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
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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)
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