Hibschite
Ca3Al2(SiO4)3-x(OH)4x (0.2 < x < 1.5)
A hydrous aluminum garnet (hydrogrossular) intermediate between grossular and katoite, common in altered calcium-rich and serpentinized rocks.
- Mohs hardness
- 6-7
- Color
- white, gray, pink, green
- Type
- mineral
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Overview
Hibschite is a hydrous member of the grossular-katoite hydrogarnet series, in which hydroxyl groups replace part of the silica. It occupies the intermediate range of the series, with more water than grossular but less than the fully hydrated end-member katoite.
Its colors include white, gray, pink, and green, and it is commonly massive or granular rather than well-crystallized. The old name 'plazolite' refers to the same hydrogrossular material.
Hibschite is widespread in low-temperature, calcium-rich alteration environments and is a key mineral for studying hydrogarnet substitution.
Formation & geology
Hibschite forms by hydrothermal and metasomatic alteration of calcium-aluminum-rich rocks at relatively low temperatures. It is especially typical of rodingites, which form where gabbroic rocks are altered during serpentinization of surrounding ultramafics.
It also occurs in skarns, altered limestones, and contact-metamorphic zones, growing alongside calcite, vesuvianite, diopside, and other calc-silicates.
The mineral was named after the geologist Josef Hibsch, and hydrogrossular material has been described from many serpentinite and skarn localities worldwide.
How to identify it
Look for massive to granular white, gray, greenish, or pink garnet-like material in rodingite, skarn, or altered calcareous rock. Hardness is about 6-7, softer than gem grossular, with a pale streak.
Massive green hydrogrossular can resemble jade; garnet's higher specific gravity and lack of jade's tough, fibrous fracture help separate them. Distinguishing hibschite from grossular and katoite within the series requires measuring the degree of hydroxyl-for-silica substitution.
Uses & significance
Massive green hydrogrossular (a hibschite-grossular mix) is sometimes cut and polished as an ornamental stone, marketed under names like 'Transvaal jade' or 'South African jade,' though it is not true jade. Otherwise hibschite has little commercial value.
Scientifically it is important for understanding water in calc-silicate systems and for tracing serpentinization-related metasomatism. Collectors value attractive massive specimens.
Frequently asked questions
Is hibschite the same as plazolite?
Yes. Plazolite is an older name for hydrogrossular material that corresponds to hibschite.
Can hibschite look like jade?
Green massive hydrogrossular resembles jade and is sold as imitation jade, but it is denser and lacks jade's fibrous toughness.
Where does hibschite form?
In low-temperature altered calcium-aluminum rocks, especially rodingites and skarns.
What is the difference between hibschite and grossular?
Hibschite contains structural water (hydroxyl replacing silica), placing it in the hydrogarnet series, while grossular is the anhydrous end-member.
Hibschite guides
In-depth guides for identifying, valuing, and understanding Hibschite.
