Mineralogy of Phosphates and Other Rare Minerals

In previous articles, we spoke about only a few mineral classes because each class had a lot of essential and common minerals. But there are mineral classes that are essential for mineralogists to understand. These are: phosphates, arsenates, vanadates, wolframates, molybdates, chromates, nitrates, and borates. These mineral classes contain only a few or, in some cases, only one mineral of note. In particular, nitrates and borates have seldom interested gemologists. As a result, a good understanding of these minerals is essential for those in the mineralogy field.

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 each other easily, sometimes forming solid solution series, like, for example, 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 belonging to these three mineral classes are rare, except apatite, a common mineral. We will also discuss monazite, turquoise, and vivianite - other significant phosphates. Monazite is an essential mineral for geochronology because it has radioactive thorium in its composition. Turquoise is a famous gemstone. Vivianite is highly appreciated 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 vanadate class is represented by vanadinite, Pb₅(VO₄)₃Cl, as one of the vanadium ores and is a popular mineral for mineral collectors. Another less common vanadate is carnotite, K₂(UO₂)₂(VO₄)₂ • 3H₂O.

Erythrite, Co₃(AsO₄)₂ • 8H₂O, is a recognized representative of arsenates and an indicator for exploring cobalt ores.

Geology of Phosphates, Arsenates, and Vanadates

The genesis of minerals is primarily exogenous, resulting in molecular water in formulas and specific physical properties. Some minerals are of magmatic, pegmatite, and hydrothermal origin. 

Physical Properties of Phosphates, Arsenates, and Vanadates

The physical properties of these three mineral classes representatives have the following mutual features. The hardness varies from 1 to 6.5, and density from 1.7 to 7.2 g/cm³, which is explained by significant variations in chemical compositions. The hardness of phosphates, arsenates, and vanadates is generally low and middle, which is undoubtedly lower than the average hardness of silicates.

Phosphates, arsenates, and vanadates crystallize in the lower and middle crystal systems. The occurrence form is usually prismatic, needle-like, lamellar crystals, concretions, granular, earthy, and lumpy masses. 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 crystal structure similarity and mutual geological occurrence. Oxyanions are represented by (WO₄)^2- for wolframates and (MoO₄)^2- for molybdates and are also in tetrahedral coordination (with an exception for wolframite with octahedral coordination). However, compared to the previous group, oxyanions tetrahedrons are flattened because W⁶⁺ and Mo⁶⁺ ions are bigger than  P⁵⁺. The most common cations are Са, Fe, Cu, Pb, and Mn.

Two groups together form a small number of mineral species (around 40), with wulfenite, PbMoO₄, scheelite, CaWO₄, and wolframite, (Fe,Mn)WO₄, dominating species, which are essential tungsten and molybdenum ores.

Chromates is also a small mineral class with 15 mineral representatives. Chromate oxyanion is (CrO₄)^2- in a tetrahedral coordination. The most common cations are Pb, Сu, Zn, Ca, K, Na, and Ba. The leading representative of the chromates class is crocoite PbCrO₄, which has no economic significance but is highly valued among mineral connoisseurs.

• 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 necessity of Ca needed for scheelite formation. Wolframite forms in quartz-rich pegmatite dykes and hydrothermal veins. It is durable, so it can also be found in placer deposits.

Chromates are primarily found in hypergenic 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 distinguishing property of wolframates, molybdates, and chromates is their yellow, orange, red, and brown colors (of course, with the exceptions). Minerals predominantly crystallize tetragonal and monoclinic crystal systems. Also, the hardness is commonly less than 5.

It is typical short-prismatic and tabular crystal morphology with common clusters of minerals for wolframates and molybdates. Chromates occur in more elongated needle-like crystal forms, forming radial sprays.

Introduction to Nitrates and Borates

Nitrates and borates minerals are rare. These two groups are specific; only a narrow group of mineralogists are working with them. However, these two classes are indicated in the Nickel-Strunz classification, and we will shortly describe them.

The nitrates' structure is characterized by a planar triangular anion (NO₃)^- similar to (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 minerals of nitrates, and all of them are very rare minerals.

Borate minerals also have 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 more than one hundred borate mineral species. In detail, 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 nitrates class are nitratine (nitratite, or soda niter), NaNO₃ , and niter (saltpeter), KNO₃. Minerals are typically found in thin layers or coatings deposited by efflorescence in dry environments.

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

Physical Characteristics of Nitrates and Borates

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

A common feature of borates is their light colors - white, gray, and light yellow. Luster varies from vitreous to dull and earthy, and transparency from transparent to primarily translucent and opaque. The hardness of borates depends on the presence of water molecules. Anhydrous borates' hardness is 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

Here, we would like to provide diagnostic characteristics to differentiate and identify phosphate, vanadates, wolframates, molybdates, chromates, and borates minerals. We are giving the most ubiquitous and economically significant ones like apatite, monazite, turquoise, vivianite, vanadinite, wolframite, scheelite, wulfenite, crocoite, and borax, emphasizing how to differentiate them from the most similarly looking minerals. The best diagnostic characteristics are highlighted in bold. 

Most minerals of these classes are of low to moderate hardness, with vitreous to dull luster, commonly translucent to opaque, but generally of lighter colors. Some additional characteristics like water solubility glow under UV can help to differentiate some mineral 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 in the peridot-like yellow-green vivid color and sapphire-blue saturated hues. However, apatite, even if it looks like many other minerals, is much softer than them. Green apatite is usually confused with beryl but can be identified due to the hexagonal dipyramid crystal termination, while beryl has flat termination commonly. Also, apatite glows under UV light with different colors, while minerals of beryl and tourmaline group are commonly 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 almost isometric habit, while monazite tends to have yellowish-brown shades and more tabular flattened crystal forms.

Turquoise

Formula: CuAl₆(PO₄)₄(OH)₈ • 4H₂O

Turquoise crystals are typically tiny and often mixed with a native rock matrix. Therefore, it is confusing as turquoise is usually known for its opaque green-blue appearance with a waxy luster, whereas its properties are often described in the literature as transparent with a vitreous luster. It is sometimes mistaken for chrysocolla but can be easily distinguished as chrysocolla is much softer.

The challenge mineralogists and gemologists 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 usually omitted in mineral encyclopedias and classical mineralogy books. However, it is popular on mineral shows and among mineral collectors because of its unique, magnificently saturated teal-to-indigo color. Vivianite is easily identifiable due to prismatic flattened and elongated crystals commonly in the form of stellate groups. Additionally, vivianite crystals are usually connected to the orangey brown earthy native rock, making this association a distinguishable feature for vivianite.

Vanadates

Vanadinite

Formula: Pb₅(VO₄)₃Cl

Vanadinite can be quite easily identified due to red-orange translucent hexagonal prismatic crystals that usually occur in small sizes, creating appealing druses and incrustations. Vanadinite can be mistaken for wulfenite because of its color. However, crystal morphology is a key here, as vanadinite occurs in short hexagonal prisms, while wulfenite is in flat tabular crystals of square form.

Wolframates

Wolframite

Formula: (Fe,Mn)WO₄

Wolframite is a name for solid solution between two end-members ferberite, (Fe)WO₄, and hübnerite, (Mn)WO₄. These minerals are hardly identifiable at first glance. A diagnostic feature of wolframite is its density. You can check it by holding it in your hand. Mineral feels harder than it is supposed to be. Wolframite has perfect cleavage and vertical striation, making it look like black minerals of the pyroxene group. The wolframite's lower hardness distinguishes it from pyroxenes (5.5-6).

Scheelite

Formula: CaWO₄

Scheelite can be identified by the combination of yellow to orange color, pseudo-octahedral crystal form, and strong fluorescence under UV light. Additionally, scheelite crystals have high heft. Scheelite can be mistaken for fluorite mostly, but higher density and hardness will 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 are astonishing because of their vibrant orange color. At first glance, it can be mistaken for vanadinite. With careful observation of crystal morphology, wulfenite can be identified due to flat tabular crystals of square form, while vanadinite crystals have a hexagonal form.

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 in the form of white powder used for household cleaning. In the form of crystals, borax occurs in opaque white prismatic crystals with dull luster as it rapidly dehydrates in air. Borax is the first white opaque mineral with a dull luster, so it is easily identified. Borax's solubility in water is another helpful characteristic for mineral identification.

References for Phosphates and Other Rare Minerals

1. 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.
2. 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/.
3. 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
4. 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.
5. Klein, C., & Dutrow, B. (2007). Manual of mineral science. John Wiley & Sons, 704 p.
6. 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
7. Pasteris, J. D., Wopenka, B., & Valsami-Jones, E. (2008). Bone and tooth mineralization: Why apatite? Elements, 4(2), p. 97-104.
8. 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.

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