When the word “mineral” is used colloquially, it is very common for it to be mixed with concepts such as rock, stone and probably many others. but in geology a mineral is not equivalent to a rock, and many to a stone—a general term used to refer to solid materials, but without much geological meaning.
The 5 requirements that define a mineral
In general, and with many nuances that we will see later, most minerals meet these 5 requirements:Is natural Is solid at normal temperature (25°C) Is inorganic Has a crystalline structure Can be described by a specific chemical formula
Exceptions and nuances
Although for many human action is completely natural, only substances that are found and formed in nature without human intervention are considered minerals. Substances that meet the other criteria but are of anthropogenic origin would be excluded.
However, until the mid-1990s, some crystalline solid substances formed by the degradation of artificial substances were accepted as minerals, and although this option in geology is completely closed today, you can find some texts where they still appear.
We all think of solid minerals, and nearly all of them are, but there are notable exceptions. The clearest example we have is mercury ore, a liquid substance to form crystals that needs temperatures below -40 ºC, but it is traditionally and officially considered a mineral, although only in some regions of Antarctica can it be found naturally in crystalline form.
Furthermore, the so-called study of liquid crystals, a state of matter with properties between conventional liquids and solid crystals, largely requires the knowledge provided by mineralogy.
Organic substances and activity of living beings
Minerals are also often defined as inorganic substances. For example, solid sugar forms crystals, but it is an organic substance and is not considered a mineral. However, there is a group of mineral hydrocarbons, a rather rare group of minerals, but included in the mineral classification of the International Association of Mineralogy (IMA).
A criterion for defining a mineral that was not mentioned above, but which is closely related to the inorganic character, is that of being an abiogenic substance, that is, that it was not formed by biological processes; for example, calcium oxalate crystals in plant tissues. But this criterion actually means that it is not formed exclusively through biological processes.
An example of a substance of biological origin that ends up forming a mineral is calcite. Many marine animals secrete calcium carbonate to form their shells, but it does not become the mineral calcite until it undergoes certain geological processes.
So what is a mineral?
A mineral can be more accurately defined as a substance with a crystalline structure and a fixed chemical formula. Furthermore, they are usually, but not always, solid, inorganic and abiogenic. More than 5,000 different minerals are known to exist on Earth, each with a different chemical formula and crystallized in a particular crystal system, although most of them are rare.
However, to determine the crystalline structure at the molecular level, X-ray tests are usually used, and there are some substances that would pass other criteria that the X-ray test does not pass, these are called mineraloids.
So it doesn’t seem like you can clearly define what a mineral is, other than going to the International Mineralogical Association’s list of approved minerals and seeing which ones have been accepted as minerals.
A rock is an aggregate of minerals
Although the definition of a mineral leaves many aspects open, it is clearly different from a rock. A rock is defined as an addition of one or more minerals. A mineral crystal would not be a rock, but an aggregate of several crystals or mineral grains would form a rock.
Most rocks are solid materials composed of a heterogeneous mixture of minerals that have undergone a transformation process.lithological cycle) in which geological factors intervene, such as pressure and temperature due to tectonic action, to the biological activity of living beings (biogeochemical cycle) .
For example, a shale is a rock of metamorphic origin in which laminar minerals predominate. In the image below you can clearly see a shale with prominent aggregates of the mineral sillimanite, an aluminum silicate with the formula AldoisSiO5 crystallized in the orthorhombic system.