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Arenys de Mar Museum. Mollfulleda Mineralogy Museum

Les propietats dels minerals

What are minerals like?

Colour

Colour is one of the properties used to identify minerals. The colour of a mineral can come from a variety of factors associated with its chemical composition and structure. So, all minerals have a characteristic colouring, which we call their idiochromatic colour. However, sometimes, one mineral can have several colours. Why does this happen? There are chemical and physical processes that can alter the normal colour of minerals and cause what is known as allochromatic colouring. There are several causes that can lead to this, such as defects in the crystal structure, called ‘colour centres’.

Allochromatic colouring can also be caused by small amounts of chemical elements such as Ti, V, Cr, Mn, Fe, Co, Ni or Cu. So, the same mineral species can have different colours depending on whether it contains these elements. Another cause of allochromatism is microscopic inclusions of other minerals that can have very intense colouring.

Did you know... Chalcopyrite (CuFeS2) is a metallic mineral that contains copper and iron, but its golden-yellow hue has sometimes caused it to be mistaken for gold. This is why it is popularly known as ‘fool’s gold’.

Shape

Minerals have an internal structure made up of atoms. The internal organisation of these atoms can be orderly, symmetrical or generate external geometric shapes (crystals). Minerals can crystallise in line with the symmetry of the 7 crystalline systems:

  • Cubic
  • Tetragonal
  • Orthorhombic
  • Hexagonal
  • Trigonal
  • Monoclinic
  • Triclinic

The external morphology of a crystal can vary widely and, depending on their shape, we speak of different crystal habits.

  • Equidimensional habit: typical of cubic minerals
  • Prismatic habit: long, column-like shapes
  • Acicular habit: very long and slender, needle-like crystals
  • Tabular habit: thin, four-sided crystals, shaped like a table or book
  • Lamellar habit: very thin, flat layers, like sheets of paper

Hardness

Hardness is how resistant minerals are to scratching. So, the harder the mineral, the more difficult it is to scratch. One way to measure this is the Mohs scale, which is a list of 10 minerals from softest to hardest that determines the hardness of minerals by comparison.

Did you know... Diamonds are the hardest known mineral and can only be scratched by another diamond. However, they are also very fragile and can be broken easily, with a hammer, for example.

Magnetism

The most well-known manifestation of magnetism is the froce of attraction or repulsion between some materials. Some iron minerals like magnetite (Fe2+Fe3+2O4) and pyrrhotite (FeS) have different types of magnetism.

Find out about magnetism! Move this magnet under the fragments of magnetite and pyrrhotite. (Aquesta frase al costat de la caixa).

OBJECTS

<p><em>Bismuth,</em>&nbsp;Valencia de Alcantara.&nbsp;Photo by Josep Rosell.</p>
Bismut
Bismut
<p><em>Columbite,</em>&nbsp;Cap de Creus Park. Arenys de Mar Museum, inventory no. 5837. Photo by Irene Masriera.</p>
Columbite (Fe)
Columbite (Fe)
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