Introduction: (Initial Observation)
Some minerals are so soft that are being used as baby powder and additive for creams and cosmetics. Some others are so hard that are being used as abrasives in making sandpapers and grinding wheels. Hardness is one of the important physical properties of minerals and sometimes is used as a way to identify minerals.
Hardness of a mineral is a measure of a mineral’s resistance to abrasion and reflects the atomic structure of a mineral.
Find out about Minerals 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.
|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:
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?
Below is a revised Mohs Hardness Scale with some everyday items listed:
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.
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.
streak plate: Unglazed piece of porcelain, such as a tile, used to test a mineral’s streak.
Streak: The color of the powder of a mineral. Streak can be tested by rubbing a mineral against a hard, white, porcelain object (streak plate), and the “streak” that remains on the swiped area is the color of the streak of the mineral.
Also use the following links for more information:
Question for this project is: How hard are various types of minerals?
We intend to learn about methods of testing hardness in minerals and perform such tests on samples that we may obtain.
Not needed for this project.
You may have different hypothesis for this project based on the application that you may have in mind for this project. Following are some examples.
- Harder minerals have higher density?
- Harder minerals can be found in rivers and beaches, because they are not easily washed away.
- Minerals with smaller molecules are harder.
Remember that hypothesis are not facts and may be proven to be incorrect.
The experiment includes collecting some minerals and testing their hardness based on known objects such as fingernail, copper and knife.
While searching for minerals we should preferably look for crystals.
Materials and Equipment:
Fingernail, copper (Penny), knife blade, steel file and samples of known minerals.
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.
Not needed usually.
Summery of Results:
Make a table with a number for each item that you test, Its hardness, Possible name and other physical properties such as color and luster.
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.
List of References