Black Kyanite

Description

Black Kyanite is a variety of the aluminum silicate mineral Kyanite (Al2SiO5). It is characterized by its typically bladed or columnar crystal habit and its distinctive black color. Unlike the more common blue kyanite, which is often translucent to transparent, black kyanite is usually opaque. The black color is generally attributed to fine inclusions of graphite or other carbonaceous material within the kyanite structure, or sometimes iron-bearing impurities. Kyanite is one of the 'aluminum silicate polymorphs' along with andalusite and sillimanite, each forming under different pressure-temperature conditions.

How to Identify

Color

Black, often with a slightly metallic or greasy sheen due to graphite inclusions.

Luster

Vitreous to sub-vitreous, sometimes pearly on cleavage surfaces. If graphite-rich, it can appear dull to sub-metallic.

Texture

Typically bladed or columnar crystals, often forming radiating or fan-like aggregates. Can also be massive.

Crystal Form

Triclinic system, typically forming elongated, bladed crystals. These blades can be very thin and flat.

Cleavage

Perfect cleavage parallel to {100} and good cleavage parallel to {010}. This results in the characteristic bladed appearance and can make it prone to splintering.

Geological Environment

Found in high-pressure, low-to-moderate temperature metamorphic rocks such as schists, gneisses, and granulites, often associated with quartz, muscovite, garnet, and staurolite. The presence of carbonaceous material in the protolith is key for the black coloration.

Key Facts

• Hardness: Anisotropic; 4.5-5 parallel to the long axis of the crystal, 6-7 perpendicular to the long axis (Mohs scale).
• Specific Gravity: 3.53-3.67 g/cm³
• Crystal System: Triclinic
• Color: Black (due to inclusions of graphite or other carbonaceous material, or iron oxides)
• Luster: Vitreous to sub-vitreous, pearly on cleavage surfaces, sometimes dull or sub-metallic.
• Transparency: Opaque to translucent (rarely).
• Fracture: Splintery to uneven.
• Cleavage: Perfect on {100}, good on {010}, poor on {110}.
• Composition: Aluminum Silicate (Al2SiO5)

Quick Check

• Color: Black
• Luster: Vitreous to sub-vitreous, sometimes dull or sub-metallic due to inclusions.
• Streak: White to gray (if pure kyanite), but can be dark gray to black if rich in graphite inclusions.

Physical Characteristics

• Crystal Habit: Typically bladed, columnar, or tabular crystals, often forming radiating or fan-like aggregates. Can also be massive.
• Cleavage Type: Perfect basal cleavage on {100}, good on {010}.
• Fracture Type: Splintery to uneven.
• Tenacity: Brittle.
• Luster Type: Vitreous to sub-vitreous, pearly on cleavage surfaces.

Formation

Kyanite forms under high-pressure, low-to-moderate temperature metamorphic conditions. It is typically found in regionally metamorphosed pelitic (aluminum-rich) rocks such as schists and gneisses. The black coloration is often due to inclusions of graphite or other carbonaceous material, or sometimes iron oxides/hydroxides, rather than an intrinsic property of the kyanite itself.

Usage

While blue kyanite is used as a gemstone and in refractories, black kyanite is primarily of interest to mineral collectors and for its metaphysical properties. Its industrial uses are limited compared to other kyanite varieties due to its color and often opaque nature.

Age Distribution

Found in metamorphic rocks of various ages, from Precambrian to Cenozoic, depending on the specific metamorphic event.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Nesosilicate

Group

Aluminum Silicate Group

Crystal System

Triclinic

Chemical Formula

Al2SiO5

Composition

Aluminum oxide (Al2O3) 62.9%, Silicon dioxide (SiO2) 37.1%. Black coloration is due to impurities/inclusions.

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