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Aragonite is a carbonate mineral, one of the two common naturally occurring crystal forms of calcium carbonate (CaCO3), the other being calcite. It crystallizes in the orthorhombic system, in contrast to calcite's trigonal system. The 'Spudnik' or 'Moroccan' variety is characterized by its distinctive radiating, acicular crystal habit, often forming spherical or hemispherical aggregates that resemble sea urchins or Sputnik satellites. These aggregates can range in size from a few millimeters to several centimeters. The color is typically white, colorless, or light brown, but can also be yellow, green, or blue due to impurities. It is generally translucent to opaque with a vitreous to resinous luster.
How to Identify
- Color
- Typically white, colorless, light brown, yellow, green, or blue. The 'Spudnik' variety is often white to cream.
- Luster
- Vitreous to resinous, sometimes greasy on fracture surfaces.
- Texture
- The 'Spudnik' variety exhibits a distinctive radiating, acicular (needle-like) crystal texture, often forming botryoidal or globular aggregates.
- Crystal Form
- Orthorhombic system. Commonly forms prismatic, acicular, or tabular crystals. Twinning is common, often forming pseudohexagonal prisms. The 'Spudnik' variety is characterized by radiating aggregates of acicular crystals.
- Cleavage
- Distinct cleavage in one direction ({010}), imperfect in two others ({110} and {011}). Often difficult to observe due to fine-grained or acicular habit.
- Geological Environment
- Found in various environments including hot springs (tufa, travertine), caves (speleothems), hydrothermal veins, marine and freshwater sediments, and as a component of mollusk shells and corals. The 'Spudnik' variety is often found in cave systems or specific hydrothermal deposits.
Key Facts
- Hardness: 3.5 - 4 on the Mohs scale.
- Specific Gravity: 2.93 - 2.95 (higher than calcite, which is 2.71). This is a key distinguishing feature.
- Crystal System: Orthorhombic.
- Color: White, colorless, light brown, yellow, green, blue.
- Luster: Vitreous to resinous.
- Transparency: Transparent to translucent to opaque.
- Fracture: Subconchoidal to uneven.
- Cleavage: Distinct on {010}, imperfect on {110} and {011}.
- Composition: Calcium Carbonate (CaCO3).
Quick Check
- Color: White to colorless, light brown, yellow, green, or blue.
- Luster: Vitreous to resinous.
- Streak: White.
Physical Characteristics
- Crystal Habit: Prismatic, acicular, tabular, columnar, fibrous, stalactitic, pisolitic, oolitic, botryoidal. Often twinned, forming pseudohexagonal prisms. 'Spudnik' variety is radiating acicular aggregates.
- Cleavage Type: Distinct in one direction ({010}), imperfect in two others ({110} and {011}).
- Fracture Type: Subconchoidal to uneven.
- Tenacity: Brittle.
- Luster Type: Vitreous to resinous.
Formation
Aragonite is a polymorph of calcium carbonate (CaCO3), meaning it has the same chemical composition as calcite but a different crystal structure. It typically forms under specific conditions of temperature and pressure that differ from those favoring calcite. These conditions include: 1. Low-temperature, near-surface environments, such as in hot springs, caves (speleothems), and as precipitates in marine and freshwater environments. 2. High-pressure, low-temperature metamorphic conditions. 3. Biogenic precipitation by marine organisms (e.g., mollusk shells, corals). 4. Hydrothermal veins. The 'Spudnik' or 'Moroccan' variety specifically refers to aragonite that forms as radiating, acicular (needle-like) crystals, often in botryoidal or globular aggregates, giving it a distinctive 'spiky' or 'urchin-like' appearance. This habit is common in certain cave environments or hydrothermal deposits.
Usage
Aragonite has limited industrial uses compared to calcite due to its relative instability. However, it is used as a decorative stone, for carving, and as a collector's mineral. Biogenic aragonite is a primary component of coral reefs and mollusk shells. In some applications, it is used in water treatment to remove heavy metals or as a soil amendment. The 'Spudnik' variety is primarily valued by mineral collectors for its aesthetic and unique crystal habit.
Age Distribution
Aragonite forms in various geological settings and can be found in rocks of many ages, from recent to ancient. Its stability is lower than calcite at surface conditions, so it often alters to calcite over geological time.
Where to Find
Morocco
The 'Moroccan' variety, often synonymous with 'Spudnik' aragonite, is famously found in various localities in Morocco, particularly in the Atlas Mountains region, where it forms spectacular radiating crystal aggregates in geodes and veins.
Spain
Aragonite is the namesake mineral for Molina de Aragón, Spain, where it was first described. Classic localities produce well-formed prismatic crystals.
Slovakia
The Ochtinská Aragonite Cave in Slovakia is a UNESCO World Heritage site known for its unique aragonite formations, including 'aragonite helictites' and 'aragonite flowers'.
United States
Found in various states, including New Mexico (e.g., Santa Eulalia District), Arizona, and California, often in association with volcanic rocks or in cave systems.
Mexico
Significant deposits, often associated with lead-zinc deposits, producing well-crystallized specimens.
Finding Tips
Look for Cave Environments
Aragonite, especially the 'Spudnik' variety, frequently forms in caves as speleothems (stalactites, stalagmites, helictites, cave flowers). Look for areas with limestone bedrock and active or ancient cave systems.
Check Hot Spring Deposits
Aragonite can precipitate from hot springs, forming tufa or travertine deposits. Examine these areas for crystalline forms.
Examine Hydrothermal Veins
In some areas, aragonite forms in hydrothermal veins, often associated with metallic ore deposits. Look for vugs or open spaces within these veins.
Identify Distinctive Crystal Habit
For 'Spudnik' aragonite, look for the characteristic radiating, needle-like crystals forming spherical or hemispherical aggregates. This habit is quite unique and helps distinguish it from other minerals.
Perform Acid Test (Carefully)
Aragonite, like calcite, will effervesce (fizz) vigorously in dilute hydrochloric acid. This distinguishes it from many other non-carbonate minerals. Always use caution with acids.
Similar Rocks
Calcite
Calcite
Also known as: Calcium Carbonate
Dolomite
Dolomite
Also known as: Calcium Magnesium Carbonate
Gypsum
Gypsum
Also known as: Selenite, Alabaster, Satin Spar
Scientific Classification
- Mineral Class
- Carbonates
- Group
- Aragonite Group
- Crystal System
- Orthorhombic
- Chemical Formula
- CaCO3
- Composition
- Calcium Carbonate
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