The Hydronium Ion
Mineral Matters #446
The H2O molecule is quite common in mineral formulas. Minerals containing this water are classed as “hydrated”.
While taking some photos earlier today, I came across one that had a H3O molecule in its formula. What is this?
The molecule H3O, also known as the hydronium ion, is quite different from H2O (water) in several ways:
Structure: H2O has a bent molecular shape with two hydrogen atoms bonded to an oxygen atom. In contrast, H3O+ has a trigonal pyramidal shape with three hydrogen atoms bonded to an oxygen atom, and it carries a positive charge.
Formation: H3O+ is formed when a proton (H+) is added to a water molecule (H2O). This process is common in acidic solutions where the protonation of water occurs.
Charge: H2O is a neutral molecule, whereas H3O+ is a positively charged ion. This positive charge significantly affects its chemical behaviour, especially in acid-base reactions.
Role in pH: The concentration of H3O+ ions in a solution determines its acidity (pH). A higher concentration of H3O+ ions means a lower pH, indicating a more acidic solution.
Chemical Behaviour: H3O+ is a key player in acid-base chemistry. It acts as an acid in aqueous solutions, donating protons to other molecules, whereas H2O can act as both an acid and a base depending on the reaction.
These differences make H3O+ an essential ion in various chemical processes, particularly in understanding the behaviour of acids in aqueous solutions.
There are not many minerals that have H3O in their formula. These are the ones that do: Aqualite, Chernikovite, Hydroniumjarosite, Hydroniumpharmacoalumite, Hydroniumpharmacosiderite, Ikranite, Ilyukhinite, Karchevskyite, Larisaite, Phosphuranylite, Schlossmacherite, Selsurtite, Slavikite, Szymanskiite, Trogerite and Tsepinite-Na.
So why do the minerals noted have the H3O molecule? It can't just be to balance the charges can it?
The presence of the hydronium ion in minerals isn’t solely for charge balance. Here are a few reasons why H3O+ might be present in these minerals:
Structural Role: In some minerals, H3O+ can replace other cations, such as potassium (K+), within the crystal structure. This substitution can influence the mineral’s stability and properties.
Formation Conditions: The presence of H3O+ can be indicative of the conditions under which the mineral formed. For example, minerals containing H3O+ might form in more acidic environments where hydronium ions are more prevalent.
Hydration and Dehydration: H3O+ can play a role in the hydration and dehydration processes of minerals. It can be involved in the exchange of water molecules within the mineral structure, affecting the mineral’s physical and chemical properties.
Chemical Behaviour: The presence of H3O+ can affect the reactivity and solubility of the mineral. It can participate in various chemical reactions, influencing how the mineral interacts with its environment.
These factors make the presence of H3O+ in minerals quite significant beyond just balancing charges.
Below: Phosphuranylite, Barker-Ferguson Mine, Keystone, Pennington Co., South Dakota, USA. Width of view 3mm.
