10.2 Phosphate Mineral Chemistry and Classification

Phosphate minerals, characterized by their tetrahedral (PO4)3- anion, play a crucial role in mineralogy. Their complex structures and varied compositions, influenced by ionic substitutions, create a diverse group of minerals with unique properties and classifications.

Understanding phosphates is key to grasping the broader sulfate, phosphate, and borate mineral families. From the common apatite group to rare earth phosphates, these minerals showcase the intricate interplay of chemistry and structure in the mineral world.

Phosphate mineral composition and structure

Phosphate anion and general formula

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• Phosphate minerals contain tetrahedral (PO4)3- anion as primary structural unit
• Oxygen atoms arranged around central phosphorus atom
• General formula A5(XO4)3Z represents phosphate minerals
  • A represents cations (Ca, Pb, rare earth elements)
  • X typically P (can be As or V)
  • Z represents anions (F, Cl, OH)
• Complex crystal structures form due to various phosphate anion bonding arrangements
• Stability and properties influenced by ionic radii and charges of cations

Structural variations and substitutions

• Isomorphous substitution occurs in phosphate minerals
  • Ions of similar size and charge replace each other in crystal structure
  • Leads to compositional variations
• Phosphate minerals form anhydrous and hydrated structures
  • Presence of water molecules affects physical and chemical properties
• Cation substitution common in phosphates
  • Elements with similar ionic radii and charge replace each other (Ca2+ substituted by Pb2+)
• Goldschmidt's rules govern extent of solid solution
  • Consider size, charge, and electronegativity of substituting ions

Classifying phosphate minerals

Classification systems

• Dana classification system organizes phosphates into Class 40
  • Based on chemical composition and crystal structure
• Strunz classification system groups phosphates under Class 8
  • Subclasses based on additional anions and water content
• Primary groups classified by dominant cation
  • Calcium phosphates
  • Lead phosphates
  • Rare earth phosphates

Structural and compositional classifications

• Degree of phosphate tetrahedra polymerization used for classification
  • Orthophosphates
  • Pyrophosphates
  • Polyphosphates
• Structural classification considers arrangement of phosphate tetrahedra and cations
  • Sheet phosphates
  • Chain phosphates
  • Framework phosphates
• Additional anions in mineral structure used for further subdivision
  • Fluorine-bearing phosphates
  • Chlorine-bearing phosphates
  • Hydroxyl-bearing phosphates

Substitution and solid solution in phosphates

Types of substitutions

• Solid solution occurs with continuous range of compositions between end-member minerals
• Isomorphous substitution of central P5+ ion
  • As5+ creates arsenate minerals
  • V5+ creates vanadate minerals
• Cation substitution involves replacement of similar ionic radii and charge
  • Ca2+ substituted by Pb2+ or rare earth elements
• Complex mineral series form due to substitution
  • Apatite group: F-, Cl-, and OH- substitute for each other

Effects of substitution

• Physical properties affected by degree of substitution
  • Crystal morphology changes
  • Optical characteristics alter
  • Solubility varies
• Formation of mineral series with varying compositions
  • Pyromorphite-mimetite series: Pb5(PO4,AsO4)3Cl
  • Variscite-strengite series: AlPO4·2H2O - FePO4·2H2O
• Substitution can lead to zoning in crystals
  • Core and rim of crystal may have different compositions

Phosphate mineral groups and characteristics

Apatite and rare earth phosphates

• Apatite group most abundant and diverse phosphate mineral group
  • General formula: Ca5(PO4)3(F,Cl,OH)
  • Hexagonal crystal system
  • Examples: fluorapatite, chlorapatite, hydroxylapatite
• Monazite group important source of rare earth elements
  • General formula: (Ce,La,Nd,Th)(PO4)
  • Monoclinic crystal system
  • Examples: monazite-(Ce), monazite-(La)

Metal phosphates and hydrated phosphates

• Pyromorphite-mimetite series demonstrates isomorphous substitution
  • Formula: Pb5(PO4,AsO4)3Cl
  • Hexagonal crystal system
• Vivianite group characterized by hydrated iron phosphates
  • Example: vivianite Fe3(PO4)2·8H2O
  • Monoclinic crystal system
• Turquoise group represents complex hydrated phosphates
  • General formula: A0-1B6(PO4)4(OH)8·4H2O (A = Cu, Fe; B = Al, Fe3+)
  • Triclinic crystal system
• Wavellite group showcases hydrated aluminum phosphates
  • Example: wavellite Al3(PO4)2(OH,F)3·5H2O
  • Orthorhombic crystal system