Pumpellyite Mineral Overview

• Mineral name: Pumpellyite
• Mineral class: Silicate (sorosilicate)
• Chemical formula: Ca₂(Mg,Fe,Al)₂(Al,Fe)(Si₂O₇)(SiO₄)(OH)₂·H₂O (variable)
• Crystal system: Monoclinic
• Mohs hardness: 5.5–6
• Color range: Green, bluish-green, dark green, brownish-green
• Luster: Vitreous to silky
• Streak: White
• Cleavage: Poor
• Fracture: Uneven to splintery
• Specific gravity: Approximately 3.1–3.3
• Transparency: Translucent to opaque
• Common locations: United States, Japan, New Zealand, Italy
• Uses: Indicator mineral, collector mineral
• Similar minerals: Epidote, prehnite, chlorite

Naming and Classification

Pumpellyite is a hydrated calcium magnesium iron aluminum silicate, named after American geologist Raphael Pumpelly.

It belongs to the sorosilicate group, meaning its structure includes both:

• Isolated silica tetrahedra
• Paired silica tetrahedra

Pumpellyite is especially important in geology as part of a specific metamorphic classification known as the:

• Prehnite–Pumpellyite facies

Physical and Optical Characteristics

Pumpellyite typically occurs as:

• Fibrous or radiating aggregates
• Fine-grained masses
• Rare small prismatic crystals

Its color is usually green, often appearing as:

• Dark green
• Bluish-green
• Brownish-green in iron-rich varieties

The luster is vitreous, sometimes appearing silky in fibrous forms.

With a Mohs hardness of 5.5 to 6, it is moderately hard.

Chemical Composition and Structure

Pumpellyite is composed of:

• Calcium (Ca)
• Magnesium (Mg)
• Iron (Fe)
• Aluminum (Al)
• Silicon (Si)
• Hydroxyl (OH)
• Water (H₂O)

Its structure is complex and hydrated, combining:

• Sorosilicate units
• Water molecules within the lattice

This contributes to its formation under specific metamorphic conditions.

Formation and Geological Occurrence

Pumpellyite forms in low-grade metamorphic environments, particularly in:

Prehnite–Pumpellyite Facies

• Transitional stage between:
  • Diagenesis (sedimentary alteration)
  • Greenschist metamorphism

Typical formation conditions include:

• Low temperatures (~200–350°C)
• Moderate pressure
• Water-rich environments

It is commonly found in:

• Altered basalt
• Metamorphosed volcanic rocks

Common Associations

Pumpellyite is often associated with:

• Prehnite
• Chlorite
• Epidote
• Albite

These minerals together indicate specific metamorphic conditions.

Common Locations

Pumpellyite is found in many regions with low-grade metamorphism:

• United States: California and Pacific Northwest
• Japan: Well-studied metamorphic belts
• New Zealand: Alpine metamorphic regions
• Italy: Alpine deposits

These areas are known for metamorphosed volcanic rocks.

Uses and Practical Significance

Pumpellyite is important for:

Geological Indicator

• Defines the prehnite–pumpellyite metamorphic facies
• Helps geologists determine:
  • Temperature
  • Pressure conditions during metamorphism

Collector Mineral

• Fibrous and radiating specimens are collected

It has no major industrial use.

Similar and Related Minerals

Pumpellyite can resemble:

• Epidote: Typically brighter green and more crystalline
• Prehnite: Lighter green and botryoidal
• Chlorite: Softer and more platy

Associations and texture help distinguish pumpellyite.

Identification Notes for Collectors

• Look for green fibrous or radiating aggregates
• Note moderate hardness (5.5–6)
• Observe vitreous to silky luster
• Check association with prehnite and chlorite
• Consider occurrence in low-grade metamorphic rocks

Pumpellyite is a hydrated sorosilicate mineral, recognized for its role as a key indicator of low-grade metamorphic conditions, making it especially important in geological studies and metamorphic petrology.