hexagonal Cyclosilicates

Benitoite

BaTiSi3O9

Crystal Structure

CDL Notation

 #! Species: Benitoite (BaTiSi3O9)
#! System: Hexagonal (-6m2, ditrigonal dipyramidal)
#! Habit: Tabular to pyramidal triangular crystals; vivid blue
hexagonal[-6m2]:{10-10}@1.0 + {11-20}@0.6 + {10-11}@0.8

Crystal System hexagonal

Point Group -6m2

Crystal Forms (3)

{10-10} {11-20} {10-11}

Quick Facts

Hardness

6.0

Specific Gravity

3.65

Refractive Index

1.757

Optical Character

Uniaxial +

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Physical Properties

Crystal Systemhexagonal

Hardness (Mohs)6.0

Specific Gravity3.65

CleavagePoor {1010}

FractureConchoidal

LustreAdamantine to vitreous

Optical Properties

Refractive Index1.757

Birefringence0.047

Optical CharacterUniaxial +

Dispersion0.044

Pleochroismstrong: Vivid blue (ordinary ray perpendicular to c-axis) / Colourless to pale blue (extraordinary ray along c-axis)

Colours

Vivid blue (primary gem colour)ColourlessRarely pinkish-orange

Localities

San Benito County, California, USA (the only commercial source worldwide; gem-quality crystals in natrolite vein in blueschist)Non-gem occurrences in Japan, Belgium, Australia, Czech Republic

Common Inclusions

Generally cleanOccasional two-phase fluid inclusionsNeptunite crystals (black associated mineral in matrix specimens)

Known Treatments

None known

Crystal Forms

dipyramidprism

Diagnostic Features

Intense blue fluorescence under shortwave UV (SWUV) — the most immediately diagnostic field test; strong dichroism (vivid blue / colourless in dichroscope); very high birefringence 0.047 (strong fire and doubling); RI above standard refractometer range (1.79 limit); only commercial source: New Idria district, San Benito County, California

Notes

BaTiSi3O9; barium titanium cyclosilicate; hexagonal point group -6m2 (ditrigonal dipyramidal). Crystals have a triangular (not hexagonal) outline due to this point group. State gemstone of California since 1985. Primary gemmological reference: Laurs, Rohtert & Gray 1997, Gems & Gemology 33(3):166-187 (DOI 10.5741/gems.33.3.166) [VERIFIED via live API]. SWUV fluorescence mechanism confirmed by Vigier, Fritsch et al. 2023, Minerals 13(1):104 (DOI 10.3390/min13010104) [VERIFIED via live API]: colour from O2- to Ti4+ ligand-to-metal charge transfer in TiO6 groups. Fluorescence characterisation by Zhang & Shen 2023 (DOI 10.3390/min13050626) [VERIFIED]. Dispersion 0.044 is equal to diamond; confirmed per VERIFIED.md Conflict 8 (no conflict; both values accurate). RI 1.757-1.804 is above standard refractometer range; SG measurable hydrostatically.