Amphibole
Chemical formula: (Ca,Na)₂(Mg,Fe,Al)₅(Al,Si)₈O₂₂(OH)₂
Amphiboles are an important and diverse group of common, dark rock-forming minerals with complex chemical compositions and characteristic elongated crystals.
Properties
- Mohs hardness
- 5-6
- Luster
- Vitreous, silky
- Streak
- White to grayish-green
- Density
- 2.9-3.6
- Cleavage
- Perfect in two directions, angles ~56° and 124°
- Fracture
- Uneven, splintery
- Transparency
- Translucent to opaque
- Crystal system
- Monoclinic
Diagnostic features
## Identification The key diagnostic feature of amphiboles is the angle between the two cleavage planes, which is approximately 124° and 56°. In practice, on fractured crystal surfaces, two sets of shiny "steps" intersecting at these angles are visible. The elongated, prismatic, or acicular crystal habit and a hardness range of 5-6 on the Mohs scale are also helpful. ## Distinguishing from Similar Minerals Amphiboles are most often confused with pyroxenes, which have a very similar appearance, hardness, and often occur in the same rocks. The primary difference is the cleavage angle: in pyroxenes, cleavage planes intersect at an angle close to 90° (approx. 87° and 93°). As a result, the cross-section of an amphibole crystal is rhombic, while that of a pyroxene is square or rectangular. Tourmaline, although also sometimes prismatic and black, is much harder (7-7.5) and has a different, more complex cross-section (often a rounded triangle). ## Crystal Forms Amphiboles form crystals with prismatic, columnar, acicular, or fibrous habits. They often occur as granular, radial (as in variolite), fibrous (nephrite, asbestos), or tangled aggregates. Well-formed, freestanding crystals are rarer and found mainly in metamorphic rocks and pegmatites.
Geological environment
## Genesis Amphiboles are minerals with a wide range of formation conditions. They crystallize as primary components from intermediate, water-rich magmas, forming rocks such as diorites, andesites, and syenites. They are also secondary products, formed by the hydration of pyroxenes (a process called uralitization). However, they are most significant in metamorphic rocks, where their presence defines the entire amphibolite facies – conditions of medium to high pressures and temperatures. They then form through the alteration of basalts, gabbros, or marls, creating rocks known as amphibolites. ## Mineral Associations As rock-forming minerals, amphiboles coexist with many other common minerals. In igneous rocks, these are most often plagioclase, potassium feldspar, quartz, and biotite. In metamorphic rocks, their typical associates are plagioclase, epidote, garnets (especially almandine), staurolite, kyanite, and sillimanite. In contact metamorphic rocks (hornfels), they occur with cordierite and andalusite. ## Localities Amphiboles are widespread globally. Classic localities for well-formed hornblende crystals include the Bancroft area in Ontario (Canada), Arendal (Norway), and Franklin, New Jersey (USA). Beautiful, green actinolite crystals come from the Zillertal Valley in Austria. Tremolite is common in metamorphic dolomites in the Campolungo region in Switzerland and in New York State (USA). Blue riebeckite in the form of crocidolite is known from South Africa, and glaucophane from the island of Syros in Greece and California (USA).
Rarity
Common
For collectors
## Quality Criteria For collectors, the most desirable specimens are those with well-formed, sharp, and lustrous crystals that are clearly distinct from the surrounding rock (matrix). Large, single crystals or aesthetic crystal groups are highly valued. In the case of hornblende and actinolite, the depth and saturation of color and the absence of damage are important. Rare varieties, such as blue riebeckite or purple richterite, are particularly sought after. For fibrous varieties, such as nephrite, color uniformity, translucency, and absence of cracks are assessed. ## Popular Localities The most prized hornblende crystals by collectors come from the Canadian province of Ontario (Bancroft and Wilberforce areas). Classic, large tremolite and actinolite crystals were found in the Alps (Switzerland, Austria). Gem-quality pargasite specimens come from Pakistan (Hunza Valley), and purple richterite from Wind Mountain in New Mexico (USA).
Care and storage
## Cleaning Amphibole specimens are relatively hard and can be cleaned with a soft brush under running water. For specimens with delicate, acicular crystals, special care should be taken, limiting cleaning to gentle rinsing or using compressed air to remove dust. Fibrous (asbestiform) varieties should not be mechanically cleaned to avoid releasing fibers into the air. ## What to Avoid Avoid contact with strong acids, which can etch the mineral surface. Despite relatively good resistance, specimens should not be exposed to sudden temperature changes. The greatest caution should be exercised with asbestiform varieties (crocidolite, amosite) – absolutely avoid inhaling dust and fibers that may be released from them. ## Storage Amphibole specimens can be safely stored in collections. Due to their excellent cleavage, they should be protected from impacts and falls, which can cause breakage along cleavage planes. Specimens of asbestiform varieties should be stored in sealed, closed containers to prevent the spread of fibers.