Introduction: (Initial Observation)
A mineral is any naturally-occurring, homogeneous solid that has a definite chemical composition and a distinctive internal crystal structure. Minerals have many uses. Minerals are the starting point for manufacturing all inorganic chemicals and all metals. In addition to that many minerals are used as gems and many others such as marble and granite are used as construction material.
Before we know what is the proper use and the value of each mineral, we need to identify that. That is what makes the mineral identification an important task of geologists and specially mineralogists. In this project you will step in the world of mineralogists and try to identify a few mineral that you collect in your local area.
Adult Supervision is required in collecting minerals. Goggles are required if you need to break minerals.
Find out about Minerals and their color and their hardness. Read books, magazines or ask professionals who might know in order to learn about testing and identifying minerals. Keep track of where you got your information from. Following is some info.
Most common minerals can be identified by inspecting or testing their physical properties. These properties are color, streak, transparency, luster, hardness, cleavage, fracture, specific gravity, and crystal form. Following are more details about each of these properties.
Usually, we notice the color of a mineral first. Some minerals are easily identified by color because they are never any other color. For example, malachite is always green. Keep in mind, however, that color by itself isn’t enough to identify a mineral. Chemical impurities can change the color of a mineral without changing its basic make-up. For example, quartz in its purest form is colorless and clear as glass. Quartz with traces of iron becomes violet (amethyst). With traces of manganese, it turns pink (rose quartz). If quartz is exposed to radiation, it turns brown (smoky quartz).
When a mineral is rubbed firmly across an unglazed tile of white porcelain (a streak plate), it leaves a line of powder. This is called the streak. The color of the streak is always the same, whether or not the mineral has impurities. For example, quartz leaves a white streak, whether it’s violet (amethyst), pink (rose quartz), or brown (smoky quartz).
Transparency describes how well light passes through a mineral sample. There are three degrees of transparency: transparent, translucent, and opaque. You can see objects through a transparent mineral. You can see light, but no objects through a translucent mineral. You can’t see anything through an opaque mineral.
This quartz is transparent.
Luster is the way the surface of a mineral reflects light. Luster should be observed on a cut or freshly broken, untarnished surface. There are two general types of luster — metallic and non-metallic. The terms used to describe luster are:
- Metallic — example: gold
- Vitreous (glassy) — example: quartz, tourmaline
- Adamantine (brilliant) — example: diamond
- Resinous (like resin or sap from a tree) — example: sphalerite
- Greasy or waxy — example: turquoise
- Pearly — example: talc
- Silky — example: asbestos
- Dull or earthy — example: bauxite
The hardness scale was established by the German mineralogist, Friedrich Mohs. The Mohs’ hardness scale places ten common or well-known minerals on a scale from one to ten. One is the softest mineral and ten is the hardest. These are the minerals used in the Mohs’ hardness scale:
|Mohs’ Hardness Scale|
To use the hardness scale, try to scratch the surface of an unknown sample with a mineral or substance from the hardness scale (these are known samples). If the unknown sample cannot be scratched by feldspar (6) but it can be scratched by quartz (7), then it’s hardness is between 6 and 7. An example of a mineral that has a hardness between 6 and 7 is pyrite (6 to 6.5).
If you don’t have minerals from the hardness scale on hand, here are some common objects and their hardness values:
|Common Objects and Their Hardness Values|
|Fingernail||Penny||Glass||Steel knife||Emery cloth|
If an unknown sample can not be scratched by your fingernail (2.5) but it can be scratched by a penny (3.5), then it’s hardness is between 2.5 and 3.5. An example of a mineral that has a hardness between 2.5 and 3.5 is calcite (3).
When a mineral sample is broken with a hammer, it breaks along planes of weakness that are part of its crystalline structure. These breaks are cleavages. Some minerals break only in one direction. Others break in two or more directions.
Some common forms of cleavage are cubic, rhombohedral, and basal. Cubic cleavages form cubes (example, halite). Rhombohedral cleavages form six-sided prisms (example, calcite). Basal cleavages occur along a single plane parallel to the base of the mineral (example, topaz).
If a mineral breaks easily and cleanly in one or more directions, its cleavage is considered perfect. For example, calcite cleaves perfectly along three planes. As the quality of the break decreases, cleavage may be described as good, distinct, and poor or none. Some minerals cleave perfectly in one direction and poorly in others. For example, gypsum cleaves perfectly on one plane and poorly along two others.
Not all minerals cleave easily. Some fracture instead. Unlike cleavages, which are usually clean, flat breaks, fractures can be smoothly curved, irregular, jagged or splintery.
The malachite pictured here is an example of a conchoidal fracture; it’s smooth and curved.
Click here to see a table of mineral tenacity and fracture.
Specific gravity is the density of a mineral. Special equipment is usually needed to find out a mineral’s exact specific gravity. With a little practice, you can guess a mineral’s specific gravity by hand. Some mineral samples will feel heavier than others, even if all your samples are the same size. The heavier ones have a greater specific gravity. Here are some examples of common minerals and their specific gravity ranges:
|fluorite, beryl||medium heavy||3-4|
|corundum, most metal oxides||heavy||4-6|
|native gold, platinum||heaviest||19|
Minerals grow in specific shapes, and usually crystallize into one of six crystal systems. The axes of the crystal, the angles at which the axes intersect, and the degree of symmetry define each system.
- Isometric — Also called the cubic crystal system. Crystals are usually shaped like blocks, with similar and symmetrical faces. The crystal has three axes of symmetry, all at right angles to each other, and all of the same length.
- Tetragonal — Typically, the crystals are shaped like four-sided prisms and pyramids. Each crystal has three axes, all perpendicular to one another. Two axes are the same length and lie on a horizontal plane. The third axis is not the same length and is at a right angle to the other two.
- Hexagonal — These crystals are usually shaped like six-sided prisms or pyramids. Each crystal has four axes of symmetry. Three lie in the same plane, are the same length, and intersect at 120° angles. The fourth axis is not the same length, and is perpendicular to other three.
- Orthorhombic — These crystals are short and stubby. Each crystal has three unequal axes, all at right angles to one another.
- Monoclinic — Crystals are short and stubby with tilted faces at each end. Each crystal has three unequal axes. Two axes lie in the same plane at right angles to each other. The third axis is inclined.
- Triclinic — Crystals are usually flat with sharp edges, but exhibit no right angles. Each crystal has three unequal axes. None are perpendicular to one another.
A good property in mineral identification is one that does not vary from specimen to specimen. In terms of reliability, hardness is one of the better physical properties for minerals. Specimens of the same mineral may vary slightly from one to another, but generally they are quite consistent. Inconsistencies occur when the specimen is impure, poorly crystallized, or actually an aggregate and not an individual crystal.
Minerals with small atoms, packed tightly together tend to be the hardest minerals. Hardness is generally consistent because the chemistry of minerals is generally consistent.
Hardness can be tested through scratching. A scratch on a mineral is actually a groove produced by microfractures on the surface of the mineral. It requires either the breaking of bonds or the displacement of atoms (as in the metallic bonded minerals). A mineral can only be scratched by a harder substance. A hard mineral can scratch a softer mineral, but a soft mineral can not scratch a harder mineral (no matter how hard you try). Therefore, a relative scale can be established to account for the differences in hardness simply by seeing which mineral scratches another. That is exactly what French mineralogist Friedrich Mohs proposed almost one hundred and seventy years ago. The Mohs Hardness Scale starting with talc at 1 and ending with diamond at 10, is universally used around the world as a way of distinguishing minerals. Simply put; the higher the number, the harder the mineral.
Below is the Mohs Hardness Scale:
|9||Corundum (ruby and sapphire)|
In order to use this scale, it is necessary to have on hand some of the minerals in the scale. If you wish to test an unknown mineral for hardness you might want to start with an ordinary specimen of apatite to see if the unknown mineral can scratch it. If the unknown mineral scratches the apatite, then you can conclude that it has a hardness of 5 or more. If the apatite can scratch the unknown mineral, then the unknown mineral has a hardness of 5 or less. If they can scratch each other, then the unknown mineral has a hardness of 5. You will need to perform other tests to narrow down the hardness. If it is softer than apatite, try calcite, etc., etc until you have narrowed down the approximate hardness. Remember, this is a relative scale and a mineral that can scratch a mineral that has a hardness of 4.5 may be given a hardness of 5, but it still might be softer than apatite.
One word of caution for inexperienced collectors: do not SCRATCH NICE CRYSTAL FACES! A fractured, cleaved or inconspicuous part of the mineral should still give a good hardness test and not damage a potentially wonderful specimen.
What if you do not have the minerals listed in the Mohs Hardness Scale?
Well, a collector might keep a few items of known hardness in a “hardness kit”; just in case they are needed.
Below is a revised Mohs Hardness Scale with some everyday items listed:
|3.5||Copper (old penny)|
|5.5||Window glass or typical knife blade|
|6.5||streak plate or good steel file|
|9||Corundum (ruby and sapphire)|
Again, the Mohs Hardness Scale is only relative. Meaning that fluorite at 4 is not twice as hard as gypsum at 2; nor is the difference between calcite and fluorite similar to the difference between corundum and diamond. An absolute hardness scale looks a little different than the relative scale. Using sensitive equipment, a comparison of the absolute hardness of minerals can be measured. It turns out that most minerals are close in hardness. But as hardness increases, the difference in hardness greatly increases as seen in the scale below.
Below is an absolute hardness scale:
The simpler, relative Mohs hardness scale is much easier to remember and use.
It is easy to see why diamond gets so much respect as the hardest natural substance know to man. The next hardest mineral, corundum, is four times softer! There are many substances that are currently being created and studied to beat diamond in hardness. But diamonds’ all carbon, extremely dense, structurally sound and tightly bonded structure is hard to beat. At present only diamonds created with isotopes of carbon have exceeded the mark of 10 on the hardness scale.
Hardness is particularly important for gemstones. Very few soft minerals are cut as gems and when they are, they generally are cut only for collectors and not for wearable jewelry. Apatite is one of the softest of gemstones. Mostly gemstones have a hardness of 7 or more. Hardness also plays a major apart in the minerals that are used for grinding, polishing and other abrasive purposes. Soft minerals can be used as high temperature lubricants, pencil lead, talcum powder, paper gloss, etc.
Here are a few hints on how to use hardness to identify minerals:
Keep in mind that most minerals have small differences in hardness according to the direction of the scratch and the orientation of the scratch and that some minerals such as kyanite and diamond, have a marked difference in hardness with respect to these factors.
A massive specimen will probably be softer than a single crystal and ideally hardness should only be used on individual crystals.
Some minerals will have a range of hardness due mostly to impurities or substitution of certain ions.
Don’t be fooled by a dust trail on a mineral after being “scratched” by a softer mineral. It may look as if the softer mineral scratched the harder mineral but in actuality the softer mineral just left a dust trail across the resistant surface of the harder mineral (always blow air over the scratch or rub across the scratch to be sure it’s a scratch).
Another clue to relative hardness is ease of scratching (both diamond and quartz scratch glass, but diamond scratches glass “. . . like a knife through butter”).
To remember the Mohs scale try remembering this phrase:
The Geologist Can Find An Ordinary Quartz, (that) Tourists Call Diamond!
Some Common Minerals and Their Properties
|Graphite||Black||Black||Metallic||1||Crystals are rare.|
|Mica||Colorless||White||Pearly||2.5-3||Flakes into sheets|
|Pyrite||Golden||Black||Metallic||6-6.5||Looks like gold|
Also use the following links for more information:
What do you want to find out? Write a statement that describes what you want to do. Use your observations and questions to write the statement.
The purpose of this project is to collect and identify certain minerals found in your area.
When you think you know what variables may be involved, think about ways to change one at a time. If you change more than one at a time, you will not know what variable is causing your observation. Sometimes variables are linked and work together to cause something. At first, try to choose variables that you think act independently of each other.
Independent variable is the location or search area.
Dependent variable is the type of mineral that you find.
Controlled variables are collection and identification method.
Based on your gathered information, make an educated guess about what types of things affect the system you are working with. Identifying variables is necessary before you can make a hypothesis.
If you live in an area known for it’s minerals and you can find publications and local mineralogy maps, you can use them and predict what minerals you will find in each area that you are planning to search. This will be your hypothesis. Following is a sample hypothesis:
I expect to find Slate (talk), marble and granite in my area, south west Vermont. My hypothesis is based on the information that I gathered from Vermont geological website and united states geological survey.
Design an experiment to test each hypothesis. Make a step-by-step list of what you will do to answer each question. This list is called an experimental procedure. For an experiment to give answers you can trust, it must have a “control.” A control is an additional experimental trial or run. It is a separate experiment, done exactly like the others. The only difference is that no experimental variables are changed. A control is a neutral “reference point” for comparison that allows you to see what changing a variable does by comparing it to not changing anything. Dependable controls are sometimes very hard to develop. They can be the hardest part of a project. Without a control you cannot be sure that changing the variable causes your observations. A series of experiments that includes a control is called a “controlled experiment.”
Visit US Geological Survey website and search for the name of your state and the word minerals. For example you may try New York Minerals and find links for geological maps and reports about New York Minerals. Find out what minerals may be found in your area.
Then Visit webmineral.com and try to find information and picture of minerals that you may find. Now you are ready to go out and search for minerals. Accompanied with an adult, visit a local state park, mountain or river to look for minerals.
In a river or a stream with clean water, minerals are washed and they appear in their natural color. That is where you can easily find good minerals. Look for homogeneous solid rocks with distinctive color and crystal structure. Note that you don’t need mud, you need rocks. Look for rocks with any distinguishable color. Place each rock in a separate plastic bag and label it with the location that you found it. Also record each collection in your journal.
Identification of minerals is done based on their physical properties. Make observations and record color, luster and transparency of each specimen in your data table. Then test the streak color, hardness and the specific gravity of each specimen and record them in your data table. Finally determine the cleavage, fracture and crystal form of each mineral and record them in your data table. Skip any property that you are not sure about. When your data table is ready, use a good field guide to identify each mineral.
If you don’t have a field guide, visit http://geology.csupomona.edu/alert/mineral/id1.htm and follow the step by step procedures to identify each of the minerals that you have collected.
Other options: You may also search for all minerals with a specific physical property. This method is not as fast, however it may be required for certain minerals. To search the database of minerals visit http://www.webmineral.com/search.shtml. The instructions on how to search and sample searches are available at the same page.
Materials and Equipment:
List of material that you use to determine the physical property such as hardness, streak color or specific gravity may vary. Final list of materials depends on your final experiment design and material and equipment that you can find. Following is just a sample:
- Streak plate
- Measuring cylinder
- copper piece
- finger nail
- piece of window glass
Results of Experiment (Observation):
Experiments are often done in series. A series of experiments can be done by changing one variable a different amount each time. A series of experiments is made up of separate experimental “runs.” During each run you make a measurement of how much the variable affected the system under study. For each run, a different amount of change in the variable is used. This produces a different amount of response in the system. You measure this response, or record data, in a table for this purpose. This is considered “raw data” since it has not been processed or interpreted yet. When raw data gets processed mathematically, for example, it becomes results.
The most important calculation that you may do is calculating the density of each sample that you collect. To do that first weight the sample. Then measure the volume of the sample by placing it in a measuring cylinder half full with water. Changes in the water level in the measuring cylinder shows the volume of your sample. Divide the weight by volume to get the density. I recommend to use metric system to calculate the density. So measure the weight based by grams and measure the volume by cubic centimeter.
Summary of Results:
Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.
It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.
Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.
Related Questions & Answers:
What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.
If you did not observe anything different than what happened with your control, the variable you changed may not affect the system you are investigating. If you did not observe a consistent, reproducible trend in your series of experimental runs there may be experimental errors affecting your results. The first thing to check is how you are making your measurements. Is the measurement method questionable or unreliable? Maybe you are reading a scale incorrectly, or maybe the measuring instrument is working erratically.
If you determine that experimental errors are influencing your results, carefully rethink the design of your experiments. Review each step of the procedure to find sources of potential errors. If possible, have a scientist review the procedure with you. Sometimes the designer of an experiment can miss the obvious.