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Smithsonite is a zinc carbonate mineral, belonging to the calcite group. It is known for its wide range of colors, often occurring in attractive botryoidal, reniform, or stalactitic masses, as well as granular or earthy aggregates. Well-formed rhombohedral crystals are rare but highly sought after. Its characteristic effervescence with dilute acids helps in its identification. It is a secondary mineral, typically found in the oxidized zones of zinc ore deposits.
How to Identify
- Color
- Highly variable: typically blue-green (due to copper impurities), yellow, pink, purple, white, gray, brown, or colorless. The most prized colors are the vibrant blue-green and pink.
- Luster
- Vitreous (glassy) to pearly on cleavage surfaces, sometimes dull or earthy in massive forms.
- Texture
- Often botryoidal (grape-like), reniform (kidney-shaped), stalactitic, mammillary, or granular. Crystalline forms are rare and typically rhombohedral.
- Crystal Form
- Rarely forms distinct rhombohedral crystals. More commonly found as botryoidal, mammillary, stalactitic, or granular aggregates. Individual crystals, when present, are usually small and curved.
- Cleavage
- Perfect rhombohedral cleavage in three directions, though often obscured by its massive or botryoidal habit.
- Geological Environment
- Found in the oxidized zones of zinc-bearing sulfide deposits, often associated with sphalerite, cerussite, anglesite, malachite, azurite, and limonite.
Key Facts
- Hardness: 3.5-4 on the Mohs scale
- Specific Gravity: 4.30-4.45 (relatively heavy for a non-metallic mineral)
- Crystal System: Trigonal
- Color: Highly variable: blue-green, yellow, pink, purple, white, gray, brown, or colorless
- Luster: Vitreous to pearly, sometimes dull or earthy
- Transparency: Transparent to translucent
- Fracture: Uneven to subconchoidal
- Cleavage: Perfect rhombohedral on {1011}
- Composition: Zinc Carbonate (ZnCO3)
Quick Check
- Color: Variable (blue-green, yellow, pink, white, gray, brown, colorless)
- Luster: Vitreous to pearly, sometimes dull/earthy
- Streak: White
Physical Characteristics
- Crystal Habit: Botryoidal, mammillary, stalactitic, reniform, granular, earthy; rarely as small rhombohedral crystals.
- Cleavage Type: Perfect rhombohedral in three directions, often difficult to observe in massive forms.
- Fracture Type: Uneven to subconchoidal.
- Tenacity: Brittle.
- Luster Type: Vitreous to pearly, sometimes dull or earthy.
Formation
Smithsonite is a secondary mineral, forming in the oxidized zone of zinc ore deposits. It typically forms from the alteration of primary zinc sulfides, primarily sphalerite (ZnS), through the action of carbonic acid-rich waters. The zinc ions (Zn2+) released from the weathering of sphalerite react with dissolved carbonate ions (CO32-) to precipitate smithsonite. It can also form directly from hydrothermal solutions in some instances.
Usage
Smithsonite is an important ore of zinc, particularly in historical mining operations. While sphalerite is the primary ore today, smithsonite was historically significant. Fine specimens are highly prized by mineral collectors for their aesthetic appeal, especially the botryoidal and stalactitic forms and vibrant colors. It has also been used as a minor gemstone, though its relative softness limits its use.
Age Distribution
Forms in the supergene (secondary) alteration zones of zinc-bearing ore deposits, thus its formation is ongoing and not tied to a specific geological age, but rather to the weathering of primary sulfide deposits.
Where to Find
Kelly Mine, Magdalena, New Mexico, USA
Famous for its vibrant blue-green smithsonite, often botryoidal.
Tsumeb Mine, Namibia
Produces exceptional specimens in various colors, including blue, green, and pink.
Lavrion (Laurium) District, Greece
Historically significant source, known for blue and green smithsonite.
Choix, Sinaloa, Mexico
Known for attractive pink and purple smithsonite.
Broken Hill, New South Wales, Australia
Produces various colors, including yellow and green.
Sardinia, Italy
Known for yellow and green smithsonite.
Finding Tips
Look for Zinc Deposits
Smithsonite is a secondary mineral, so search in the oxidized zones of known or historical zinc mining districts. Look for areas where primary zinc sulfides (like sphalerite) have been exposed to weathering.
Identify Associated Minerals
Smithsonite often occurs with other secondary minerals such as cerussite, anglesite, malachite, azurite, and limonite. The presence of these minerals can indicate a favorable environment.
Test for Effervescence
Smithsonite is a carbonate and will effervesce (fizz) when a drop of dilute hydrochloric acid is applied. This is a key diagnostic test, though it may be slow if the mineral is dense.
Observe Habit and Color
Look for characteristic botryoidal, mammillary, or stalactitic forms. The wide range of colors, especially the distinctive blue-green, pink, or yellow, can be a strong indicator.
Check Hardness
Smithsonite is relatively soft (3.5-4 on the Mohs scale), so it can be scratched by a steel knife but not by a copper coin.
Similar Rocks
Hemimorphite
Hemimorphite (Zn4Si2O7(OH)2·H2O)
Also known as: Calamine (historically, often confused with smithsonite)
Calcite
Calcite (CaCO3)
Also known as: Calcium Carbonate
Aragonite
Aragonite (CaCO3)
Also known as: Calcium Carbonate
Dolomite
Dolomite (CaMg(CO3)2)
Also known as: Calcium Magnesium Carbonate
Scientific Classification
- Mineral Class
- Carbonates
- Group
- Calcite Group
- Crystal System
- Trigonal
- Chemical Formula
- ZnCO3
- Composition
- Zinc Carbonate
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