💎Mineralogy Unit 7 – Native Elements and Sulfides in Mineralogy

Key Concepts and Definitions

• Native elements minerals consist of a single element in their pure form
• Sulfides minerals contain sulfur combined with one or more metallic elements
• Crystal structure refers to the ordered arrangement of atoms in a mineral
• Bonding types in minerals include metallic, covalent, and ionic bonds
• Luster describes the way a mineral reflects light from its surface (metallic, submetallic, nonmetallic)
• Streak is the color of a mineral when it is powdered or scratched on a streak plate
• Specific gravity measures the density of a mineral relative to water
• Tarnish is a thin layer of corrosion that forms on the surface of some minerals due to chemical reactions with the environment

Native Elements: Characteristics and Examples

• Native elements occur naturally in their pure elemental form without combining with other elements
• Examples of native elements include gold (Au), silver (Ag), copper (Cu), and sulfur (S)
• Native elements often have distinctive physical properties related to their crystal structure and bonding
  • Gold has a cubic close-packed structure and metallic bonding, resulting in high ductility and malleability
  • Sulfur has a complex orthorhombic structure and covalent bonding, leading to low hardness and brittleness
• Many native elements have high economic value due to their rarity and unique properties
• Some native elements form under specific geological conditions
  • Native copper can form in the oxidized zones of copper deposits or in basaltic lava flows
  • Native sulfur often forms in volcanic environments or as a result of bacterial reduction of sulfates

Sulfides: Formation and Properties

• Sulfides form when sulfur combines with one or more metallic elements
• Common sulfide minerals include pyrite (FeS2), galena (PbS), and sphalerite (ZnS)
• Sulfides can form in various geological environments, such as hydrothermal veins, contact metamorphic zones, and sedimentary deposits
• Many sulfides have a metallic luster and high specific gravity due to their dense crystal structures
• Sulfides often have distinct crystal habits, such as the cubic form of pyrite or the octahedral form of galena
• Some sulfides, like pyrite and marcasite, can exhibit polymorphism, where they have the same chemical composition but different crystal structures
• Sulfides are important ore minerals for extracting valuable metals, such as lead, zinc, and copper

Crystal Structures and Bonding

• The crystal structure of a mineral is determined by the arrangement of its atoms in a repeating pattern
• Native elements and sulfides can have various crystal systems, such as cubic, hexagonal, or orthorhombic
• The type of bonding in a mineral influences its physical properties and behavior
  • Metallic bonding, common in native elements like gold and silver, involves a sea of delocalized electrons surrounding positively charged atomic cores
  • Covalent bonding, found in elements like sulfur and some sulfides, involves the sharing of electrons between atoms
  • Ionic bonding, present in some sulfides, involves the attraction between positively and negatively charged ions
• The strength and directionality of bonding affect properties like hardness, cleavage, and electrical conductivity

Identification Techniques

• Native elements and sulfides can be identified using a combination of physical properties and chemical tests
• Color, streak, luster, and specific gravity are important diagnostic properties
  • Native copper has a distinctive reddish color and metallic luster
  • Galena has a lead-gray color and perfect cubic cleavage
• Hardness tests, using the Mohs scale, can help distinguish between minerals with similar appearances
• Chemical tests, such as acid reactions or flame tests, can provide additional information
  • Calcite (a carbonate) effervesces in hydrochloric acid, while most sulfides do not react
  • Native copper produces a green flame color when heated
• X-ray diffraction and electron microprobe analysis can definitively identify mineral species and determine their chemical composition

Economic Importance and Uses

• Many native elements and sulfides are valuable ore minerals, serving as sources of important metals
• Gold and silver are precious metals used in jewelry, electronics, and investment
• Copper, extracted from native copper and sulfide ores like chalcopyrite, is used in electrical wiring, plumbing, and alloys
• Lead, primarily obtained from galena, is used in batteries, radiation shielding, and formerly in gasoline additives
• Zinc, sourced from sphalerite, is used in galvanizing steel, making brass and bronze alloys, and in rubber and paint production
• Pyrite, though not a major ore of iron, is used to produce sulfuric acid for various industrial applications
• The presence of certain native elements or sulfides can guide mineral exploration and indicate potential ore deposits

Environmental Impact and Geochemistry

• The weathering and oxidation of sulfide minerals can lead to the formation of acid mine drainage
  • Pyrite exposed to water and oxygen generates sulfuric acid, which can lower the pH of nearby water sources
  • Acid mine drainage can mobilize heavy metals and adversely affect aquatic ecosystems
• Some native elements and sulfides are associated with geochemical anomalies that can be used in mineral exploration
  • Elevated levels of arsenic or mercury in soil or water may indicate the presence of gold mineralization
  • The oxidation of sulfides can create gossans, iron-rich weathered zones that cap underlying ore deposits
• Understanding the geochemical behavior of native elements and sulfides is crucial for predicting their environmental impact and designing effective remediation strategies

Related Mineral Groups and Associations

• Native elements and sulfides are often associated with other mineral groups in various geological settings
• Sulfides are commonly found with oxides, hydroxides, and carbonates in hydrothermal ore deposits
  • Galena often occurs with sphalerite, pyrite, and calcite in Mississippi Valley-type lead-zinc deposits
  • Chalcopyrite is frequently associated with bornite, pyrite, and molybdenite in porphyry copper deposits
• Native elements can occur with silicates, such as quartz or mica, in pegmatites or hydrothermal veins
• Some native elements and sulfides form as secondary minerals in the oxidized zones of ore deposits
  • Native silver can form as a result of the oxidation of silver sulfides like acanthite
  • Native sulfur can form from the oxidation of hydrogen sulfide gas in volcanic environments
• Understanding these associations helps in interpreting the genesis and evolution of mineral deposits and guides exploration efforts