hexagonal Cyclosilicates

Goshenite

Be3Al2Si6O18

Crystal Structure

CDL Notation

 #! Species: Goshenite (Beryl, colourless variety)
#! System: Hexagonal (6/mmm)
#! Habit: Prismatic hexagonal crystals; colourless
hexagonal[6/mmm]:{10-10}@1.0 + {0001}@0.6

Crystal System hexagonal

Point Group 6/mmm

Crystal Forms (2)

{10-10} {0001}

Quick Facts

Hardness

7.5

Specific Gravity

2.68

Refractive Index

1.564

Optical Character

Uniaxial -

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Physical Properties

Crystal Systemhexagonal

Hardness (Mohs)7.5

Specific Gravity2.68

CleavageImperfect basal

FractureConchoidal

LustreVitreous

Optical Properties

Refractive Index1.564

Birefringence0.005-0.008

Optical CharacterUniaxial -

Dispersion0.014

PleochroismNone

Colours

Colourless (pure; no chromophore)

Localities

Goshen, Massachusetts, USA (type locality)Brazil (Minas Gerais; major source)Pakistan (Gilgit-Baltistan)Nigeria, NamibiaChina

Common Inclusions

Two-phase fluid inclusionsGrowth tubes parallel to c-axisOccasional negative crystals

Known Treatments

Not routinely treated; occasionally irradiated to produce aquamarine or Maxixe-type colour

Crystal Forms

prismpinacoid

Diagnostic Features

Colourless beryl; uniaxial negative; RI 1.564-1.600; SG 2.68-2.80; hardness 7.5-8; separates from colourless topaz (biaxial +; SG 3.50), colourless sapphire (uniaxial -; much higher SG 4.00), and phenakite (uniaxial +; lower SG 2.95)

Notes

Colourless variety of beryl (Be3Al2Si6O18). Chemistry per VERIFIED.md conflict resolution F-01: correct cyclosilicate formula Be3Al2Si6O18 (NOT Be3Al2(SiO3)6). Beryl formula confirmed by Read 7th ed. (DOI 10.4324/9780080507224) and Turner & Groat (ch. 5) (DOI 10.3749/9780921294696.ch05) [VERIFIED]. Goshenite is the pure beryl end-member, colourless due to absence of chromophores. Named after Goshen, Massachusetts. May contain colourless beryl seed cores in synthetic emerald Lechleitner overgrowth production.