9 Important Physical Properties of Minerals
There are quite a few physical properties of minerals. Many of those physical properties I have found in my years as a rockhounder/amateur geologist, do not draw as much curiosity as other properties.
I have found that there are some great physical properties of minerals that are really helpful in defining and identifying minerals when I am looking for some knowledge or guidance. This is not to say that all the physical properties behind a mineral are not important, but just that some carry more interest and seem to be great defining points of a mineral.
There are 9 important physical properties of minerals:
- Hardness
- Luster
- Color
- Streak
- Specific Gravity
- Cleavage
- Fracture
- Tenacity
- Crystal System
Many of us do not want to get down to all the minute details of minerals as we study and learn about them. That is well and okay and really we can leave those greater details to the geologists and those who aspire to go down that career.
In this article I will get into some further detail on these important physical properties of minerals you should know about.
Mineral Hardness
The hardness of a mineral is defined by using the Mohs mineral hardness scale. Mineral hardness is the resistance or difficulty in scratching a mineral. Mohs mineral hardness scale has a ranking of 1 to 10 associated with 10 minerals to help determine the hardness of a mineral.
Mineral hardness is important because it is one of the most useful physical properties in identifying minerals. Mineral hardness tools are often used in conjunction with the Mohs scale of hardness to assist in identifying minerals in the field or samples brought home.
Some common mineral hardness tools can include your fingernail, copper penny, glass/knife, steel, and even a masonry drill bit. If you are testing hardness of a mineral as part of a hobby or interest, you will want to put together a simple kit using these common mineral hardness tools.
As a great example in using hardness tools and the Mohs mineral hardness scale, you can figure out the difference between quartz or gypsum. Both can look fairly similar and that is where the confusion can happen. Both these minerals are a defining mineral on the Mohs scale of hardness. Gypsum is a 2 on the scale, very soft, and quartz being a 7, very hard.
If you can take your fingernail and scratch the surface of the mineral, then you may have gypsum, if not, you may have quartz.
Mineral Luster
Mineral luster is a physical property that indicates how well light is reflected from a mineral. The common mineral luster categories are adamantine, dull, glassy, greasy, pearly, resinous, silky, vitreous and waxy.
This physical property of a mineral can be quite helpful in identification. Many minerals that might look the same will have a very different light reflective surface.
Some examples of mineral luster can include chert which has a luster of dull or waxy, and calcite which can be dull, but is often vitreous to pearly. Both minerals can look like common rock you might find on the ground, but both will reflect light very differently.
For more references on some contrasting minerals that have different luster, see my articles “What is Chert”, which has a waxy dull look, and “All About Topaz” which has a very vitreous reflective surface. If topaz is on the ground and the sun catches it just right, you see a very bright reflection of light.
The Physical Property of Color
Mineral color is quite possibly the best know physical property of a mineral available. Most people interested in minerals will focus on this physical property immediately, as finding the color of a mineral only requires the use of your eyes.
The color of a mineral is definitely not a catch all though. Many people end up diagnosing a minerals identity incorrectly based on color. For example, many green minerals can often get identified incorrectly as there are a good handful of popular and well known green minerals.
Dioptase and emerald can often be confused with each other based on their color. They both can be very shinny and both can have a very nice deep green color. Many will often identify dioptase as an emerald because it really looks like an emerald. Many years ago I was even fooled for a moment on this one.
For a reference example of dioptase see my article on “All About Dioptase”. An article on emerald also is available here “All About Emerald”.
There are many colors of minerals in nature. These colors of minerals can range in black, white, browns, grays, greens, reds, blues, yellows, purple, and oranges. Don’t forget your metal type colors as well with copper, silver, and gold.
Mineral color is a great way to help identify a mineral, but remember, don’t use it as a catch all or you might end up getting it very wrong.
A Mineral Has Streak
Mineral streak is where you take a mineral and scratch it across a porcelain streak plate. When doing this, you produce a colorful mineral powder left behind on the porcelain surface. This colorful powder, or streak, is an important physical property of a mineral.
The streak test is helpful in identifying a mineral because of the unique colorful powder that is left behind on the streak plate. With the streak test and a couple of other helpful physical properties, you can more easily identify a mineral.
One of my favorite examples of mineral streak is the streak lapis lazuli creates and the streak sodalite creates. Many people will quickly confuse these two minerals straight away. Both are blue with white inclusions and at times can visually look quite similar. If you do a streak test on lapis lazuli you will get a light blue, and on sodalite you will get white.
For reference on both of these minerals I have these great articles “All About Lapis Lazuli” and “All About Sodalite”. These articles will go into further detail about the two minerals.
Specific Gravity of a Mineral
The specific gravity of a mineral requires measuring the mass of the mineral to the equal mass in volume of water. This measurement is a way to measure the density of minerals as minerals can have varying densities.
This particular physical property of a mineral is not as quickly obtainable as some of the other properties. Not many have the scale(s) to do such a measurement, however this measurement is a great way to help identify a mineral.
There are some different ways to calculate the specific gravity. The common one I know of is to do the weight of the mineral in air divided by the weight of the mineral in air minus the weight of the mineral in water.
Mineral Cleavage
The physical property of mineral cleavage is the ability for a mineral to break smoothly along the minerals plane of weakness. This weakness is in the chemical bonding along a plane or multiple planes of the mineral.
I know, often people think of something else when it comes to the word cleavage… I will avoid going there. Cleavage is a pretty good way of identifying a mineral as minerals will often have different planes of weakness in their chemical makeup.
Take the mineral fluorite for instance. It has four perfect cleavage planes that can easily create an octahedral shape. Some use fluorite as the best example of a…, eh-hum…, a perfect cleavage, as it breaks off perfectly along the planes that it does.
For more information and pictures of fluorite you can visit my article on fluorite here “All About Fluorite”
In contrast, Topaz will break easily on one plane. This plane is horizontal to the crystal and often you can see topaz cracked or broken along this weaker bonded plane.
Mineral Fracture
Mineral fracture much like mineral cleavage, is a break in the mineral. With a mineral facture the break is not just smooth, but can be conchoidal, sub-conchoidal, rough, fibrous, and uneven. A fracture break is a break that is not along one of the planes.
A mineral fracture is caused by a force to the mineral which breaks it in a way other than on a natural plane. You can help identify a mineral if you can find a fracture in it not existing along one of its natural planes. This fracture can have a unique texture to it which can identify with a specific mineral.
Mineral Tenacity
Mineral tenacity is simply the behavior of a mineral when broken. Tenacity categories of a mineral are brittle, ductile, elastic, plastic, malleable and sectile.
This physical property of a mineral is used to help further identify a mineral as various minerals will behave differently when they are broken.
For a more precise definition of each tenacity category you can find that at Wikipedia “Tenacity”
Crystal System of a Mineral
The mineral crystal system is an important physical property of a mineral. The crystal system of a mineral is categorized into seven different crystal systems; cubic, tetragonal, orthorhombic, monoclinic, triclinic, hexagonal, and trigonal.
Learning the crystal systems may be somewhat difficult. Not only are there crystal systems but also crystal group/classes under those crystal systems. This is where book and internet references can help with this particular physical property.
When we take this physical property to the eye level, we can quickly note with many different crystals that there is an obvious difference in how the crystal looks.
Physical Properties of Minerals Conclusion
We have noted that there are plenty of physical properties of minerals available to us. These properties are used for identifying a mineral and also defining a mineral.
I have gone over what I believe to be the most important physical properties of minerals that people should pay attention to. I by no means say other physical properties have no merit. The ones I discussed have the best use for me. If you are interested in finding other useful ways to identify your rocks and or minerals, I have written a pretty good article on several ways you can do just that. “7 Ways to Identify My Rock or Mineral”.
