Indicolite

Description

Indicolite is the blue variety of the mineral Elbaite, which belongs to the Tourmaline group. Tourmalines are complex borosilicate minerals characterized by a trigonal crystal system and a distinctive prismatic habit, often with striations parallel to the c-axis. Indicolite's color can range from light, sky-blue to deep, intense blue, sometimes with greenish or violet overtones. It exhibits strong pleochroism, meaning its color can appear different when viewed from various angles. Like other tourmalines, it is pyroelectric and piezoelectric. Its hardness and resistance to chemical alteration make it suitable for jewelry.

How to Identify

Color

Ranges from light blue to deep blue, often with greenish or violet tints. Strong pleochroism is characteristic, showing different shades of blue or even blue-green depending on the viewing angle.

Luster

Vitreous (glassy) to resinous.

Texture

Smooth to slightly rough on crystal faces, often exhibiting prominent striations parallel to the length of the crystal.

Crystal Form

Typically forms elongated, prismatic crystals with a characteristic triangular cross-section (due to the trigonal crystal system). Crystals often have rounded edges and may be terminated by complex pyramidal or pedion faces. Can also occur as massive or columnar aggregates.

Cleavage

Indicolite, like other tourmalines, has very poor to indistinct cleavage, making it prone to conchoidal or uneven fracture rather than clean breaks along crystallographic planes.

Geological Environment

Primarily found in complex granitic pegmatites, which are intrusive igneous rocks. Also occurs in some hydrothermal veins and regionally metamorphosed rocks, particularly schists and gneisses, where boron-rich fluids were present.

Key Facts

• Hardness: 7-7.5 on the Mohs scale
• Specific Gravity: 3.03-3.10 (variable depending on composition)
• Crystal System: Trigonal
• Color: Blue, often with pleochroism (different colors from different angles)
• Luster: Vitreous to resinous
• Transparency: Transparent to translucent
• Fracture: Conchoidal to uneven
• Cleavage: Very poor to indistinct
• Composition: Complex borosilicate of aluminum, lithium, and iron, with varying amounts of other elements (Na, Ca, Mg, F, OH)

Quick Check

• Color: Blue (light to deep, sometimes greenish or violet)
• Luster: Vitreous to resinous
• Streak: White

Physical Characteristics

• Crystal Habit: Prismatic, often elongated, with vertical striations on prism faces; typically forms three-sided or six-sided prisms with rounded edges; terminations can be complex pyramidal or pedion.
• Cleavage Type: Very poor to indistinct, typically parting along basal planes or rhombohedral planes, but rarely observed.
• Fracture Type: Conchoidal to uneven, brittle.
• Tenacity: Brittle
• Luster Type: Vitreous (glassy) to resinous

Formation

Indicolite, a variety of Elbaite tourmaline, forms primarily in complex granitic pegmatites, which are coarse-grained igneous rocks. It crystallizes from highly evolved, volatile-rich magmatic fluids that are residual after the main crystallization of granite. These fluids are enriched in incompatible elements such as boron, lithium, aluminum, and fluorine, which are essential components of tourmaline. Hydrothermal veins and metamorphic rocks, particularly those associated with metasomatism, can also host Indicolite, though pegmatitic occurrences are most significant for gem-quality material. The blue color is typically attributed to the presence of iron (Fe2+ and Fe3+) and sometimes titanium (Ti4+) through intervalence charge transfer (IVCT) mechanisms.

Usage

Primarily used as a gemstone in jewelry due to its attractive blue color, transparency, and durability. Collector's specimens are also highly valued, especially well-formed crystals. It has limited industrial use beyond its aesthetic appeal.

Age Distribution

Formed during various geological epochs, primarily associated with the late stages of granitic intrusion and pegmatite formation, ranging from Precambrian to Cenozoic.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Silicates

Group

Tourmaline Group (specifically Elbaite subgroup)

Crystal System

Trigonal

Chemical Formula

Na(Li,Al)3Al6(BO3)3Si6O18(OH)4 (simplified for Elbaite, with Fe2+/Fe3+ substituting for Al in Indicolite)

Composition

Sodium lithium aluminum borosilicate hydroxide. The blue color in Indicolite is primarily due to iron (Fe2+ and Fe3+) and sometimes titanium (Ti4+) through intervalence charge transfer (IVCT).

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