Kyanite is an aluminum silicate mineral with the chemical formula \(\text{Al}_2\text{SiO}_5\), typically forming under the high-pressure conditions of metamorphic rocks like schists and gneisses. Its name comes from the Greek word “kyanos,” meaning “dark blue,” which points to its most recognizable coloration. This mineral is a polymorph, sharing its chemical composition with andalusite and sillimanite, but its unique crystalline structure results in distinct physical and visual characteristics.
Primary Visual Traits
Kyanite is most famous for its rich, sapphire-like blue color, which is caused by trace amounts of iron and titanium within its crystal lattice structure. While blue is the most common and prized hue, the mineral can also be colorless, white, gray, green, or even a rare orange due to manganese inclusions. The color intensity can vary across a single specimen, sometimes showing patches or zones of light and dark blue.
The mineral ranges from transparent to translucent, allowing light to pass through it. Kyanite typically displays a vitreous, or glassy, luster on its main crystal faces, but cleavage planes often exhibit a pearly sheen. A significant visual identifier is pleochroism, where the color of the crystal appears to change when viewed from different angles. For blue kyanite, this effect is often strong, showing shades from colorless or pale blue to deep violet-blue.
Form and Growth Habit
The most distinctive feature of kyanite is its characteristic “bladed” habit, where it forms long, flattened, and thin tablet-like crystals. These crystals are typically elongated and columnar, growing in the triclinic crystal system. This unique structure gives the mineral a somewhat striated or lined appearance along its length.
These blades rarely occur in isolation and often form intergrown or aggregate masses. They can arrange themselves in radiating sprays or dense, tangled clusters within the host rock. The mineral exhibits a perfect cleavage parallel to the long axis of the crystal, meaning it tends to split cleanly along this plane. This prominent cleavage contributes to its splintery fracture pattern.
Anisotropy: Kyanite’s Defining Physical Property
Kyanite possesses a physical property known as anisotropy, which means its hardness varies depending on the direction it is measured. This dual hardness is the single most definitive way to identify the mineral. The difference in scratch resistance is a direct result of the mineral’s internal atomic structure.
When a scratch test is performed parallel to the length of the crystal blade, kyanite registers a relatively low hardness of approximately 4.5 to 5 on the Mohs scale. However, when the same test is applied perpendicular to the blade’s length, the mineral’s hardness increases to between 6.5 and 7.
The stronger atomic bonds run across the width of the crystal, while weaker bonds run along the length. This causes the substantial variation in its resistance to scratching.