Mineralogy of Phosphates and Other Rare Minerals

Phosphates, arsenates, vanadates, wolframates, molybdates, chromates, nitrates, and borates may contain only a few or, in some cases, only one mineral of note. In particular, nitrates and borates have seldom interested gemologists. Nevertheless, it is essential for mineralogists to understand these mineral classes.

Introduction to Phosphates, Arsenates, and Vanadates

Phosphates, arsenates, and vanadates are mineral classes with complex anions of (PO₄)^3-, (AsO₄)^3-, and (VO₄)^3-, respectively. Anion complexes occur in tetrahedral coordination, meaning each atom of phosphorus, arsenic, and vanadium is surrounded by four oxygen atoms.

These three groups are usually studied together because P⁵⁺, As⁵⁺, and V⁵⁺ can substitute for each other easily. They sometimes form solid solution series, like pyromorphite (Pb₅(PO₄)₃Cl), mimetite (Pb₅(AsO₄)₃Cl), and vanadinite (Pb₅(VO₄)₃Cl). The main cations of these mineral classes are Fe, Al, Ca, Mn, Zn, Pb, Ni, Co, Li, U, and Na.

• Phosphates— (PO₄)^3- anion 
• Arsenates — (AsO₄)^3- anion 
• Vanadates— (VO₄)^3- anion 

Examples of Phosphates, Arsenates, and Vanadates

Most minerals in these three classes are rare, except for apatite. We will also discuss other significant phosphates, such as monazite, turquoise, and vivianite. Monazite is an essential mineral for geochronology because it has radioactive thorium in its composition. Turquoise is a famous gemstone. Vivianite is highly prized among mineral collectors.

Other examples of phosphates are triphylite (Li(Fe²⁺,Mn²⁺)PO₄), lithiophilite (Li(Mn²⁺, Fe²⁺)PO₄), amblygonite (LiAl(PO₄)(F,OH)), lazulite (MgAl₂(PO₄)₂(OH)₂), scorzalite (Fe²⁺,Mg)Al₂(PO₄)₂(OH)₂), and wavellite (Al₃(PO₄)₂(OH, F)₃ • 5H₂O).

The most well-known member of the vanadate class is vanadinite (Pb₅(VO₄)₃Cl). It serves as an ore for the element vanadium, and collectors look for large, well-formed crystals of this mineral. A less well-known vanadate is carnotite (K₂(UO₂)₂(VO₄)₂ • 3H₂O).

Erythrite (Co₃(AsO₄)₂ • 8H₂O) is a well-known arsenate and serves as an indicator for the exploration of cobalt ores.

Geology of Phosphates, Arsenates, and Vanadates

The genesis of these minerals is primarily exogenous, resulting in molecular water in their formulas. Some minerals are of magmatic, pegmatitic, and hydrothermal origin. 

Physical Properties of Phosphates, Arsenates, and Vanadates

Phosphates, arsenates, and vanadates have the following physical properties. Their hardness ranges from 1 to 6.5, and their density from 1.7 to 7.2 g/cm³. They have such a wide range of values because their chemical composition varies significantly. The hardness of phosphates, arsenates, and vanadates is generally low and middle range, which is lower than the average hardness of silicates.

Phosphates, arsenates, and vanadates crystallize in the lower and middle crystal systems. They usually form prismatic, needle-like, or lamellar crystals or granular, earthy, and lumpy masses or concretions. The minerals are transparent to opaque with a vitreous luster.

Introduction to Wolframates, Molybdates, and Chromates

Wolframate (also called tungstate) and molybdate classes are also usually studied together because of their similar crystal structure and mutual geological occurrence. Oxyanions are (WO₄)^2- for wolframates and (MoO₄)^2- for molybdates and occur in tetrahedral coordination (except for wolframite, which occurs with octahedral coordination). However, compared to the previous group, these oxyanion tetrahedrons are flattened because W⁶⁺ and Mo⁶⁺ ions are bigger than  P⁵⁺. The most common cations are Са, Fe, Cu, Pb, and Mn.

Combined, wulframates and molybdates include around 40 mineral species. Wulfenite (PbMoO₄), scheelite (CaWO₄), and wolframite ((Fe,Mn)WO₄) are the most significant. They are essential tungsten and molybdenum ores.

There are only 15 mineral species in the chromate class. Chromate's oxyanion is (CrO₄)^2- in a tetrahedral coordination. The most common cations are Pb, Сu, Zn, Ca, K, Na, and Ba. The most well-known chromate is crocoite (PbCrO₄). Although crocoite has no economic significance, mineral collectors value it very highly.

• Wolframates — (WO₄)^2- anion
• Molybdates— (MoO₄)^2- anion 
• Chromates— (CrO₄)^2- anion 

Geology of Wolframates, Molybdates, and Chromates

Wolframates and molybdates can form during similar processes and occur together in hydrothermal veins or, more rarely, in granitic pegmatites. Wulfenite occurs in the oxidation zone of hydrothermal Pb deposits. Scheelite occurs predominantly in skarn deposits because of the Ca needed for scheelite formation. Wolframite forms in quartz-rich pegmatite dykes and hydrothermal veins. Since wolframite is durable, it can also be found in placer deposits.

Chromates are found primarily in hypergenic (weathering) environments. Crocoite, in particular, is formed in the oxidation zone above hydrothermal Pb deposits hosted by chromite-bearing rocks.

Physical Properties of Wolframates, Molybdates, and Chromates

The most distinguishing feature of wolframates, molybdates, and chromates is their yellow, orange, red, and brown coloration. (Of course, there are exceptions). These minerals usually crystallize in the tetragonal and monoclinic crystal systems. Their hardness is usually less than 5.

Wolframates and molybdates usually form as short prismatic and tabular crystals and clusters. Chromates usually occur as more elongated needle-like crystals, forming radial sprays.

Introduction to Nitrates and Borates

Nitrate and borate minerals are rare, and only a specialized group of mineralogists usually studies them.

Nitrate structure is characterized by a planar triangular anion (NO₃)^- similar to the (CO₃)^2- of carbonates. However, the forces between N and O are stronger, so nitrates are harder to dissolve in acids. Cations are mainly represented by Na and K. There are only eight known nitrate minerals, all of them very rare.

Borate minerals also have a simple triangular oxyanion (BO₃)^3-. However, this oxyanion can polymerize, linking the (BO₃)^3- complexes into extended units such as double triangles, triple rings, sheets, and chains. Cations are mostly Mg, Na, and Ca.

• (BO₃)^3- triangular  anion
• (BO₄)^5- tetrahedral anion
• [B₃O₃(OH)₅]^2- complex anion, with one triangular and two tetrahedrons 
• [B₄O₅(OH)₄]^2- quadrangle group, two triangles and two tetrahedrons

There are over one hundred borate mineral species. We will cover borax (Na₂B₄O₅(OH)₄ • 8H₂O). Other less common minerals are kernite (Na₂B₄O₆(OH)₂ • 3H₂O), ulexite (NaCaB₅O₆(OH)₆ • 5H₂O), and colemanite (Ca₂B₆O₁₁ • 5H₂O).

Geology of Nitrates and Borates

Two of the most common minerals of the nitrate class are nitratine — nitratite or soda niter — (NaNO₃), and niter or saltpeter (KNO₃). These minerals are typically found in thin layers or coatings deposited by efflorescence in dry environments.

Borates are predominantly sedimentary in origin. They are deposited as evaporites from seawater in isolated basins with chlorides, carbonates, and sulfites.

Physical Characteristics of Nitrates and Borates

Nitrate minerals are light-colored, soft (1-2 on the Mohs scale), and water-soluble. Therefore, they can only be found in crystal forms in arid zones.

A common feature of borates is their light coloration — white, gray, and light yellow. Their luster varies from vitreous to dull and earthy. They can also display transparency but are more commonly translucent or opaque. The hardness of borate minerals depends on the presence of water molecules. Anhydrous borates have a hardness of 5-6, while aqueous borates are much softer (2-4) and easily soluble in water.

Most borates crystallize in lower crystal systems and occur in needle-like and fibrous aggregates and earthy masses.

Diagnostic Characteristics of Phosphates, Vanadates, Wolframates, Molybdates, Chromates, and Borates

Below, you'll find diagnostic characteristics for differentiating and identifying phosphates, vanadates, wolframates, molybdates, chromates, and borates. Included here are the most ubiquitous and economically significant members. We emphasize how to differentiate them from minerals with very similar appearances. The best diagnostic characteristics are highlighted in bold. 

Most minerals in these classes have low to moderate hardness, vitreous to dull luster, and generally light colors. They are usually translucent to opaque. Additionally, characteristics like water solubility and ultraviolet (UV) luminescence can help differentiate some species. 

Phosphates

Apatite

Formula: Ca₅(PO₄)₃(F,Cl,OH)

Apatite occurs in various colors and crystal forms, making it hard to identify at first glance. You may find apatite with yellow-green colors like peridot or saturated blues like sapphire. However, apatite is much softer than these and many other minerals it may resemble. Green apatite crystals are often confused with beryls but can be distinguished due to their hexagonal dipyramid terminations (ends). Beryls usually have flat terminations. Apatite also fluoresces different colors under UV light. Beryl and tourmaline usually do not.

Monazite

Formula: (Ce,La,Nd,Th)PO₄

Monazite commonly occurs in tiny brown crystals as accessory phases in rock. It is frequently confused with zircon. Zircon has more reddish hues and occurs in prismatic forms, while monazite tends to have yellowish brown colors and forms in more tabular, flattened forms.

Turquoise

Formula: CuAl₆(PO₄)₄(OH)₈ • 4H₂O

Rare turquoise crystals are usually tiny. This mineral occurs more commonly mixed with native rock matrix. Be aware that turquoise is widely known for its opaque green-blue appearance and waxy luster, but the literature often describes it as transparent with a vitreous luster. These are the characteristics of its rare crystalline form. Turquoise is sometimes mistaken for chrysocolla, but chrysocolla is much softer.

However, the challenge mineralogists and gemologists typically face with turquoise isn't distinguishing it from natural chrysocolla but from numerous other artificial imitations. Consult our turquoise treatments and synthetics guide for more information.

Vivianite

Formula: Fe²⁺₃(PO₄)₂ • 8H₂O

Vivianite is popular at mineral shows and among mineral collectors because of its unique, magnificently saturated teal-to-indigo color. Vivianite is easily identifiable because it occurs in prismatic, flattened and elongated crystals commonly in the form of stellate groups. Additionally, vivianite crystals are usually connected to orangey brown, earthy native rock. This association is a distinguishing feature of vivianite.

Vanadates

Vanadinite

Formula: Pb₅(VO₄)₃Cl

Vanadinite can be identified easily due to its red-orange, translucent, hexagonal, prismatic crystals that usually occur in small sizes. These can create appealing druzy formations and incrustations. Vanadinite can be mistaken for wulfenite because of its color. However, crystal morphology is a key distinguishing feature in this case. Vanadinite occurs in short, hexagonal prisms, while wulfenite occurs in flat, tabular squares.

Wolframates

Wolframite

Formula: (Fe,Mn)WO₄

Wolframite is the name for a solid solution series between two end-members: ferberite ((Fe)WO₄) and hübnerite ((Mn)WO₄). At first glance, these minerals are hardly distinguishable. A diagnostic feature of wolframite is its density. You can check it by holding it in your hand. It will feel heavier than it should. Wolframite also has perfect cleavage and vertical striation, which makes it look like a black mineral of the pyroxene group. However, wolframite's lower hardness distinguishes it from pyroxenes (5.5-6).

Scheelite

Formula: CaWO₄

Scheelite can be identified by its combination of yellow to orange color, pseudo-octahedral crystal form, and strong fluorescence under UV light. Scheelite crystals also have high heft. It can sometimes be mistaken for fluorite, but higher density and hardness make scheelite easily distinguishable from fluorite.

Yellow scheelite crystal with a white quartz crystal and small calcite crystals, under white light and UV light, Yaogangxian Mine, Yizhang Co., Chenzhou, Hunan, China, 4.9 x 3.3 x 2.0 cm. © Rob Lavinsky, www.iRocks.com. Used with permission.

Molybdates

Wulfenite

Formula: PbMoO₄

Wulfenite crystals have astonishing, vibrant orange colors. At first glance, you might mistake them for vanadinites. However, wulfenite can be distinguished due to its flat, tabular, square-shaped crystals. Vanadinite crystals have hexagonal forms.

Chromates

Crocoite

Formula: PbCrO₄

Among mineralogy students, crocoite is a favorite mineral for identification. Its vibrant color and acicular crystals make it easily identifiable during tests.

Borates

Borax

Formula: Na₂B₄O₅(OH)₄ • 8H₂O

Borax is commonly known as a white powder used for household cleaning. As a crystal, borax occurs in opaque, white, prismatic shapes with dull luster and dehydrates rapidly in air. As a white, opaque mineral with a dull luster, borax is easily identified. Borax's solubility in water is another helpful characteristic for mineral identification.

References for Phosphates and Other Rare Minerals

• Anovitz, L. M., & Grew, E. S. (Eds.). (2018). Boron: Mineralogy, Petrology, and Geochemistry. Reviews in Mineralogy and Geochemistry, (33). Walter de Gruyter GmbH & Co KG. 884 p.
• Anthony, J. W., Bideaux,R. A., Bladh, K. W., &  Nichols, M C. (2001). Handbook of Mineralogy, Mineralogical Society of America, Chantilly, VA 20151-1110, USA. http://www.handbookofmineralogy.org/.
• Deer, W. A., Howie, R. A., & Zussman, J. (2013). An introduction to the rock-forming minerals. Mineralogical Society of Great Britain and Ireland, 498 p. https://doi.org/10.1180/DHZ
• Kohn, M. J., Rakovan, J., & Hughes, J. M. (Eds.). (2002). Phosphates: geochemical, geobiological and materials importance. Reviews in Mineralogy and Geochemistry,  (48). Walter de Gruyter GmbH & Co KG. 742 p.
• Klein, C., & Dutrow, B. (2007). Manual of mineral science. John Wiley & Sons, 704 p.
• Okrusch, M. & Frimmel, H. E. (2020). Mineralogy: An introduction to minerals, rocks, and mineral deposits. Springer Nature, 719 p. https://doi.org/10.1007/978-3-662-57316-7
• Pasteris, J. D., Wopenka, B., & Valsami-Jones, E. (2008). Bone and tooth mineralization: Why apatite? Elements, 4(2), p. 97-104.
• Strunz, H., & Nickel, E. H .(2001). Strunz mineralogical tables. Schweizerbart, Stuttgart. 869 p.

Olena Rybnikova, PhD

Olena Rybnikova is a gemologist and mineralogist. She has a PhD in mineralogy and petrology specializing in beryllium minerals and is a certified Applied Jewelry Professional accredited by the Gemological Institute of America. Her passion is actively promoting knowledge and appreciation of nature, geology, and gemstones.

View All Articles