Crystal Heulandite

Chemical Formula

(Ca,Na)_2-3Al₃(Al,Si)₂Si₁₃O₃₆ · 12H₂O

The above is the general Heulandite formula. The formula for the individual members categorized by the IMA are:
Heulandite-Ca: (Ca,Na)_2-3Al₃(Al,Si)₂Si₁₃O₃₆ · 12H₂O
Heulandite-Na: (Na,Ca)_2-3Al₃(Al,Si)₂Si₁₃O₃₆ · 12H₂O
Heulandite-K: (K,Na,Ca)_2-3Al₃(Al,Si)₂Si₁₃O₃₆ · 12H₂O
Heulandite-Ba: (Ba,Ca,K,Na,Sr)_2-3Al₃(Al,Si)₂Si₁₃O₃₆ · 12H₂O
Heulandite-Sr: (Sr,Na,Ca)_2-3Al₃(Al,Si)₂Si₁₃O₃₆ · 12H₂O

Composition

Hydrous calcium sodium aluminum silicate. The ratio of calcium to sodium is variable, and may also be substituted for potassium, and rarely strontium or barium. The ratio of aluminum and silica combined with the first set of metallic elements may also vary slightly.

Variable Formula

(Ca,Na,K,Sr,Ba)_2-3Al₃(Al,Si)₂Si₁₃O₃₆ · 12H₂O

Color

White, colorless, beige, light yellow, brown, pink, and gray. Less commonly green, red, burgundy, orange, or black.

Streak

Colorless

Hardness

3.5 - 4

Crystal System

Monoclinic

Crystal Forms 
and Aggregates

Crystals, which are usually tabular, are often in a distinctive form reminiscent of old coffins, with a wider center, narrower edges, and flat top and bottom. Crystals usually form in curved aggregates of such crystals. Crystals may also be barrel-shaped, in groups of thick radiating crystals, saddle or fan-shaped, chisel-tipped, in foliated groups, in rosettes, and grainy. Also in elongated interconnected groups of crystals and in geode-like formations.

Transparency

Transparent to translucent

Specific Gravity

2.2

Luster

Vitreous, pearly

Cleavage

1,1

Fracture

Uneven

Tenacity

Brittle

In Group

Silicates; Tectosilicates; Zeolite Group

Striking Features

Crystal habits and association with other zeolites.

Environment

Most often in cavities of volcanic basalt. Occasionally in altered metamorphic gneiss.

Rock Type

Igneous

Popularity (1-4)

2

Prevalence (1-3)

2

Demand (1-3)

2