Gormanite vs. Other Phosphate Minerals: A Comparison

Posted on Mon 03 February 2025 in Blog

Phosphate minerals are a diverse group of compounds containing the phosphate anion (PO4)3-. They play crucial roles in various geological, biological, and industrial processes. While many phosphate minerals are relatively common, some, like Gormanite, are rarer and possess unique characteristics. This post delves into a detailed comparison of Gormanite with other notable phosphate minerals, highlighting its distinct properties and placing it within the broader context of phosphate mineralogy.

What is Gormanite?

Gormanite, with the chemical formula Fe2+3Al4(PO4)4(OH)6·2H2O, is a rare hydrated iron aluminum phosphate mineral. It belongs to the souzalite group. It was first discovered in 1977 in the Rapid Creek and Big Fish River area, Yukon Territory, Canada, and is named after Professor Donald Herbert Gorman (1922-2016) of the University of Toronto, for his contributions to mineralogy. Mindat.org - Gormanite.

Gormanite typically occurs as blue-green to green, vitreous to dull, translucent to opaque masses, crusts, or rarely, small, tabular crystals. It's often found in association with other phosphate minerals in iron-rich sedimentary formations, particularly those that have undergone low-grade metamorphism. Its relatively low hardness (around 4-5 on the Mohs scale) and specific gravity (around 3.1) are characteristic features.

Key Properties of Gormanite:

  • Chemical Formula: Fe2+3Al4(PO4)4(OH)6·2H2O
  • Crystal System: Triclinic
  • Color: Blue-green, green, rarely yellowish-green
  • Luster: Vitreous to dull
  • Hardness: 4-5 (Mohs scale)
  • Specific Gravity: 3.1 - 3.2
  • Streak: White to pale green
  • Cleavage: Good on {010}
  • Fracture: Uneven
  • Occurrence: Secondary mineral in metamorphosed iron-rich sedimentary rocks.

Comparing Gormanite to Other Phosphate Minerals

To understand Gormanite's unique position, we'll compare it to several other phosphate minerals, categorized for clarity:

1. Apatite Group Minerals (Apatite, Fluorapatite, Hydroxylapatite, Chlorapatite):

The apatite group is arguably the most well-known and abundant group of phosphate minerals. These minerals are essential components of bones and teeth in vertebrates and are widely used in fertilizers and industrial applications.

  • Chemical Formula: Apatite is a group name, with variations like Ca5(PO4)3(F,Cl,OH) (Fluorapatite, Chlorapatite, Hydroxylapatite).
  • Crystal System: Hexagonal
  • Hardness: 5 (Mohs scale)
  • Specific Gravity: 3.1 - 3.2
  • Key Differences from Gormanite: Apatite minerals are calcium phosphates, while Gormanite is an iron aluminum phosphate. Apatite has a hexagonal crystal system, whereas Gormanite is triclinic. Apatite is generally harder and often found in a wider range of geological environments, including igneous, metamorphic, and sedimentary rocks. Gormanite's formation is more restricted to specific metamorphosed iron-rich sedimentary settings. Apatite is often found as well-formed crystals, while Gormanite is more commonly found as masses or crusts. Mindat.org - Apatite

2. Vivianite:

Vivianite (Fe2+3(PO4)2·8H2O) is another iron phosphate, but it differs significantly from Gormanite in its composition and properties.

  • Chemical Formula: Fe2+3(PO4)2·8H2O
  • Crystal System: Monoclinic
  • Hardness: 1.5 - 2 (Mohs scale)
  • Specific Gravity: 2.6 - 2.7
  • Key Differences from Gormanite: Vivianite is a hydrated ferrous phosphate, lacking the aluminum present in Gormanite. It's significantly softer than Gormanite and has a monoclinic crystal system. Vivianite is famous for its color change upon exposure to light, often starting as colorless or pale green and darkening to deep blue or blue-green. Gormanite does not exhibit this dramatic color change. Vivianite often forms in organic-rich environments, such as clay sediments and fossil bone deposits, while Gormanite's occurrence is more specific to metamorphosed iron formations. Mindat.org - Vivianite

3. Lazulite:

Lazulite ((Mg,Fe2+)Al2(PO4)2(OH)2) is a magnesium aluminum phosphate that shares some similarities with Gormanite in terms of containing aluminum and having a blue-green color.

  • Chemical Formula: (Mg,Fe2+)Al2(PO4)2(OH)2
  • Crystal System: Monoclinic
  • Hardness: 5.5 - 6 (Mohs scale)
  • Specific Gravity: 3.0 - 3.1
  • Key Differences from Gormanite: Lazulite contains magnesium as a primary cation, while Gormanite contains ferrous iron. Lazulite is generally harder than Gormanite. Lazulite has a monoclinic crystal system, while Gormanite is triclinic. Lazulite is often found in metamorphic rocks like quartzites and schists, while Gormanite's occurrence is more restricted to iron-rich sedimentary formations. Mindat.org - Lazulite

4. Scorzalite:

Scorzalite ((Fe2+,Mg)Al2(PO4)2(OH)2) is the iron-dominant analogue of Lazulite and forms a solid solution series with it. It is even more closely related to Gormanite.

  • Chemical Formula: (Fe2+,Mg)Al2(PO4)2(OH)2
  • Crystal System: Monoclinic
  • Hardness: 6 (Mohs scale)
  • Specific Gravity: 3.3
  • Key Differences from Gormanite: Scorzalite, like Lazulite, is harder than Gormanite. Scorzalite is monoclinic, while Gormanite is triclinic. While both contain iron and aluminum, the ratio and hydration state differ, leading to distinct structural and physical properties. Gormanite has a more complex formula with additional water molecules. Mindat.org - Scorzalite

5. Wavellite:

Wavellite (Al3(PO4)2(OH)3·5H2O) is an aluminum phosphate, but it differs from Gormanite in its iron content and crystal structure.

  • Chemical Formula: Al3(PO4)2(OH)3·5H2O
  • Crystal System: Orthorhombic
  • Hardness: 3.5 - 4 (Mohs scale)
  • Specific Gravity: 2.3 - 2.4
  • Key Differences from Gormanite: Wavellite is a pure aluminum phosphate, lacking the iron found in Gormanite. It's generally softer and has a lower specific gravity. Wavellite typically forms radiating, spherical aggregates, a habit rarely seen in Gormanite. Wavellite has an orthorhombic crystal system, while Gormanite is triclinic. Mindat.org - Wavellite

6. Turquoise:

Turquoise (CuAl6(PO4)4(OH)8·4H2O) is a copper aluminum phosphate, known for its distinctive blue-green color.

  • Chemical Formula: CuAl6(PO4)4(OH)8·4H2O
  • Crystal System: Triclinic
  • Hardness: 5 - 6 (Mohs scale)
  • Specific Gravity: 2.6 - 2.9
  • Key Differences from Gormanite: Turquoise contains copper, which is absent in Gormanite. Turquoise is often found in arid environments as a secondary mineral in copper deposits, a very different geological setting from Gormanite's typical occurrence. While both are triclinic, their specific crystal structures and unit cell parameters differ. Mindat.org - Turquoise

7. Dufrenite: Dufrenite (Fe2+Fe3+4(PO4)3(OH)5·2H2O) is another iron phosphate mineral.

  • Chemical Formula: Fe2+Fe3+4(PO4)3(OH)5·2H2O
  • Crystal System: Monoclinic (pseudo-orthorhombic)
  • Hardness: 3.5 - 4.5
  • Specific Gravity: 3.2 - 3.4
  • Key Differences from Gormanite: Dufrenite contains both ferrous (Fe2+) and ferric (Fe3+) iron, while Gormanite is primarily ferrous. Dufrenite lacks the aluminum present in Gormanite. Dufrenite is typically monoclinic, while Gormanite is triclinic. Dufrenite often forms botryoidal or radial aggregates, while Gormanite is more commonly massive or crust-like. Mindat.org - Dufrenite

8. Rockbridgeite: Rockbridgeite (Fe2+Fe3+4(PO4)3(OH)5) is closely related to Dufrenite.

  • Chemical Formula: Fe2+Fe3+4(PO4)3(OH)5
  • Crystal System: Orthorhombic
  • Hardness: 4.5
  • Specific Gravity: 3.3 - 3.5
  • Key Differences from Gormanite: Similar to Dufrenite, Rockbridgeite contains both ferrous and ferric iron and lacks aluminum. It has an orthorhombic crystal system, unlike Gormanite's triclinic system. The hydration state also differs. Mindat.org - Rockbridgeite

9. Childrenite and Eosphorite: Childrenite (Fe2+Al(PO4)(OH)2·H2O) and Eosphorite (Mn2+Al(PO4)(OH)2·H2O) form a solid solution series.

  • Chemical Formula: (Fe,Mn)Al(PO4)(OH)2·H2O
  • Crystal System: Orthorhombic
  • Hardness: 4.5 - 5
  • Specific Gravity: 3.1 - 3.2
  • Key Differences from Gormanite: Childrenite-Eosphorite contains only one aluminum atom per formula unit, compared to Gormanite's four. The crystal system is orthorhombic, and the hydration state is different. Mindat.org - Childrenite Mindat.org - Eosphorite

10. Souzalite: Souzalite ((Mg,Fe)3(Al,Fe)4(PO4)4(OH)6·2H2O) is a member of the same group as Gormanite.

  • Chemical Formula: (Mg,Fe)3(Al,Fe)4(PO4)4(OH)6·2H2O
  • Crystal System: Triclinic
  • Hardness: 5.5 - 6
  • Specific Gravity: 3.08
  • Key Differences from Gormanite: Souzalite contains magnesium, which is absent or present only in minor amounts in Gormanite. The iron-to-aluminum ratio can also vary. Mindat.org - Souzalite

Summary and Significance of Gormanite

Gormanite, while rare, holds a significant place in mineralogy due to its unique composition and occurrence. It represents a specific set of geochemical conditions involving iron-rich sedimentary rocks undergoing low-grade metamorphism. Its presence can provide valuable insights into the geological history of a particular region.

The comparisons above highlight that while Gormanite shares some characteristics with other phosphate minerals (like containing phosphate, iron, or aluminum), its specific combination of elements, crystal structure, and formation environment make it distinct. It's not a common mineral like apatite, nor does it exhibit the dramatic color changes of vivianite. Its rarity and specific geological context contribute to its scientific interest.

The search term "Gormanite" is best served by focusing on its unique properties: its triclinic crystal system, its Fe2+3Al4(PO4)4(OH)6·2H2O formula, its association with metamorphosed iron formations, and its distinction from more common phosphates like apatite and vivianite, as well as its closer relatives like souzalite, scorzalite, and lazulite. This detailed comparison provides a comprehensive understanding of Gormanite within the broader context of phosphate mineralogy.