tetragonal Tectosilicates

Scapolite (Prismatic)

(Na,Ca)4Al3(Al,Si)3Si6O24(Cl,CO3,SO4)

Crystal Structure

CDL Notation

 #! Species: Scapolite (Marialite-Meionite series)
#! System: Tetragonal (4/m)
#! Habit: Prismatic tetragonal crystals
tetragonal[4/m]:{110}@1.0 + {100}@0.6 + {001}@0.4

Crystal System tetragonal

Point Group 4/m

Crystal Forms (3)

{110} {100} {001}

Quick Facts

Hardness

5.5

Specific Gravity

2.5

Refractive Index

1.536

Optical Character

Uniaxial -

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Physical Properties

Crystal Systemtetragonal

Hardness (Mohs)5.5

Specific Gravity2.5

CleavageDistinct prismatic in two directions {100} and {110}

FractureConchoidal to uneven

LustreVitreous

Optical Properties

Refractive Index1.536

Birefringence0.004-0.037

Optical CharacterUniaxial -

Pleochroismweak: Yellow to orange / Colourless to pale yellow

Colours

Yellow (most common gem colour; colour centres)Pink/lilac/violet (Tanzania)Colourless (marialite-rich)Cat's-eye (chatoyant; parallel hollow tubes)

Localities

Myanmar (Burma; fine cat's-eye scapolite)Madagascar (Ihosy; yellow transparent)Tanzania (pink and violet)Sri Lanka (various including cat's-eye)Brazil (colourless to yellow)Kenya

Common Inclusions

[object Object]Two-phase fluid inclusions; growth planes[object Object]

Known Treatments

Not routinely treated; some material reportedly irradiated to enhance colour

Crystal Forms

prismdipyramidpinacoid

Diagnostic Features

Uniaxial negative; strong LWUV fluorescence (yellow to orange in yellow stones; diagnostic — chrysoberyl cat's-eye does not fluoresce strongly); moderate birefringence; cat's-eye variety from Myanmar is celebrated; SG 2.50-2.78

Notes

Na-Ca aluminosilicate solid solution series (marialite to meionite); tetragonal 4/m. Yellow scapolite from Madagascar confirmed by Superchi et al. 2010, Gems & Gemology 46(4):274-279 (DOI 10.5741/gems.46.4.274) [VERIFIED]. Cat's-eye and unusual properties from Sri Lanka confirmed by Zwaan 1996, Gems & Gemology 32(4):262-269 (DOI 10.5741/gems.32.4.262) [VERIFIED]. Colour cause in yellow stones: colour centres (F-centres; not transition metal ions) — Fritsch & Rossman 1988 (DOI 10.5741/gems.24.1.3) [VERIFIED]. Strong LWUV fluorescence (yellow-orange) is a key diagnostic. Two prismatic cleavage directions complicate faceting.