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Arenys de Mar Museum. Mollfulleda Mineralogy Museum

The systematic collection

The systematic collection room. Photo: Museu d'Arenys de Mar.

The systematic collection room. Photo: Museu d'Arenys de Mar.

MINERAL CLASSIFICATION

Minerals are classified by scientific criteria based on their chemical composition and crystal structure.

There are several systems used to classify them, but the most commonly used are the Dana and Nickel-Strunz systems. Both classifications are equally valid and accepted, although they differ in small but substantial ways.

For this exhibition, we have used a simplified version of the Dana classification system, which distinguishes between the following classes of minerals:

Class I - NATIVE ELEMENTS

 

Class II - SULPHIDES, ARSENIDES, TELLURIDES, SELENIDES and SULFOSALTS

Class III - OXIDES and HYDROXIDES

Class IV - HALIDES

Class V- CARBONATES, NITRATES and BORATES

Class VI- SULPHATES, CHROMATES AND SELENATES

Class VII- PHOSPHATES, ARSENATES, VANADATES, MOLYBDATES and TUNGSTATES

Class VIII - SILICATES

Subclass NEOSILICATES

Subclass SOROSILICATES

Subclass CYCLOSILICATES

Subclass INOSILICATES

Subclass PHYLLOSILICATES

Subclass TECTOSILICATES

Class IX - ORGANIC MINERALS

I - NATIVE ELEMENTS

This class is made of minerals comprised of only one chemical element (gold, copper, silver, arsenic, diamond, etc.) and their natural alloys.

II - SULPHIDES, ARSENIDES, TELLURIDES, SELENIDES and SULFOSALTS

Sulphurs are minerals made up of a metal element and sulphur. Also included in this class are arsenides and selenides, which are the combination of metal elements and arsenic or selenium, respectively. Sulfosalts, however, are more complex compounds made up of metals or semi-metals and arsenic, antimony or selenium.

III - OXIDES and HYDROXIDES

The minerals in this class are found in abundance in the Earth's crust. The oxides contain oxygen and the hydroxides, water. Some of the most characteristic minerals in this class are iron oxides like haematites and aluminium oxides like corundum, which can have a variety of colours, including red (rubies) and blue (sapphires).

IV - HALIDES

This class is made up of minerals combining metals and the chemical elements in the halogen group: fluorine, chlorine, bromine and iodine. Some of these minerals are very common, like fluorite and halite, which is also known as 'table salt'.

V- CARBONATES, NITRATES and BORATES

This class includes numerous species, among them the minerals made up of the carbonate (CO32-), nitrate (NO3-) and borate (BO33-) chemical groups. Some of these minerals, such as calcite and aragonite, can form in many different geological settings and have a wide range of colours and shapes.

VI- SULPHATES, CHROMATES AND SELENATES

This class comprises minerals made up of chemical anions: sulphate (SO42-), chromate (CrO42-) and selenate (SeO42-). Some very common minerals belong in this group, such as gypsum (CaSO4 · 2H2O), which has a wide range of applications in the industrial sector.

VII- PHOSPHATES, ARSENATES, VANADATES, MOLYBDATES and TUNGSTATES

The minerals in this large group are often very colourful and come in a wide range of shapes. They are species that combine metals and the chemical groups: phosphate (PO43-), arsenate (AsO43-), vanadate (VO43-), molybdate (MO43-) and tungsten (WO43-), respectively.

VIII - SILICATES

Silicates are the most numerous group of mineral species and the most abundant in the rocks that make up the Earth's crust. They comprise a combination of any of a variety of chemical elements and the silicate group (SiO44-). The crystalline structure of silicates is organised in the tetrahedron shape of the silicate group, which can combine in different ways. According to their morphology, this combination of SiO4 tetrahedrons are broken down into 6 subclasses of silicates: neosilicates, sorosilicates, cyclosilicates, inosilicates, phyllosilicates and tectosilicates.

VIII - Subclass NEOSILICATES

The SiO4 tetrahedrons are isolated and combine with other chemical elements through ionic bonds. The most representative minerals in the neosilicate subclass include species in the olivine group (forsterite and fayalite) and the garnet group (uvarovite, grossular, andradite, pyrope, almandine and spessartine).

VIII – Subclass SOROSILICATES

In sorosilicates, the SiO4 tetrahedrons join in pairs with a covalent bond. This pair of tetrahedrons then combines with other chemical elements to form minerals like axinite, epidote and hemimorphite.

VIII – Subclass CYCLOSILICATES

In this subclass, the SiO4 tetrahedrons form highly characteristic circular structures. Representatives of the cyclosilicates include minerals in the tourmaline group (schorl, elbaite, dravite, etc.) and beryl, which can come in very colourful and valued varieties like aquamarines and emeralds.

VIII – Subclass INOSILICATES

The structure of the inosilicates is characterised by its SiO4 tetrahedrons laid out in rows or chains. The chains can be single or double if they have two columns of tetrahedrons joined together. Some characteristic minerals in this subclass are the species in the pyroxene group (augite, enstatite, diopside, etc.) and the amphiboles (hornblende, tremolite, actinolite, etc.).

VIII – Subclass PHYLLOSILICATES

In phyllosilicates, the SiO4 tetrahedrons form layers or sheets of tetrahedrons joined together by weaker bonds. The layered structure means most minerals in this subclass, externally, are laminar. This characteristic is easy to see in some species in the mica group like biotite, muscovite and phlogopite.

VIII – Subclass TECTOSILICATES

The minerals in this subclass have a highly compact structure made up of a three-dimensional framework of SiO4 tetrahedrons joined by covalent bonds. The tectosilicates include species in the feldspar group (orthoclase, microcline, albite, etc.) and those in the zeolite group (stilbite, heulandite, natrolite, mesolite, etc.).

VIII – Subclass TECTOSILICATES (Quartz)

Quartz (SiO2) is one of the most abundant minerals in the Earth's crust. It is colourless but can have impurities giving it a variety of hues: purple (amethyst), red (haematoid), yellow (citrine), etc. There are also other varieties of quartz classified by the shape and size of their crystals, like flint and chalcedony. The crystalline structure of quartz is that of a tectosilicate, but chemically quartz is a silicon dioxide. The Dana classification prioritises structure over chemical make-up in this case and considers it a tectosilicate. However, in other classifications, such as the Nickel-Strunz system, priority is given to the chemical composition and quartz is put in the oxides class.

IX - ORGANIC MINERALS

This class incorporates minerals made through organic activity that may be components of some rocks, such as amber.

RADIOACTIVE MINERALS

Radioactivity is a naturally occurring physical phenomenon. It happens when the atoms in some chemical elements are unstable and transform spontaneously into other, more stable nuclei.

The nuclei of the atoms achieve this stability by giving off particles (there are two different types: alpha α and beta β) accompanied in some cases by radiation (gamma rays γ).

Some radioactive elements, such as uranium and thorium, form mineral species that can be used to extract these elements and use them for many applications: nuclear power, medicine, etc.

PYRITES

Pyrite is an iron disulphide that forms cubic crystals. It is one of the most abundant sulphides in nature and humans have been using it since antiquity to extract sulphur, iron and other elements like gold, which it may contain in some cases as impurities.

In mineral collections, pyrite poses a huge problem, as it reacts to the oxygen and humidity in the air and often breaks apart and dissolves. This is known as 'pyrite decay'.

OBJECTS

<p>Photo: Arenys de Mar Museum</p>
Dolomite
Dolomite
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