Hydroandradite
Ca3Fe3+2(SiO4)3-x(OH)4x
A hydrous, iron-rich garnet of the hydrogarnet group in which hydroxyl groups substitute for silica within the andradite structure.
- Mohs hardness
- 6-7
- Color
- yellow, brown, greenish
- Type
- mineral
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Overview
Hydroandradite is the iron-bearing member of the hydrogarnet group, the hydrous counterpart of andradite. In its structure, hydroxyl groups (OH) substitute for some of the silica tetrahedra, a feature shared with hibschite and katoite in the broader hydrogarnet series.
This water content tends to lower the density and hardness relative to anhydrous andradite. It is isometric and usually massive or fine-grained, with yellow, brown, or greenish tones.
The mineral forms in low-temperature, water-rich alteration environments and is mainly of interest to mineralogists studying hydrogarnet substitution.
Formation & geology
Hydroandradite forms under relatively low-temperature, hydrous conditions where calcium and ferric iron are available, such as during hydrothermal alteration, skarn formation, and the serpentinization-related metasomatism that also produces rodingites.
The hydrogarnet substitution (OH replacing SiO4) is favored by abundant water and lower temperatures, conditions that suppress full silica incorporation.
It occurs alongside other hydrogarnets, calcite, hydrous calc-silicates, and serpentine-group minerals in altered mafic and calcareous rocks.
How to identify it
Look for massive to fine-grained yellow, brown, or greenish garnet in altered, calcium- and iron-rich rocks. Hardness is roughly 6-7, slightly softer than anhydrous andradite, with a pale streak.
Hydroandradite is difficult to distinguish from andradite and other hydrogarnets without analysis, since the hydroxyl content is the key difference; infrared spectroscopy or chemical and density measurements are used. Its low-temperature alteration setting is a useful contextual clue.
Uses & significance
Hydroandradite has no commercial gem or industrial use. It is studied scientifically as part of the hydrogarnet group, which is important for understanding water storage in calc-silicate minerals and the chemistry of low-temperature alteration.
It is collected by specialists interested in the garnet and hydrogarnet series. There is no established metaphysical tradition specific to it.
Frequently asked questions
What is a hydrogarnet?
A garnet in which hydroxyl groups replace some silica tetrahedra, adding structural water; hydroandradite is the iron-rich example.
How is hydroandradite different from andradite?
It contains structural water (OH replacing SiO4), which lowers its hardness and density relative to ordinary andradite.
Where does hydroandradite form?
In low-temperature, water-rich settings such as hydrothermal alteration, skarns, and rodingites.
Is hydroandradite used in jewelry?
No. It is typically massive and fine-grained, of interest only to mineralogists and collectors.
Hydroandradite guides
In-depth guides for identifying, valuing, and understanding Hydroandradite.
