with a list of the mineral names.
• Equipment for determination of mineral properties: Streak plate, Glass plate, Copper Penny, and a Magnet.

INTRODUCTION

The study of Geology literally means to study the Earth. The Earth is made of a variety of materials.  In order to understand the Earth, we must understand its materials, such as rocks and their components – Minerals (Figure 1.). You have probably used several dozen minerals today without thinking specifically about them.  Pencils use graphite, wallboard is made from gypsum and there are many minerals in your toothpaste, cosmetics, and electronic devices. There are approximately 4400 known minerals.  Each differs in its chemical composition and the atomic arrangement of its atoms.  Earth scientists use distinctive physical properties that result from the different chemistries and crystal arrangements to identify individual minerals. Common minerals are identified by testing them for general or specific physical properties. This exercise introduces many of the common minerals and the properties that are used to distinguish them.

1.                                             B.                                          C.

Figure 1 A. Image of the metamorphic rock Gneiss.  It is a combination of several visibly distinct minerals as is seen in the bands of colored minerals.  Rocks are aggregates of minerals. B. Image of the mineral Quartz as three individual crystals. C. Fluorite mineral showing distinct habit. Images A & B from Western Washington University OER Commons CC BY SA P Stelling opengeology.org/textbook. Image C Rob Lavinsky, iRocks.com – CC-BY-SA-3.0, https://commons.wikimedia.org/w/index.php?curid=10139918

What is a Mineral?

Geologists define minerals by five criteria:

Naturally occurring. Minerals form by natural geologic processes.  Therefore, minerals created in the lab are not considered true minerals.

Generally inorganic. Minerals such as Quartz, that form in the Earth through crystalline processes are considered minerals.  Generally, an organic based product such as sugar formed from plant activity is generally not considered a mineral.  Some organic products such as peat and coal that have been altered by geologic processes are found in the sedimentary rock classification.

Solid. Minerals must be solid and in a crystalline form at general Earth temperatures and conditions. Therefore, ice is considered a mineral but not liquid water.

Characteristic chemical composition. Each mineral can be represented by a unique chemical formula.  The formula contains specific elements such as Silicon (Si) and Oxygen (O) as well as a specific ratio between them.  The mineral Quartz has a formula of SiO2. This unique formula produces a one to two ratio between the Si and the O. If this ratio changes it becomes a different mineral.

Orderly internal arrangement of atoms.  Minerals are said to be crystalline in form. Their atoms form repetitive and orderly arrangements (Figure 2.).  This frequently produces specific properties related to their hardness or solubility. Some minerals do not form ordered crystalline patterns and are considered amorphous, such as Opals.  These are considered mineraloids.

The variations in chemical formulas and crystalline structures often determine a mineral's outward appearance and properties.  We use the different properties to help identify minerals.

1.                                                                                                       B.

Figure 2. A. Graphite/Diamond shows the difference in atomic arrangement between two minerals Graphite (left) and Diamond that have the same chemical formula Carbon (C) – but different crystalline structures. The variation in the strength of the bond’s accounts for the variation in hardness between these two minerals. B. Orderly crystalline arrangement of the atoms in the mineral Halite (NaCl). Images courtesy of Timothy Davis CC BY license.

In-Class Activity: Is it a Mineral?

Use the five-part geologic definition of a mineral to determine if the items listed below in Table 1. are true minerals.  Recall that all five parts of the definition are required for the item to be a mineral.

Table 1. Which items are true minerals?

Physical Properties used for Identification of Minerals:

Geologists use many properties to identify minerals in hand samples.  Minerals have unique chemical compositions and crystalline structures that frequently impart a distinct set of physical properties.  Though all samples of the mineral calcite CaCO3 will have similar hardness and reactions to Hydrochloric Acid (HCl), each individual mineral may vary slightly due to impurities and variations.  The many physical properties can be separated into groups such as visual and optical properties based on light interaction with a mineral, strength, feel, shape and breakage. You should be able to utilize these physical properties to identify the minerals in this lab.  

Let’s Explore the Samples

Take the samples out of the box.  While you are removing them, explore them visually and physically with your hands - tactile.  Take note of how they feel and look.  Notice the similarities and differences.  

Discuss with your lab partner how the minerals look and feel differently.  Your teacher will guide a discussion about how we tell minerals apart and how to use the many physical properties to identify the samples.  The names of the samples will be used throughout the lab as we explore the samples and properties.

Optical properties based on light:

Luster

Luster is the appearance of how light reflects from a fresh surface of the mineral.  It is a descriptive property.  There are two main categories: Metallic and Nonmetallic.  Minerals with Metallic luster appear shiny and reflective like a new coin.  Minerals with Nonmetallic luster vary from vitreous/glassy, waxy to greasy, pearly/silky and earthy/dull (Figure 3.).  A list of terms and the description is below:

Vitreous/Glassy – resembles the luster of freshly broken glass.  It is also a common feature of the mineral quartz.

Waxy/Greasy – resembles the luster of a candle.  This luster is common in the mineral talc.

Pearly/Silky – resembles the luster of a pearl.  This luster is common in the mineral gypsum.

Earthy/Dull – the surface lacks reflection like dry soil.  This luster is common in the minerals, hematite and limonite.

Resinous – the surface looks like amber or plastic.  

Figure 3. Image of several luster examples: Earthy (dull) Kaolinite, Metallic Pyrite, Vitreous/Glassy Quartz. Images: Kaolinite federal government public domain in USA, Pyrite CC BY SA R Harris oer.galileo.usg.edu and S. Earle opentextbc.ca/geology/Canada, Quartz Didier Descouens CC BY in oer.galileo.usg.edu

Practice the Luster Property

Add the names of several mineral samples that have unique or diagnostic Lusters.

Images: CC BY-SA 2.5 wikicommons Metallic mineral - John Chapman Wikimedia CC BY-SA, Vitreous mineral, Didier Descouens CC BY in oer.galileo.usg.edu. Pearly mineral CC BY-SA Raike Wikicommons, Earthy mineral By U.S. Geological Survey and the Mineral Information Institute - US Government, Public Domain, https://commons.wikimedia.org

Diaphaneity

Transmission of light through the sample (Figure 4.). There are three basic levels of light transmission: Opaque no light is transmitted, Translucent when light but not a clear image is transmitted.  This is common in glass bricks used in bathrooms and entry ways. Transparent when both light and an image are clearly visible. Note most minerals are opaque.  Therefore, the ability to transmit light is unique to only a few of the minerals in your sample box.  Can you find and name them?  Hold the sample up to a light source to determine diaphaneity.    

Figure 4.   Examples of Diaphaneity. This Quartz mineral is transparent, the Corundum mineral showing opacity, while the Gypsum mineral shows translucency. Images: CC BY SA USGOV.jpg public domain in the USA

Let’s Explore-Diaphaneity

Hold the samples up to a light source and pass your hand between the sample and the light source.  Are you able to detect a change?  If not, your sample is most likely opaque.  This is the most common diaphaneity.  Try to find a few that are transparent or translucent.

Make a list of the few minerals that are transparent or translucent since this is now a unique property that can be used to identify certain minerals. Add the names of a few opaque minerals.

Color

Some minerals have a very stable and distinctive color, such as the mineral Sulfur (figure 5). Other minerals' color varies across a wide spectrum, such as Quartz (figure 6). Variable color in a mineral such as Quartz can result from the presence of trace elements, minor inclusion, or crystallization environment.  For example, rose quartz gets its pink color from the presence of minor amounts of the element Manganese, while Milky quartz gets its white or cloudy color from millions of microscopic fluid-filled inclusions with the mineral.  Although color can be variable between mineral samples it can also be very distinctive and utilized as a valuable identifying property. 

Although color is the most obvious physical characteristic of many minerals, it is infrequently considered a definitive diagnostic property.  

        If it is an uncertain property, why should we consider it?  

        Discuss in class why color may or may not be diagnostic.

        Try to group the minerals by color. Discuss as a class why this is difficult.

Figure 5. The mineral Sulfur shows a very distinct and diagnostic yellow color.  It is one of many minerals that can use the physical property of color as a diagnostic tool. Image: Openpress.usask.ca.physical geology CC BY NC SA 4.0 R Weller/Chochise college

Figure 6. Quartz mineral and its various colors.  All Quartz minerals are comprised of the elements Silicon and Oxygen SiO2.  The color variations are due to a variety of factors such as trace elements, minor inclusion, and crystallization environment.  Image: Openpress.usask.ca.physical geology CC BY NC SA 4.0 R Weller/Chochise college, K Panchuk.

Practice Identifying Color as a Unique Property

Make a list of minerals with variable color, including the common colors.

Make a list of minerals with distinct and identifiable color.

Streak

The color of a substance as a solid mineral often differs from its ground or powdered color.  To obtain a powered sample rub the mineral on a piece of unglazed porcelain, called a streak plate.  Some minerals will have a white powdered streak while others leave a unique color.  Minerals with a white powdered streak are generally non-metallic minerals such as orthoclase and pyroxene.  If you have difficulty seeing the powdered color, wipe the powder with your finger or use a black streak plate instead of the traditional white plate.  A colored powder represents a positive streak test and often indicates a metallic, sulfide, oxide or hydroxide mineral. The streak test is very useful when distinguishing different varieties of the same mineral such as Hematite, as the color of the powder will be similar even if the samples look different (figure 7).  

Figure 7. The mineral Hematite can look very different in luster and color, but its streak powdered color is similar. Image Openpress.usask.ca.physical geology K Panchuk CC BY 4.0

Generally metallic minerals produce a colored streak, not white powered.  The streak powdered color test is specifically useful when attempting to distinguish between several dark colored minerals, as each mineral will have a distinct streak color (figure 8).  This unique property can act as a distinctive identifying property.  The streak plate has a hardness of 6.5 on the Mohs hardness scale and will powered the porcelain, so it cannot be used for very hard mineral samples.

Figure 8. Similar dark-grey minerals with varying degrees of metallic sheen leave different colored streaks. The minerals are from upper left clockwise: hematite, magnetite, sphalerite, and galena. Image: Karla Panchuk CC BY 4.0 Openpress.usask.ca.physical geology

Let’s Practice Determining a Mineral’s Streak-Powdered Color

Test the minerals and divide them into colored streaks and white streak.  Colored streaks can be diagnostic.

Colored streak minerals                                        White powdered streak

Crystal form or Habit

Recall from the 5-part mineral definition that all minerals are crystalline, and when they grow in unrestricted environments, such as in air or fluids, they can develop external geometric shapes.  This is usually what you think about when you imagine a mineral crystal.  Most museum displays show minerals having perfect crystal forms such as a quartz crystal. This external form is the result of the orderly internal arrangement of the atoms, but in most natural cases, it does not develop due to the lack of space. Minerals in a rock tend to intergrow together restricting the expression of the external form.  Certain groups of minerals such as zeolites and garnet commonly grow to exhibit unique shape or habits.  These minerals tend to form characteristic habits such as bladed (flat knife-like), blocky (square), or, or 12-sided dodecahedrons (Figure 9).

Crystal form/Habit is distinct from cleavage.  Cleavage results from planes of weakness and strength (next segment) versus habit which forms from free growth.  They are easily confused.  Minerals that show habit will not cleave or break along those same line or faces when stress or forces are acting on them.  So please be careful with the mineral samples that your instructor uses for this property.  Many are easily damaged.  

Figure 9. Blue bladed habit of the mineral Kyanite, Cubic samples of Pyrite, Dodecahedrons form in Garnet, and cubic Halite. Image opentextbc.ca S Earle Image Rob Lavinsky, iRocks.com CC BY-SA. Pyrite image Carles Millan CC BY-SA, Garnet image Eurico Zimbres (FGEL/UERJ) and Tom Epaminondas (mineral collector). CC BY-SA.

Mineral Strength:

Tenacity

Several minerals have unique tenacity, which is the ability to bend while resisting breakage.  They are considered malleable or elastic.  Such as the mica group: Biotite and muscovite mica. Try to find them among your samples.

Hardness

The hardness of a mineral is its resistance to abrasion.  It is one of the most diagnostic properties. Hardness is determined by rubbing the mineral in order to scratch another substance of known hardness.  Hardness is measured on a relative scale called the Mohs hardness scale.  This scale was developed by a German mineralogist Friedrich Mohs (1773-1839) as a qualitative scale with Talc equal to the softest mineral ranked 1 and Diamond the hardest mineral at 10.  Mohs hardness scale is not linear as seen in Figure 10 below.  A harder mineral will damage or scratch a softer mineral.  Hardness can also be tested by using common objects of a known hardness such as a copper penny 3.5, or a glass plate at 5.5. Mineral hardness can be easily separated into four classifications as seen in the table 2 below.

Table 2. Hardness ranges using 3 common objects

Figure 10.  Mohs Hardness Scale with relative common objects. Image CC nps.gov

Using the minerals provided, test them for relative hardness.  Separate each mineral into one of the four hardness groups: Write their names in the groups.

Let’s Practice Determining a Minerals Hardness Level

Hardness Below 2.5:

Hardness Between 2.5 and 3.5

Hardness Between 3.5 and 5.5

Hardness Above 5.5

Try to determine a relative hardness of the minerals that scratch the glass by determining how easily they seemed to damage the glass plate.

Cleavage and Fracture

Cleavage is the tendency of a mineral to break along planes of weakness (Table 3).  The flat surfaces are called cleavage planes and are parallel to the direction of weakness within the crystal structure.  Recall that minerals have an internal orderly arrangement of their atoms – crystallinity. Not all minerals have planes of weaker bonds.  In this case they will break in a random or irregular fashion.   Irregular surfaces are fracture surfaces. You can identify a cleavage plane by turning the mineral sample in the light and seeing the flash or wink of a cleavage plane.  Note that the plane may not feel smooth and there may be many parallel planes, but the planes are parallel.  Planes that are parallel are counted as one plane.  When there are two or more cleavage planes the angle between them can be measured.  It is usually described as angles at 90 degrees a right angle or not 90 degrees.

Note: Crystal form or habit occurs as a mineral grows, while cleavage only forms as a mineral breaks.

Table 3. Cleavage. Images CC wiki Public Domain Fluorite opentextbc.ca CC BY-SA S. Earle

Density and Specific Gravity

Density is a measure of an object's mass divided by its volume (in cubic centimeters per grams) cm3/g.  Specific Gravity is the ratio of a mineral's weight to the weight of an equal volume of water (Figure 11.).  A mineral with a specific gravity of 2 would weigh twice as much as an equal volume of water. Most minerals are heavier than water.  Your Instructor will demonstrate this property.

Figure 11.   How specific gravity is determined. Image CC BY Timothy Davis

Let’s Practice Specific Gravity

For purposes of identifying minerals in hand samples without significant testing we observe the minerals ‘heft’.  To do this place the sample in the palm of your hand not in between your fingers and feel the weight.  Most samples have an average expected specific gravity.   For example, Halite feels very light and has a specific gravity of 2.2 while Galena feels very heavy and has a specific gravity of 7.6.  Specific gravity is the ratio of the density of a sample divided by the density of water.  Since water has a density of 1 gram/cm3 and the units cancel out, specific gravity is the same number as density but without any units.  

Try to separate your samples into groups that feel lighter than expected for their size and those that feel heavier.  Please note that most have an average heft.  

Write the names of the minerals that have below average specific gravity.

Write the names of the minerals that have above average specific gravity.

Do many of the samples that have high specific gravity also have metallic luster? OR a positive colored streak test?  Note their names as these are diagnostic properties.  

Other Unusual properties:

Magnetism

Very few minerals are strongly magnetic without heating – Magnetite and Pyrrhotite are two naturally occurring magnetic minerals.  Magnetism is easily tested by using a magnet (figure 12.).  

Try to find the mineral magnetite.

Figure 12.  Image of magnetic property. Image wiki commons. Public Domain

Taste

Some minerals have distinct tastes.  Halite is particularly salty and Sylvite is bitter.  

Caution: Do not taste any of the minerals in the box. Your instructor will point out Halite.

Tactile

The feel of a mineral is the impression gained by handling or rubbing the sample.  This is a descriptive property.  Find and make note of any minerals that have an unusual feel such as soapy or waxy or grainy.

Name the samples that you have identified with a diagnostic feel.

Reaction to dilute Hydrochloric Acid

Few minerals are reactive to dilute hydrochloric acid (HCl).  Many minerals will react to warm acid or stronger acids.  Your instructor will demonstrate the vigorous fizzing of the mineral Calcite to dilute HCl (Figure 13.).  

Figure 13.  Calcite mineral reacting vigorously to dilute HCl. Image Wiki commons. Public Domain

Safety note: Do not individually test the samples with acid. Your instructor will demonstrate this property.

Mineral Groups

There are more than 4400 known minerals.  Fortunately, less than 100 are considered common.  Many of the most common minerals are considered rock-forming minerals.  You will be able to notice many of them in the following rock labs.  Many minerals are essential economic minerals and form the foundation of construction materials, pharmaceuticals, electronic industry, and the agriculture industry.  There are more than a dozen minerals groups.  Silicates are the dominant mineral group in the earth’s crust and Non-Silicate groups include Carbonates, Oxides, Hydroxides and others (Table 4.). Minerals are the foundation of many industries.  You have probably used more than 50 different minerals already today.  Below is a common method for organizing and exploring uses for individual minerals.

Silicates: represents the dominant mineral group

Olivine – Olivine is an important group of minerals that are a major constituent of dark-colored Mafic igneous rocks that make up much of the Earth’s mantle.  Due to its formation in a high temperature and high-pressure environment such as the Earth’s mantle, Olivine is often a major component in high temperature foundry ovens used in metal production of steel.  Olivine is also a refractory (high temperature) material for use in making refractory bricks and casting sands.  It is commonly known as the gemstone ‘Peridot”.  Peridot Image below: CC BY Siim Sepp.

Non-Silicates: Include many groups such as Carbonates and Oxides, along with many others.

Table 4. A few economically important minerals

USING THE LIST OF MINERALS PROVIDED BY YOUR INSTRUCTOR NOTE WHICH PROPERTIES ARE USEFUL FOR IDENTIFYING EACH OF THE MINERALS. NOTE: ONLY FOCUS ON THE FEW PROPERTIES THAT ARE UNIQUE TO THAT MINERAL.