Is water a mineral? I understand a mineral needs to be naturally occurring, inorganic, solid, have a defined chemical composition, and a crystalline structure. Water is naturally occurring, and inorganic, but it’s not solid at room temperature and pressure. Also, its chemical composition (H2O) is well-defined. So, considering these factors, can you delve deeper and explain why water is generally not considered a mineral, even though it fulfills some of the criteria? Specifically, I’m interested in how the state of matter (liquid vs. solid) significantly impacts its classification and whether there are any exceptions or circumstances where ice (solid H2O) is regarded as a mineral. And could you clarify what constitutes a “crystalline structure” in the context of mineral definition, and how this applies or doesn’t apply to liquid water versus ice?
Answer
Is water a mineral?
Generally, no.
Why isn’t water considered a mineral?
The classic definition of a mineral, according to the International Mineralogical Association (IMA), requires five criteria to be met:
- Naturally occurring: The substance must be formed by natural geological processes.
- Inorganic: It must not be a product of living organisms or processes.
- Solid: At standard temperature and pressure, it must be a solid.
- Definite chemical composition: It must have a specific chemical formula or a relatively narrow range of chemical formulas.
- Ordered crystalline structure: Its atoms must be arranged in a highly ordered, repeating three-dimensional pattern.
Water fails to consistently meet all these criteria, particularly the solid state requirement. Water is most commonly found in liquid form at Earth’s surface.
When can water be considered a mineral?
Water can be considered a mineral when it is in its solid, crystalline form, as ice. Specifically, the mineral name for ice is ice Ih.
Why is ice Ih considered a mineral?
Ice Ih meets all five criteria:
- Naturally occurring: Ice forms naturally through freezing in cold climates.
- Inorganic: It is not produced by living organisms.
- Solid: It is a solid.
- Definite chemical composition: Its chemical formula is H₂O.
- Ordered crystalline structure: Water molecules in ice are arranged in a specific, repeating hexagonal lattice.
What about other forms of ice?
There are many different crystalline forms of ice, known as ice polymorphs (ice II, ice III, etc.). These forms exist under different pressure and temperature conditions, usually in laboratory settings or in extraterrestrial environments. While they are crystalline and have a definite chemical composition, whether they are considered "minerals" is more debatable. This is because they are rarely, if ever, found at Earth’s surface under natural conditions. The IMA definition emphasizes naturally occurring formation, and many of these high-pressure ice forms don’t naturally occur on Earth’s surface.
Does the presence of dissolved substances affect whether ice is a mineral?
No, the presence of small amounts of dissolved substances does not disqualify ice from being considered a mineral. Minerals can have minor elemental substitutions in their crystal structure. However, if the amount of the dissolved substance is so high that it significantly alters the chemical composition and crystal structure, then it might not be considered pure ice, and therefore not the mineral ice Ih.
What about water in fluid inclusions within minerals?
Water trapped as fluid inclusions inside other minerals is typically not considered a mineral itself. It is considered a fluid inclusion – a small pocket of fluid enclosed within the mineral. The host mineral is still the defined and named mineral.
Why is this distinction important?
The distinction between water and ice Ih is important for scientific accuracy and classification. Mineralogy is a precise science, and it’s crucial to use clear and consistent definitions to avoid confusion. In geological contexts, understanding whether water is present as a liquid, a solid, or a component within another mineral has significant implications for interpreting Earth processes.
Are there any exceptions or ongoing debates regarding water as a mineral?
The classification of water, particularly ice, can be a point of discussion within the scientific community. Some argue that the strict adherence to the "naturally occurring" criterion can be overly restrictive, especially when considering extraterrestrial environments where certain ice polymorphs might be common. However, the standard definition remains that only naturally occurring ice Ih on Earth is officially recognized as the mineral ice.