Physical properties of minerals are crucial for identification and understanding their formation. Luster, color, and streak are key visual characteristics that provide valuable clues about a mineral's composition and structure.

These properties can vary based on chemical makeup, crystal structure, and environmental factors. While color can be misleading, luster and streak often offer more reliable diagnostic information for mineral identification in the field and lab.

Mineral Luster Types

Metallic and Submetallic Luster

• Luster describes the appearance or quality of light reflected from the mineral surface
• Metallic luster exhibits high reflectivity similar to metals
  • Typically observed in minerals containing metallic elements or high concentrations of heavy elements
  • Examples include pyrite (fool's gold) and galena (lead ore)
• Submetallic luster falls between metallic and nonmetallic
  • Often seen in some sulfides and oxides
  • Examples include hematite and cuprite

Nonmetallic Luster Varieties

• Vitreous luster resembles the shine of glass
  • Common in many silicate minerals (quartz, feldspars)
• Resinous luster mimics the appearance of resin or plastic
  • Observed in some amber and certain sulfur specimens
• Pearly luster displays an iridescent, pearl-like sheen
  • Characteristic of minerals with perfect cleavage (micas)
• Silky luster exhibits a fibrous, silk-like appearance
  • Seen in minerals with fibrous crystal habits (asbestos, gypsum variety satin spar)
• Adamantine luster shows an exceptionally bright, diamond-like shine
  • Typical of minerals with high refractive indices (diamond, cerussite)
• Dull luster lacks any shine or reflectivity
  • Common in earthy or massive minerals (kaolinite, chalk)

Factors Affecting Luster

• Chemical composition directly influences a mineral's luster
  • Metals and metal-rich minerals tend to have metallic luster
  • Silicates often display vitreous or pearly luster
• Crystal structure impacts how light interacts with the mineral surface
  • Closely packed structures may result in higher reflectivity
• Surface properties affect luster appearance
  • Weathering can alter luster from shiny to dull
  • Crystal face orientation may produce different luster on various surfaces
• Impurities can modify luster from its typical appearance
  • Trace elements may enhance or diminish reflectivity

Factors Influencing Color

Chemical Composition and Crystal Structure

• Mineral color results from light interaction with the mineral's electronic structure
• Essential chemical components determine color in idiochromatic minerals
  • Copper in azurite creates its characteristic blue color
  • Chromium in emerald produces its distinctive green hue
• Crystal structure influences how light is absorbed and reflected
  • Atomic arrangement affects electron energy levels and light absorption
• Trace impurities can dramatically alter color in allochromatic minerals
  • Iron impurities in quartz can produce amethyst (purple) or citrine (yellow)
• Structural defects may create color centers
  • Radiation-induced defects in smoky quartz cause its dark coloration

Optical Phenomena and Environmental Factors

• Pleochroism causes minerals to display different colors when viewed from various crystallographic directions
  • Observed in anisotropic minerals (tourmaline, cordierite)
  • Results from differential light absorption along different crystal axes
• Color zoning occurs when different parts of a crystal exhibit varying colors
  • Can result from changes in chemical composition during crystal growth
  • Often seen in minerals like tourmaline and fluorite
• Environmental factors can alter a mineral's surface color
  • Weathering may produce a patina or tarnish (copper minerals)
  • Oxidation can change the color of iron-bearing minerals (pyrite to limonite)
  • Hydration can affect color (anhydrite to gypsum)

Limitations of Color in Identification

• Lighting conditions significantly impact perceived color
  • Natural vs artificial light can alter color appearance
  • Color temperature of light sources affects mineral hue
• Observer variability introduces subjectivity in color perception
  • Individual differences in color vision can lead to discrepancies
• Presence of nearby minerals may influence perceived color
  • Contrast effects can alter how a mineral's color is interpreted
• Relying solely on color for identification can lead to errors
  • Many minerals exhibit a wide range of colors
  • Different minerals may share similar colors

Streak for Identification

Streak Characteristics and Production

• Streak represents the color of a mineral in powdered form
• Obtained by rubbing the mineral across an unglazed porcelain plate (streak plate)
• Generally more consistent for a given mineral species than apparent color
  • Provides a reliable diagnostic tool for identification
• Streak color often differs from the mineral's visible color in hand specimen
  • Hematite appears metallic gray-black but produces a distinctive red-brown streak
• Intensity and consistency of streak offer insights into mineral properties
  • Softer minerals produce more intense streaks
  • Streak consistency can indicate mineral tenacity

Streak Applications and Limitations

• Particularly useful for distinguishing minerals with similar appearances
  • Pyrite (brass-yellow) and chalcopyrite (golden) both have metallic luster but different streaks
• Especially valuable for identifying metallic and near-metallic minerals
  • Surface tarnishing or alteration may obscure true color, but streak remains consistent
• Minerals with hardness greater than 7 on the Mohs scale may not produce visible streak
  • Diamond and corundum typically do not leave a streak on standard porcelain plates
• Streak color can indicate presence of certain elements
  • Copper minerals often produce greenish streaks
  • Manganese minerals typically have dark brown to black streaks
• Environmental factors have minimal impact on streak
  • Weathering or oxidation that affects surface color usually doesn't alter streak

Idiochromatic vs Allochromatic Minerals

Idiochromatic Mineral Characteristics

• Exhibit consistent and characteristic color due to essential chemical components
• Color directly related to chemical formula, typically consistent across different samples
• Essential elements act as chromophores, integral to crystal structure
• Examples of idiochromatic minerals:
  • Azurite (always blue due to copper)
  • Malachite (always green due to copper)
  • Rhodochrosite (pink to red due to manganese)
• Color serves as a reliable diagnostic feature for identification
• Variations in shade may occur but overall hue remains consistent

Allochromatic Mineral Characteristics

• Can exhibit wide range of colors due to trace impurities or structural defects
• Color-causing agents not essential to mineral's chemical formula or crystal structure
• Same mineral species can occur in various colors
• Examples of allochromatic minerals:
  • Quartz (colorless, purple amethyst, yellow citrine, pink rose quartz)
  • Corundum (red ruby, blue sapphire, and other colors)
  • Beryl (green emerald, blue aquamarine, pink morganite)
• Color unreliable for identification without additional diagnostic tests
• Impurities can create gem varieties of otherwise common minerals

Significance in Mineral Study and Formation

• Distinction crucial for accurate mineral identification
• Provides insights into geologic conditions of mineral formation
  • Presence of certain impurities indicates specific environmental factors
• Aids in understanding trace element geochemistry
  • Allochromatic minerals can serve as indicators of elemental availability during formation
• Influences mineral exploration and prospecting strategies
  • Idiochromatic minerals may indicate presence of specific ore deposits
• Impacts gemology and the value of gemstones
  • Rarity of certain colors in allochromatic minerals can greatly affect their worth