Actinolite Mineral Guide

Actinolite

Crystal system · Monoclinic

Actinolite is a silicate mineral recognized among collectors for its crystal form and distribution, with several world-class Chinese localities.

About Actinoliteextended article

Elemental Composition (by mass)

Element	Mass %	Visual
O Oxygen	37.52%
Fe Iron	29.77%
Si Silicon	23.95%
Ca Calcium	8.54%
H Hydrogen	0.21%

Computed from simplified end-member formula. Solid-solution series, water content, and trace substitutions cause real-world variation.

IMA Abbreviation (Whitney-Evans 2010)

Act

→ Actinolite

Ca-Fe amphibole

Standard symbol from American Mineralogist (Whitney & Evans, 2010). Used in thin-section labeling, phase diagrams, and IMA-style species records.

Pronunciation

/ækˈtɪnəlaɪt/

↔ ak-TIN-uh-lite

Greek "ray stone"

Cleavage & Fracture

Cleavage:

perfect 2 directions ~56°/124°

Fracture:

splintery

Nephrite jade is fibrous interlocked actinolite.

Collector tier: Micromount / Niche

Best appreciated at thumbnail or smaller scale — often dull-colored, sub-millimeter, or radioactive. Specialist appeal.

Mohs 5–6

Vickers (~) 540 HV

Knoop (~) 620 HK

Geological setting

Metamorphic

Element composition by mass

Formula: Ca₂(Mg,Fe)₅Si₈O₂₂(OH)₂ · molar mass: 891.2 g/mol

O	43.09%
Si	25.21%
Fe	15.67%
Ca	8.99%
Mg	6.82%
H	0.23%

Computed from atomic weights (IUPAC 2021). Site-occupancy groups (Fe,Mn) split equally.

GroupAmphibole Group

Related members: Hornblende · Tremolite

Optical Effects

Cat\'s eye

Mohs Hardness 5–6

1
Talc 2
Gypsum 3
Calcite 4
Fluorite 5
Apatite 6
Orthoclase 7
Quartz 8
Topaz 9
Corundum 10
Diamond

Actinolite sits at 5–6 on the Mohs scale — just hard enough to scratch glass.

Colors:

Streak
White

Crystal system
Monoclinic

SilicatesSilicates (Inosilicates — Amphiboles)

TL;DR · 1 min read

Actinolite (Ca₂(Mg,Fe)₅Si₈O₂₂(OH)₂) is the iron-bearing intermediate member of the tremolite-ferro-actinolite calcic amphibole series. Its name from Greek "aktinos" (ray) refers to its radiating fibrous habit.

Actinolite (Ca₂(Mg,Fe)₅Si₈O₂₂(OH)₂) is the iron-bearing intermediate member of the tremolite-ferro-actinolite calcic amphibole series. Its name from Greek “aktinos” (ray) refers to its radiating fibrous habit. Actinolite forms in greenschist-facies metamorphic rocks and serpentinized ultramafic bodies. Compact green Actinolite aggregates produce nephrite jade alongside Tremolite.

More minerals to explore

About Actinolite

Actinolite is classified as a silicate mineral in the amphibole group (calcium amphibole — actinolite-tremolite series) and has the chemical formula □Ca2(Mg4.5-2.5Fe0.5-2.5)Si8O22(OH)2. It crystallizes in the monoclinic system and holds a steady position among silicate species. Its combination of structural character and global distribution make it a recognized species in both systematic and aesthetic collections.

Identification & care

Actinolite typically forms bladed, prismatic crystals; fibrous and asbestiform; massive, radiating columnar; compact (smaragdite). Its color is typically bright green to dark green and bluish-green. The luster is vitreous, silky (fibrous), the streak is white, and specimens range from transparent to opaque. The cleavage is perfect on {110} at ~56° and 124° (amphibole cleavage). The fracture is uneven, splintery, which aids identification.

Collector context

How it forms

In terms of geology, Actinolite forms in metamorphic rocks in the greenschist facies (the 'actinolite zone' of progressive metamorphism); also in altered mafic rocks (ophiolites, greenstone belts); blueschist-facies glaucophane-actinolite assemblages. It is commonly found in association with chlorite, epidote, albite, tremolite, talc, magnetite, serpentine.

Classic Chinese localities

Huanggang Fe-Sn deposit is an important Chinese source for the species.

Why collectors care

Collectors pursue Actinolite for the clarity of its crystal form and, in good material, saturated color that reads instantly across a display case. A well-terminated actinolite on clean matrix photographs well, identifies quickly, and anchors a cabinet piece. Top Chinese specimens over the last two decades have reset the bar for what actinolite looks like at collector grade.

What affects value

Value in Actinolite is assessed, in typical order of weight, against: (1) locality provenance; (2) size relative to the species norm; (3) crystal form and termination sharpness; (4) color saturation and zoning; (5) transparency and internal clarity; (6) matrix quality and aesthetic balance; (7) condition (absence of damage, chips, or repair). Cleaning quality and verified locality documentation act as multipliers across the above.

Naming history

The name Actinolite has a specific etymological and historical context — see Mindat's reference entry for provenance details. We have retained naming data at the record level; published prose is paraphrased from factual fields rather than copied from source.