Hydrogrossular Mineral Overview

• Mineral name: Hydrogrossular
• Mineral class: Silicate (nesosilicate, garnet group)
• Chemical formula: Ca₃Al₂(SiO₄)₃₋ₓ(OH)₄ₓ (variable; hydroxyl replaces silica)
• Crystal system: Cubic (isometric)
• Mohs hardness: 6.5–7
• Color range: Green, white, gray, pink
• Luster: Waxy to vitreous
• Streak: White
• Cleavage: None
• Fracture: Conchoidal to uneven
• Specific gravity: Approximately 3.3–3.5
• Transparency: Opaque to translucent
• Common locations: South Africa, United States, Canada, China
• Uses: Gemstone (often sold as jade substitute), ornamental stone
• Similar minerals: Grossular, jadeite, serpentine

Naming and Classification

Hydrogrossular is a variety of grossular garnet, distinguished by:

• Partial replacement of silica (SiO₄) with hydroxyl (OH)

The name reflects this:

• “Hydro” = water-related (hydroxyl content)
• “Grossular” = its parent garnet species

It is part of the garnet group, which are nesosilicate minerals with similar crystal structures.

Physical and Optical Characteristics

Hydrogrossular typically occurs as:

• Massive, compact material
• Fine-grained aggregates

It rarely forms well-defined crystals.

Its color is often:

• Green (most common)
• White or gray
• Pink in some cases

The luster is usually waxy to dull, especially in massive forms.

With a Mohs hardness of 6.5–7, it is relatively durable.

Texture and Appearance (Key Feature)

Hydrogrossular is known for:

• Smooth, compact, jade-like appearance

Because of this, it is often:

• Mistaken for jade
• Sold as “Transvaal jade” or similar trade names

Chemical Composition and Structure

Hydrogrossular is composed of:

• Calcium (Ca)
• Aluminum (Al)
• Silicon (Si) (partially replaced)
• Hydroxyl (OH)

It retains the garnet crystal structure, but with:

• Substitution of hydroxyl groups for silica tetrahedra

This substitution affects:

• Density
• Optical properties

Formation and Geological Occurrence

Hydrogrossular forms in metamorphic and hydrothermal environments, particularly:

Altered Calcium-Rich Rocks

• Contact metamorphism of limestone

Serpentinized Environments

• Associated with ultramafic rock alteration

Typical formation conditions include:

• Presence of water-rich fluids
• Calcium- and aluminum-rich environments

It is commonly associated with:

• Serpentine
• Vesuvianite
• Other garnets

Common Locations

Hydrogrossular is found in several regions:

• South Africa: Transvaal (classic locality)
• United States: California
• Canada: Quebec
• China: Various deposits

South African material is especially well known in the gem trade.

Uses and Practical Significance

Hydrogrossular is used for:

Gemstone and Ornamental Use

• Cabochons and carvings
• Often marketed as jade substitute

Collector Interest

• Unique variety of garnet

It has no major industrial use.

Similar and Related Minerals

Hydrogrossular can resemble:

• Jadeite: More valuable and different structure
• Nephrite: Fibrous and tougher
• Serpentine: Softer and less dense

Testing is often needed to distinguish these materials.

Identification Notes for Collectors

• Look for green, massive, jade-like material
• Note hardness (6.5–7)
• Observe waxy luster
• Check lack of visible crystal structure
• Consider association with serpentine or metamorphic rocks

Hydrogrossular is a variety of grossular garnet, recognized for its hydroxyl-rich composition and jade-like appearance, making it a popular ornamental stone and a commonly mistaken substitute for true jade.