Hydroandradite Identification Guide

What Hydroandradite Looks Like

Hydroandradite is the hydrous, OH-bearing member of the andradite garnet series, where (OH)4 groups substitute for some SiO4. It is part of the hydrogarnet group:

• Color: yellow-brown, greenish, honey, to brownish-orange
• Luster: vitreous to resinous
• Transparency: translucent to nearly opaque; rarely transparent
• Habit: isometric dodecahedra/trapezohedra, but very often massive, granular, or as compact aggregates

Step-by-Step Field ID Checklist

1. Confirm garnet character. Look for equant, rounded isometric crystals or a hard, glassy granular mass with no cleavage.
2. Check for a brown-green-honey hue, typical of the andradite line.
3. Test hardness. Roughly Mohs 6.5–7, hard enough to scratch glass.
4. Look for conchoidal fracture rather than flat cleavage planes.
5. Note the setting. Hydrogarnets form in altered, hydrated calcium-rich rocks (skarns, rodingites, altered basic igneous rocks).

Key Diagnostic Tests

• Hardness: ~6.5–7 (slightly softer than dry andradite because of OH substitution).
• Cleavage: none; fracture is conchoidal to uneven.
• Streak: white to pale.
• Density: ~3.4–3.8 g/cm3, lower than fully anhydrous andradite (~3.8–3.9) because water lowers density.
• No acid reaction (silicate).
• Isotropic under crossed polars (distinguishes garnet from most look-alikes), though hydrogarnets can show weak anomalous birefringence.

Lower density and refractive index relative to ordinary andradite are the practical clues to the hydrous component; precise identification needs lab analysis.

Common Look-Alikes

• Ordinary andradite/demantoid: denser and harder, higher luster and dispersion; chemistry separates them.
• Grossular/hydrogrossular: the calcium-aluminum hydrogarnet; hydroandradite is the iron-bearing counterpart and tends browner/greener.
• Hibschite and katoite: more strongly hydrated hydrogarnets; lab work needed.
• Idocrase (vesuvianite): similar skarn settings and color but is anisotropic and often prismatic.
• Epidote: prismatic, one good cleavage, distinctly pistachio-green.

Where It Is Found

Hydrogarnets like hydroandradite occur in hydrothermally altered calcium-rich rocks: skarns, rodingites, serpentinized and altered mafic intrusions, and contact-metamorphic zones. Specimens are reported from altered ophiolite and skarn localities worldwide, generally as a minor component requiring analytical confirmation.