Introduction: How to Identify a Mineral
Identifying minerals is like playing a sport. You are given a specific set of principles or rules to follow. To be good at a sport like basketball, for instance, one must not only know the rules, but he/she must also practice. The same concepts applies for mineral identification. Once you know the guidelines, then you must apply what you have learned and develop a skill for it. Especially considering there are over 4,000 different minerals in the world.
What is a mineral?
There are five requirements that must be met in order for a substance to be deemed a mineral:
· Naturally occurring
· Definite chemical composition
· Ordered crystal structure
“Naturally occurring” means that it was created in nature. “Inorganic” means that it is not made by an organism. “Solid” means that it is not a liquid or gas at standard temperature and pressure. “Definite chemical composition” means that all occurrences of that mineral have a chemical composition identical within a specific limited range. “Ordered crystal structure” means that the atoms in a mineral are arranged in a systematic and repeating pattern.
Mineral identification and the collection of gemstones date back to the early Greco-Roman empire, early China, and ancient Babylonia. The first recorded document on this topic dates back to 77 AD with the Greek philosopher Pliny the Elder. He not only discusses his love of gemstones and various other minerals, but also many of its properties.
In 1556 AD Georgius Agricola wrote De re Metallica (metals) and De re Fossilium (natural rocks), which took a systematic study of minerals and their various properties. Mineralogy based on its crystal structure and optical properties was founded in the 17th century with the invention of the microscope.
· Hand lens (10X)
· Rock hammer
· HCL (hydro-chloric acid)
· Mineralogy book
· Scratch plate
· 1 U.S. copper penny
· Piece of glass (2x2 inches)
· 1 Nail
· Pocket knife
· UV light
· 1 Paper clip
· Inexpensive digital scale
· 1 plastic cup ( I prefer to use a small cup)
Description of Required Tools: Hand lens: a device used as a hand held microscope. Rock hammer: a durable hammer that has a flat head on one side of the end and a pick on the other side of the end, usually with a shock absorbent cushion used for breaking dense minerals/rocks. HCL (hydrochloric acid): an acid that will fizz upon contact of certain minerals due to the mineral chemical composition. Mineralogy book: used to look up a mineral based on its specific set of properties. Scratch plate: a dense plate used to observe minerals in powered form. Digital scale: used to take weight measurements usually smaller than 300 grams.
Step 1: Pick Your Mineral
The first step in this process is to a pick a mineral. Grab a notepad and a pencil so that you may record the properties of your mineral based on:
· Specific gravity
Step 2: Color
Color can be essential in mineral identification, but it can also be rather complicated. Experts use color all the time, but only because they have had sufficient practice at identifying minerals, and usually know the exceptions for common minerals. If you are a beginner, use color to help identify, but do not depend on it.
Color is pretty reliable on opaque and metallic minerals. For instance Galena always has a bluish gray metallic color and pyrite (fool’s gold) is always brass-yellow metallic.
For minerals that are transparent or translucent, color is usually not a good indicator. Color is the result of impurities. When it comes to identification by color and recording it on your notepad, try to be as precise as possible. It can even help to relate the color to a more common object that is associated with a specific color (fire truck red or blueberries).
Step 3: Hardness
For this step you will need:
· 1 copper penny
· 1 knife
· 1 piece of glass (2x2 inch)
· 1 nail
The hardness of a mineral can really help narrow down your choices as which mineral you are testing. The 10-point Mohs scale for hardness is what Geologists use word-wide. Most of the minerals you will encounter will be between 2 and 7.
If a mineral can be scratched with a copper penny, but cannot scratch glass, then your mineral has a hardness between 3-6.
Step 4: Luster
The luster of a mineral is very essential to mineral identification. It can help narrow down the type of mineral, whether it is a sulfide, a carbonate, a silicate, etc. Luster is the way light is reflected off the mineral. Be sure to be in a well-lit area to accurately measure luster.
The main types of luster include:
· Metallic: having the look of a polished metal
· Submetallic:having the look of metal that is dulled by weathering
o Adamantine: having a hard, sparkly look of a diamond
o Resinous: having a look of yellow, dark orange, or brown that is slightly reflective
o Vitreous: having the look of glass
o Pearly: having the look of a pearl
o Greasy: having the look of an oil coated surface
o Dull: having a plain looking surface
o Earthy: having the look of soil or clay
o Silky: having the look of fine, parallel fibers
Step 5: Specific Gravity
For this step you will need:
· Inexpensive digital scale
· Plastic cup filled with water
· Paper clip
The specific gravity (SG) of a mineral refers directly to its density (mass compared to volume).
It indicates how many times more the mineral weighs compared to an equal amount of water (SG 1).
So if you have a bucket of sliver (SG 10), it would weigh 10 times as much as a bucket of water.
Follow these simple instructions to determine the specific gravity of your mineral:
1. Push the “Power” button on the digital scale. The scale should read 0.0 with nothing on it.
2. Place your dry mineral on the scale and record its weight.
3. Remove the mineral.
4. Place a water-filled container on the scale and push the “tare” button to “zero” it out.
5. Unwind paper clip and wrap it around the mineral, leaving enough of the paper clip to hold with your two fingers.
6. Submerge the mineral in the water, but do not let it touch the bottom (important: be sure that your fingers are not touching or submerged in the water with your mineral specimen).
7. Record the weight of the mineral submerged in the water.
8. Take the original weight of the mineral and divide it by the weight of the mineral submerged in the water.
The quotient (answer) of the two weights is a mineral’s specific gravity.
Step 6: Streak
For this step you will need a scratch plate.
The streak of a mineral is the color of the mineral when it is powdered.
This can be produced by taking the mineral and scrapping it across the surface of a streak plate or something harder depending on the hardness of your mineral. The mineral’s color in powdered form can be a better indicator than its original color. When you have successfully achieved a powdered streak from your mineral, record its color (try to be as specific as possible). Some minerals like hematite can be black in color, but can have a reddish-brown streak.
Step 7: Smell
Once you successfully get a streak from a mineral now is a good time to smell it. Some minerals that contain sulfur, for instance, have a very distinct smell (rotten eggs). Try to relate the smell of the mineral to other common smells such as rotten eggs, garlic, meatballs, etc.
To properly smell the mineral, stick your nose close to the powdered streak, and take a light whiff. Be careful not to smell too hard, or you will suck the powder up your nose. That can be potentially dangerous depending on the mineral.
Step 8: Taste
As strange as it may sound some minerals can be readily identified by their taste. Now this may not always be a good idea and should only be used in specific circumstances. Minerals that are translucent or transparent are good candidates for a taste test. Halite, for example, has a very salty taste. It is made up of sodium and chlorine (NaCl), which is also referred to as rock salt.
To taste the mineral simply stick out your tongue and touch the tip of it to the mineral.
Step 9: Magnetism
For this step you will need a magnet.
Some minerals that contain enough iron or other metals can generate an electromagnetic force. Magnetite, for example, is strongly magnetic and its key property indicator is the fact that it is magnetic.
Use a small magnet and run it across the mineral. If the magnet is attracted to the mineral then your mineral is magnetic.
Step 10: The "Acid" Test
For this step you will need HCl (hydrochloric acid).
Some minerals, specifically the carbonates group, contain the compound calcium carbonate (CaCO3). Take your HCl bottle and drop a small amount on the mineral. If it fizzes, then it is part of the carbonate group.
Step 11: Cleavage/Fracture
For this step you will need a rock hammer.
All minerals have a specific crystal structure and when enough stress is applied the mineral will break along certain planes of weakness. Take your rock hammer and try to break a piece off of your mineral. If it breaks in a planar manner then it has cleavage. The number of cleavage planes differ from mineral to mineral. Some have 1 cleavage angles, 2 cleavage angles, 3 and 4 cleavage angles.
· Perfect: produces smooth surfaces
· Imperfect: produces planes that are not smooth
· Poor: less regular
Some minerals do not readily break along its cleavage planes. This type of breakage is called a fracture. How a mineral fractures can also be used an indicator.
· Conchoidal: fracture surface is a smooth curve, often bowl-shaped (common in glass)
· Hackly: produces sharp jagged edges
· Uneven: surface is rough and irregular
· Fibrous: surface shows fibers or splinters
Step 12: Other Minor Properties
For this step you will need:
· UV light
There are some properties that are not common in many minerals. These properties can be a key indicator as to what exactly your mineral is. These include:
· Birefringence: it is based on a mineral’s crystal structure and how light is transmitted through it. This only works if the mineral is transparent or translucent. A common mineral that displays birefringence is Calcite.
To determine if your mineral has birefringence do the following:
1. Take a pencil and draw one line on a sheet of paper.
2. Take your mineral and set it on the sheet of paper where you drew the line.
3. If two lines appear when the mineral is on top then your mineral is birefringent.
· Fluorescence: some minerals emit visible light when exposed to ultraviolet light.
Step 13: Apply the Data
Now that you have successfully tested and recorded all of your mineral properties, it is time to open up your mineralogy book and correlate your data with the information available. Good Luck!
Step 14: Troubleshooting
A mineral flow chart will help you to understand how to relate your data with a mineralogy book.