minerals, crystals…
Minerals can be described as crystallised substances, characterised by a regular internal structure consisting of regularly arranged atoms in space.
Under special, favourable conditions, they can develop in the form of regular geometric bodies – polyhedra, bounded by flat faces that are the external reflection of their regular internal structure.
Such formations represent one of the most beautiful creations in nature, and we call them CRYSTALS.
The word crystal comes from the Greek word kryos, meaning “ice-cold”.
In ancient times, the ancient Greeks believed that colourless quartz crystal, rock crystal, was formed from the purest water in high mountains, which under conditions of extreme cold turned into a permanent form of ice.
In older Croatian literature, the word used for crystal was – ledac!
For many, encountering crystals of a mineral is an exceptional moment, whether because of the incredible symmetry and regularity of form, their clarity or the lustre of smooth faces, or simply because of the enchanting beauty created by nature itself.
As our well-known mineralogist Fran Tućan beautifully described it at the very beginning of his exploration of the world of minerals back in 1934:
“I stood before something unknown, something never seen before, something that captivated me, something full of beauty, tenderness and grace. Large mineral crystals in all kinds of colours, red, purple, green, yellow, some completely colourless, and some black as soot. Strong light reflected from their flat faces and everything shimmered, sparkled and glittered.”
Fran Tućan: “Among Minerals and Rocks”
There are many crystal polyhedra, and all of them are characterised by a certain degree of symmetry.
By studying crystals, primarily by measuring the angles between crystal faces, the basic set of symmetry elements, centre of symmetry, planes of symmetry and axes of symmetry, possessed by a particular crystal polyhedron can be revealed.
It is also possible to determine the set of symmetrically identical faces developed on the crystal, which are mutually connected by symmetry elements, and these are called CRYSTAL FORMS.
When describing a crystal, we state which forms are present on it, for example hexahedron, octahedron, prism or pyramid, and to what extent they have developed. We say that we are determining the MORPHOLOGY of the crystal of a mineral, morphology – Greek μορφή, morphé = form.
An instrument called a goniometer is used to measure the angles between individual faces on a crystal.
For this purpose, the Mineralogical and Petrographic Department of the Croatian Natural History Museum uses a two-circle reflection goniometer, an instrument almost one hundred years old.
It is very precise, and in ideal cases it reaches an angular measurement accuracy of about 1 minute, 1′!
Different minerals can crystallise in the same crystal forms, for example halite, fluorite, galena and pyrite can crystallise in hexahedral forms, cubes.
A single mineral can crystallise in different, but symmetrically permitted, forms dictated by its internal structure.
The mineral calcite can crystallise in one form, for example a scalenohedron or rhombohedron, but very often it can also develop in combinations of several different forms on a single crystal.
Crystal forms are marked with so-called Miller indices in curly brackets { }.
There are 32 possible combinations of symmetry elements that define the external symmetry of crystals, so we can say that all mineral species, as well as an incomparably greater number of all crystallised substances, can be arranged into 32 groups, or 32 CRYSTAL CLASSES.
Crystal polyhedra related to these 32 crystal classes can also be classified and divided among 6 different coordinate, or CRYSTAL SYSTEMS.
We will list them as such, and alongside them mention well-known examples from the world of minerals and the most common crystal forms that occur on them.
Cubic crystal system
Tetragonal crystal system
Hexagonal crystal system
Orthorhombic crystal system
Monoclinic crystal system
Triclinic crystal system