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Heulandite

[(Ca,Na)2-3Al3(Al,Si)2Si13O36] *12H2O

Chemical name

Heulandite

Chemical formula

[(Ca,Na)2-3Al3(Al,Si)2Si13O36] *12H2O

Crystal system

monoclinic

Crystal Class

monoclinic-prismatic (2/m)

Space Group

C2/m

Unit cell

a [Å]:17.71

b [Å]:17.84

c [Å]:7.46

Molecular weight [g/mol]

2,825.14

Formula units

Z=4

Frequent crystal surfaces

{010}, {100}, {001}, {20-1}, {110}, {111}

Twinning

on {100}

Mohs hardness

3.5 - 4

Density [g/cm3]

2.2

Cleavage

perfect on {010}

Fracture / Tenacity

subconchoidal to uneven

Color

colorless, white, gray, yellow, rose to red, orange, brown, black

Streak

white

Transparency

transparent to translucent

Luster

vitreous, pearly on cleavage planes

Optical character

biaxial (+)

Refractive Index

nα=1.476-1.506; nβ=1.479-1.510; nΥ=1.479-1.170

Birefringence δ

0.003 - 0.011