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Apophyllite with Stilbite refers to a common and aesthetically pleasing mineral association where crystals of Apophyllite (typically fluorapophyllite-K or hydroxyapophyllite-K) are found growing alongside or on top of Stilbite-Ca. Apophyllite often forms sharp, pyramidal, or tabular crystals, while Stilbite-Ca typically forms sheaf-like, radiating, or tabular aggregates. This combination is highly sought after by mineral collectors due to the contrasting forms, colors, and lusters of the two minerals, creating visually striking specimens.
How to Identify
- Color
- Apophyllite: Colorless, white, green, yellow, pink, brown. Stilbite: White, pink, orange, brown, yellowish-brown.
- Luster
- Apophyllite: Vitreous to pearly on cleavage surfaces. Stilbite: Vitreous to pearly.
- Texture
- Apophyllite: Smooth, glassy crystal faces. Stilbite: Often fibrous or radiating aggregates, sometimes with a silky feel.
- Crystal Form
- Apophyllite: Tetragonal, typically forming pseudo-cubic, pyramidal, or tabular crystals. Stilbite: Monoclinic, commonly forming sheaf-like aggregates, bow-tie shapes, or tabular crystals.
- Cleavage
- Apophyllite: Perfect basal cleavage {001}. Stilbite: Perfect cleavage {010}.
- Geological Environment
- Vesicles and amygdules in basaltic and other mafic volcanic rocks, hydrothermal veins, and less commonly in granitic pegmatites.
Key Facts
- Hardness: Apophyllite: 4.5-5; Stilbite: 3.5-4
- Specific Gravity: Apophyllite: 2.3-2.5; Stilbite: 2.1-2.2
- Crystal System: Apophyllite: Tetragonal; Stilbite: Monoclinic
- Color: Apophyllite: Colorless, white, green, yellow, pink; Stilbite: White, pink, orange, brown
- Luster: Vitreous to pearly
- Transparency: Transparent to translucent
- Fracture: Apophyllite: Uneven; Stilbite: Uneven to subconchoidal
- Cleavage: Apophyllite: Perfect basal {001}; Stilbite: Perfect {010}
- Composition: Apophyllite: Hydrated potassium calcium silicate fluoride/hydroxide; Stilbite: Hydrated sodium calcium aluminum silicate
Quick Check
- Color: Apophyllite: Colorless, white, green. Stilbite: White, pink, orange.
- Luster: Vitreous to pearly.
- Streak: White for both.
Physical Characteristics
- Crystal Habit: Apophyllite: Pseudo-cubic, pyramidal, tabular, massive. Stilbite: Sheaf-like, radiating, tabular, fibrous, granular.
- Cleavage Type: Apophyllite: Perfect in one direction (basal). Stilbite: Perfect in one direction.
- Fracture Type: Apophyllite: Uneven. Stilbite: Uneven to subconchoidal.
- Tenacity: Brittle for both.
- Luster Type: Vitreous to pearly.
Formation
Apophyllite and Stilbite are both secondary minerals that form in the vesicles (gas bubbles) and fractures of basaltic and other mafic volcanic rocks, as well as in some granitic pegmatites and hydrothermal veins. They precipitate from low-temperature hydrothermal solutions rich in silica, calcium, sodium, and aluminum, often after the primary volcanic rock has cooled and undergone alteration. The presence of both minerals together indicates a specific range of chemical and temperature conditions during their formation.
Usage
Primarily a collector's mineral due to its aesthetic appeal, crystal forms, and color. It is also used in some metaphysical practices. No significant industrial uses.
Age Distribution
Typically Cenozoic to Mesozoic, associated with volcanic activity.
Where to Find
Deccan Traps, India
World-renowned for producing exceptional specimens of Apophyllite and Stilbite, particularly from the quarries near Nashik, Pune, and Mumbai in Maharashtra. These occurrences are within the vast basaltic lava flows of the Deccan Traps.
Jalgaon, Maharashtra, India
Specifically known for producing large, well-formed, often green Apophyllite crystals associated with pink or orange Stilbite.
Iceland
Found in basaltic cavities, often with other zeolites.
Faroe Islands
Similar to Iceland, found in volcanic rocks.
Nova Scotia, Canada
Occurs in basaltic flows along the Bay of Fundy.
Oregon, USA
Found in some basaltic occurrences.
Finding Tips
Look in Volcanic Rocks
Focus on areas with basaltic lava flows, especially those that have undergone hydrothermal alteration. Look for vugs (cavities) or amygdules within the rock.
Check for Associated Minerals
Apophyllite and Stilbite often occur with other zeolites (e.g., Heulandite, Scolecite, Chabazite) and sometimes Calcite or Quartz. The presence of these can indicate a favorable environment.
Examine Crystal Habits
Learn to recognize the distinct crystal forms: Apophyllite's pseudo-cubic or pyramidal crystals and Stilbite's characteristic sheaf-like or bow-tie aggregates.
Observe Luster and Cleavage
Apophyllite's pearly luster on its perfect basal cleavage and Stilbite's vitreous to pearly luster are key identifiers.
Safety Precautions
When collecting in quarries or volcanic terrains, be aware of unstable rock, sharp edges, and potential for falling debris. Always wear appropriate safety gear, including eye protection and gloves.
Similar Rocks
Heulandite with Stilbite
Heulandite Group and Stilbite-Ca
Also known as: Heulandite-Stilbite
Cavansite with Stilbite
Cavansite (Ca(VO)Si4O10·4H2O) and Stilbite-Ca
Also known as: Cavansite-Stilbite
Scolecite with Stilbite
Scolecite (CaAl2Si3O10·3H2O) and Stilbite-Ca
Also known as: Scolecite-Stilbite
Scientific Classification
- Mineral Class
- Apophyllite: Phyllosilicate; Stilbite: Tectosilicate (Zeolite Group)
- Group
- Apophyllite Group; Zeolite Group
- Crystal System
- Apophyllite: Tetragonal; Stilbite: Monoclinic
- Chemical Formula
- Apophyllite Group: KCa4(Si8O20)(F,OH)·8H2O; Stilbite-Ca: NaCa2(Si7Al2)O18·7H2O
- Composition
- Apophyllite: Potassium, Calcium, Silicon, Oxygen, Fluorine/Hydroxyl, Water; Stilbite: Sodium, Calcium, Aluminum, Silicon, Oxygen, Water
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