Heulandite: The zeolite mineral heulandite information and pictures

About

Heulandite is one of the more common zeolite minerals. It has a very distinctive crystal form and has a noticeable pearly luster on cleavage surfaces. Since its original classification, Heulandite was always regarded as a single mineral species with a variable elemental makeup. In 1997, the Zeolite Subcommittee of the IMA divided this mineral into individual sub-species, thereby regarding Heulandite as a series of four members. The series is defined by the presence of several variable elements in the following order: calcium, sodium, potassium, strontium, and barium. Calcium-dominating Heulandite is called Heulandite-Ca, and is by far the most prevalent form of this series. Sodium-dominating Heulandite is the next most common, and is known as Heulandite-Na, followed by potassium-dominating Heulandite known as Heulandite-K. Strontium-dominating Heulandite is known as Heulandite-Sr, and is very uncommon. A new barium-dominating Heulandite was classified in 2002, and is known as Heulandite-Ba.

Almost all Heulandite specimens in collections are Heulandite-Ca. A distinction among the different Heulandite types is rarely made, and the members are generally just termed Heulandite without further breakdown. Heulandite is named after English mineral dealer Henry Heuland (1778-1856), who would frequent trips to Iceland collecting Heulandite and other zeolites.

Heulandite shares its chemical structure with the closely related mineral Clinoptilolite. Clinoptilolite is a rare mineral very similar to Heulandite with a higher potassium and silicon ratio. Although regarded by the IMA as a distinct mineral species, many consider Clinoptilolite to be a variety of Heulandite.

Identification

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.
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
Luster: Vitreous, pearly
Cleavage: 1,1
Fracture: Uneven
Tenacity: Brittle
Transparency: Transparent to translucent
Specific Gravity: 2.2
Group: Silicates; Tectosilicates; Zeolite Group
Rock Type: Igneous

Rankings

Popularity (1–4): 2
Prevalence (1–3): 2
Demand (1–3): 2

Crystal Forms & 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.

Striking Features

Crystal habits and association with other zeolites.

Environment

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

Uses

Large and well-shaped Heulandite crystals are of interest to collectors. Heulandite as a zeolite is also used in water purifiers and chemical filters.

Noteworthy Localities

Heulandite is a relatively common mineral, and is found in most zeolite occurrences. Some of the most outstanding crystals come from the state of Maharashtra in India, at the localities of Pune, Jalgaon, Aurangabad, and Nasik. The Shakur Quarry in Nasik is especially known for its dark green Heulandite, formed by inclusions of Celadonite.

Well-formed and lustrous Heulandite crystals have come from Teigarhorn, Berufjördur, Iceland; and dark red crystals from Buffaure, Val d'Fassa, Trentino, Italy. Small crystals lining basalt vugs are found in Lisbon, Portugal. In Australia, fine orange to reddish colored crystals come from Gunnedah and Garrawilla Station, Pottinger Co., New South Wales.

In the U.S., some of the best Heulandite crystals, with good luster and in various colors, are found in Paterson and Prospect Park, Passaic Co., New Jersey. Interconnected grayish-green crystals were also found nearby at Fanwood, Union Co., New Jersey. Highly lustrous, pink to peach-colored Heulandite, associated with Mordenite, comes from the Rats Nest claim, Challis, Custer Co., Idaho. In Canada, good crystals come from several localities in Nova Scotia near the Bay of Fundy, and across the bay on Wasson's Bluff and Partridge Island.

Common Mineral Associations

Stilbite, Apophyllite, Calcite, Laumontite, Chabazite, Analcime, Quartz, Celadonite, Prehnite, Datolite

Distinguishing Similar Minerals

Stilbite - Forms in a different crystal shape; otherwise difficult to distinguish.
Apophyllite - Forms in a different crystal shape, luster is usually more glassy, and often has striations.