On a lazy sunday morning, after waking up an hour later than usual - not because I'm a long sleeper, but because daylight savings time took an entire hour off my day - and walking around in my apartment, my eyes fell on the minerals that decorate my kitchen table.
I thought: "Minerals. I love minerals. And I love learning more about them. I could do a series about minerals! I could call it "MineralMonday". I could post a new mineral on my blog each Monday and share some information about them worth knowing. This way, I'll refreshen my own knowledge and enlarge my personal database PLUS I'll share it with other people and show them how beautiful minerals can be."
A few hours later, I'm sitting at my laptop and typing this first article of my new series: MineralMonday.
If you're anything like me, you need a structure in your head to categorize new information accordingly. This helps me understand and memorize new knowledge much much better.
So, before I throw all that new information about the various minerals we can find on our planet at you, I want to briefly explain a few basic terms first.
This glossary will be expanded as MineralMonday will take shape!
Crystals are solid objects composed of atoms, molecules and ions. These basic parts form a three-dimensional structure, a lattice. The most primitive structure inside this lattice is the unit cell, which is repeated periodically, constructing a more or less symmetrical geometric object: the crystal lattice.
Crystals are anisotropic. This means that certain physical and chemical properties are dependent on the direction they pass through the crystal.
The way in which the above unit cells are repeated determines the crystal's symmetry and therefore its physical and chemical properties. There exist a few basic symmetrie operations: translation, rotation, reflection, inversion and rotoinversion.
Crystal Systems & Classes
Symmetrie operations can only be combined infinitely. To be precise there exist 32 unique ways different symmetry operations can be combined. These are called the 32 crystal classes.
These crystal classes are categorized into 7 crystal systems, according to their individual charateristic set of symmetry operations and level of symmetry.
The 7 crystal system are (from lowest to highest symmetry):
Minerals are physically and chemically homogenous solid objects found on Earth, the Moon and other celestial objects. They are anorganic, this basically means non-living (as opposed to e.g. plants or sugar) and also crystalized.
Minerals form crystals.
Almost all minerals are crystals, but not all crystals are minerals. E.g. sugar has a crystalline structure, but chemically it is considered to be organic. Therefore, it is not mineral. Unlike salt, which is made up of Sodium (Na) and Chlorine (Cl) in a crystalline lattice, hence considered inorganic and a mineral.
They are held together by chemical bonds: covalent bonds, strong ionic bonds (=electrostatic bonds), weak Van der Waals bonds or metallic bonds.
Minerals with the same chemical composition and crystal structure belong to one mineral group. However, these minerals can differ in colour, (trace-)chemistry, shape or form (morphology). These are called mineral varieties.
Rocks are heterogenous objects. They are made up of different minerals and form mineral aggregates.
Cleavage, Streak and Mohs Scale
Some imporant features that allow the distinction of minerals without a microscope are mineral properties such as colour, cleavage, streak and hardness.
Describes the way a mineral breaks or splits apart along certain lines of weakness defined by the internal crystal order. Weaknesses may occur due to weak bonds between atomic layers inside the lattice. Greater lattice spacing (the distance between two ions) usually accompanies weaknesses. Some minerals can be easily recgnized by their quality, the angles and direction of cleavage. Not all minerals display cleavage.
When grinding into powder (e.g. when using the mineral to "write" on another surface such as a porcelain plate) a mineral may show a different colour than it did before.
Minerals possess differing grades of hardness by which they can be identified.
When determining a mineral's hardness, its resistance to scratching is examined. Will the test mineral be able to scratch the reference mineral or will it be scratched by the reference mineral?
The Mohs Scale is defined by 10 reference minerals:
- Talc (can be scratched with a fingernail)
- Gypsum (can be scratched with a fingernail)
- Apatite (window glass = 5,5)
- Feldspar (can scratch glass)
This order isn't linear. The scale is ordinal, which means that the distances between the categories can vary. F.e. corundum is twice as hard as topaz, but diamond is four times harder than corundum.
USGS. Note: A sample of azurite, the blue mineral, and malachite, the green mineral. Both azurite and malachite are copper minerals that were once used as pigments but are now mostly valued as collectors minerals.
Borchardt-Ott, Walter und Sowa, Heidrun (2018): Kristallographie, Eine Einführung für Studierende der Naturwissenschaften. 9- Aufl. Springer Spektrum. Berling Heidelberg 2018.