Science Festival at the Croatian Natural History Museum
SCIENCE FESTIVAL at the Croatian Natural History Museum
9 – 13 April 2019
The colour of minerals – natural formations of a specific chemical composition and crystal structure, formed as a result of natural physical and chemical processes and stable under certain temperature and pressure conditions, is one of the many properties of minerals.
Because of their different chemical composition and structure, minerals differ from one another in a whole range of physical properties such as shape, colour, cleavage, fracture, hardness and density. In addition, some minerals are characterized by specific magnetic and electrical properties, as well as fluorescence, phosphorescence or radioactivity.
Along with shape, colour is the first feature noticed on a mineral. It reflects the nature of the interaction between electromagnetic radiation in the visible spectrum and the electrons of atoms, ions or molecules from which a particular mineral is built. The colour of a mineral is the result of selective absorption and reflection of light rays from the surface of the mineral.
Minerals can have their own characteristic colour if, as essential components, they contain elements that absorb a certain part of the visible light spectrum. Such minerals are called idiochromatic minerals, and their colour is a constant and predictable property of the mineral. Examples of idiochromatic minerals are azurite, which is always blue, cinnabar, which is red, and malachite, which is green.
The colour of some minerals is caused by inclusions of other minerals, the content of trace elements or structural defects, which is most often variable. Such minerals are called allochromatic minerals, and their colour is a variable and unpredictable property of the mineral.
An example of an allochromatic mineral is quartz, which occurs in nature in differently coloured varieties such as purple amethyst, yellow citrine, brown smoky quartz, black morion, pink rose quartz and others.
In certain types of minerals, the colour is apparent and is caused by changes on or below their surface due to the diffraction of light. Such minerals are called pseudochromatic minerals, which in a free translation would mean falsely coloured minerals.
In pseudochromatic minerals, the colour is variable, but this variability is a unique property of the individual mineral. The best-known examples are opal, which has a unique “play of colours”, and labradorite, which has unique light reflections known as the “schiller effect”.
In mineralogy, colour is one of the primary diagnostic properties of minerals, and in order to determine the true colour of a particular mineral, mineralogists also determine the streak of the mineral.
The streak is the powdery trace left by a mineral when it is rubbed across a rough ceramic surface, and it corresponds to the true colour of the mineral.
More at: http://www.festivalznanosti.hr/2019/zagreb/