cubic Tectosilicates

Hauyne

(Na,Ca)4-8Al6Si6O24(SO4,S)1-2

Crystal Structure

CDL Notation

 #! Species: Hauyne (sodalite group)
#! System: Cubic (-43m)
#! Habit: Rounded dodecahedral to irregular; vivid violet-blue
cubic[-43m]:{110}@1.0 + {100}@0.4

Crystal System cubic

Point Group -43m

Crystal Forms (2)

{110} {100}

Quick Facts

Hardness

5.5

Specific Gravity

2.4

Refractive Index

1.494

Optical Character

Isotropic

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Physical Properties

Crystal Systemcubic

Hardness (Mohs)5.5

Specific Gravity2.4

CleavageImperfect dodecahedral {110}

FractureConchoidal

LustreVitreous to greasy

Optical Properties

Refractive Index1.494

Optical CharacterIsotropic

PleochroismNone (isotropic)

Colours

Vivid violet-blue to cornflower blue (primary gem colour)Rarely white, grey, green

Localities

Eifel district, Germany (primary gem-quality source; volcanic ejecta in leucitite tuffs, Laacher See region)Vesuvius and Monte Somma, Italy (historically important; crystals in volcanic rocks)Czech Republic, Bohemia (minor)Lapis lazuli deposits (Afghanistan, Chile; hauyne as component mineral in some lapis — not gem-grade)

Common Inclusions

Generally clean; occasional fluid inclusions

Known Treatments

None known; extremely rare gem

Crystal Forms

dodecahedronrhombdodecahedron

Diagnostic Features

Vivid violet-blue colour combined with very low SG 2.40-2.50 (lower than aquamarine); low RI 1.494-1.509; isotropic (no birefringence); orange or cream LWUV fluorescence (characteristic but not universal); dissolves in HCl (destructive test)

Notes

Sodium calcium aluminosilicate sulfate; cubic -43m; sodalite group member. Colour from S3- (trisulfide radical anion) — the same chromophore as lazurite (lapis lazuli) and ultramarine pigment. Confirmed by Farsang, Caracas et al. 2023, American Mineralogist 108:2234-2243 (DOI 10.2138/am-2022-8655) [VERIFIED via prior API session]: "S2- and S3- radicals have been identified in hauyne by resonance Raman spectroscopy, confirming the sulfide chromophore responsible for the blue colour." Sodalite-group structural context confirmed by Chukanov et al. 2020, Minerals 10:363 (DOI 10.3390/min10040363) [VERIFIED]. Gemmological constants (SG, RI) from Read 7th ed. (DOI 10.4324/9780080507224) [PARTIALLY_SUPPORTED]. Very low SG 2.40-2.50 is the key physical separator from virtually all other blue gems. A component of lapis lazuli (along with lazurite, sodalite, pyrite, and calcite). Extremely rare as a faceted gem; primarily collector's gem.