Mineralogy of Halides, Carbonates + Sulfates

Introduction to Halides, Carbonates, and Sulfites

Mineralogists study halides, carbonates, and sulfites (not "sulfides") together because most of the minerals in these classes form in similar environments and are chemically deposited from water. Sulfites' connection to halides and carbonates will help you remember how to differentiate them from sulfides.

Halides, carbonates, and sulfites have many similar physical properties. Most of these minerals are light colored (colorless or white, gray, ivory, yellow, orange), transparent to translucent, with vitreous luster, low density (specific gravity), and low (2 for gypsum) to medium (4.5 for magnesite) hardness. This makes differentiating mineral species within these classes quite challenging. These minerals also occur in various crystal forms, from perfectly symmetrical, well-formed crystals to nodular, botryoidal, stalactitic, columnar, fibrous, granular, and massive aggregates. Crystal twinning is common.

Evaporite Minerals

Evaporites are salt rocks originally precipitated from a saturated surface or near-surface brine in hydrologies driven by solar evaporation. (Warren, 2016) "Salt" refers to the chemical compound, not just common table salt. Hydrologies are different water reservoirs like seas, lakes, rivers, and their separate components.

Many halides, carbonates, and sulfites form due to water evaporation processes, which creates expansive rock bodies with volumes of thousands of square kilometers. Therefore, halides, sulfites, and carbonates are rock-forming minerals, as one mineral species can make up massive amounts of rocks. These minerals are also sedimentary since they precipitated from water.

Around 80 mineral species can be found in evaporites. These consist mainly of halides, sulfites, carbonates, and borates. However, the ten most common evaporite minerals are halite, sylvite, carnallite, gypsum, anhydrite, langbeinite, polyhalite, kieserite, calcite, and dolomite. We will discuss most of these later.

Evaporites and Seawater Composition

Understanding the composition of seawater will help you understand the composition of evaporite minerals. The most common cations in seawater are (in order of decreasing amount):

• Sodium (Na⁺),
• Magnesium (Mg²⁺),
• Calcium (Ca²⁺), and
• Potassium (K⁺).

While the most common anions are:

• Chloride ion (Cl^-),
• Sulfate ion (SO₄^2-),
• Bicarbonate ion (HCO₃^-)

So, by combining the most common cation (Na⁺) with the most common anion (Cl^-), we get the mineral halite with the formula NaCl.

Mineral Precipitation Order

You must know this simplified mineral precipitation order:

• The first mineral to precipitate is calcite (CaCO₃). This happens when the initial water volume is twice reduced.
• Gypsum (CaSO₄•2H₂O) and anhydrite (CaSO₄) are the next minerals to precipitate. This happens when one-fifth of the initial water volume remains. Whether gypsum or anhydrite forms depends on factors such as water temperature and salinity. Anhydrite is likely in elevated temperature and salinity conditions.
• Halite (NaCl) is the third mineral to precipitate. This happens when around one-tenth of the initial water volume remains.
• Sulfites and chlorides of Mg and K are the last minerals to precipitate.

You might ask why halite isn't the first mineral to precipitate, since the components for its formation are the most abundant. Water solubility plays a critical role here. Since halite is highly soluble in water, it takes time for all ions and water molecules to balance out so halite can precipitate and not dissolve.

Halides

You most likely have a halide representative at home: halite or table salt. You already know that this mineral is easily soluble in water. It's a pity halite is soluble and soft and cannot be used in jewelry. (Usually). If you have coarse table salt, you can see that halite forms in the cubic or isometric crystal system. However, with any type of table salt you can experience its principal diagnostic feature: a salty taste.

Let's learn more about the halide mineral class.

The minerals of this class are compounds of the large halogen anions Cl⁻, F⁻, Br⁻, and J⁻ and pretty large low-valence cations Na⁺, K⁺, Ca²⁺, and Mg²⁺. The bond character is predominantly ionic. When anions combine with relatively large, weakly polarized, low-valence cations, both ions acquire an almost perfect spherical shape. The arrangement of spherical ions leads to the high symmetry of halide structures. The most common halides (halite, sylvite, and fluorite) have cubic crystal systems and occur in the form of cubes (hexahedrons) and octahedrons. Sometimes, a water molecule (nH₂O) is present in the mineral formula.

Examples of Halides

Halides have numerous important industrial applications. In the chemical industry, halite is essential for metallic Na, soda, chlorine gas, and hydrochloric acid production. Sylvite, another halide, acts as a starting product for premium fertilizers. In metallurgy, fluorite is essential for glass manufacturing and as a flux melting agent.

Some less common halide minerals include carnallite (KMgCl₃ · 6H₂O), bischofite (MgCl₂ · 6H₂O), cryolite (Na₃AlF₆), and chlorargyrite (AgCl).

Geology of Halides

Most halides form during evaporation due to precipitation from a water solution. Sometimes, halide and sylvite occur as sublimation products of active volcanoes. On the other hand, fluorite commonly occurs in hydrothermal veins deposited from hydrothermal solutions and as an accessory phase in many other igneous and metamorphic rocks.

Physical Properties of Halides

Because of ion configurations, halide crystal structure is highly symmetrical, resulting in cubic crystal symmetry and isometric crystals. Halide is allochromatic, so chemically pure halides are colorless. All other possible halide colors — blue, violet, pink, and green — result from foreign ions, mineral inclusion, or various lattice defects.

In general, halides have low densities, low refractive indices, and a vitreous to dull luster when massive. Some of them, like halite and sylvite, are soluble in water.

Carbonates

Carbonates are minerals composed of carbon oxyanions (CO₃)^2- with cations like Ca, Mg, Fe, Mn, Zn, Ba, Sr, Pb, and Cu. A carbonate ion (CO₃)^2- is triangular in coordination. This means a carbon atom is located at the center, and three oxygen atoms are at the corners of an equilateral triangle. Bonding between C and O atoms in a carbonate ion is much stronger than between cation and oxyanion. The triangular carbonate anions are the basic building units of all carbonate minerals and are primarily responsible for the properties particular to this class.

Other mineral classes we will study later also have oxyanions and very similar crystal structures, like sulfates (SO₄)^2-, nitrates (NO₃)^-, phosphates (PO₄)^3-, chromates (CrO₄)^2-, wolframates (WO₄)^2-, and arsenates (AsO₄)^3-.

Carbonate Subdivisions

The carbonate class is subdivided into calcite, aragonite, and dolomite groups, and monoclinic carbonates with (OH)^- anions. We will study the most economically significant carbonates: calcite, magnesite, dolomite, siderite, rhodochrosite, azurite, and malachite.

Calcite's structure can be described as a deformed halite lattice, set up at a corner and compressed along the diagonal space, where Na is replaced by Ca and Cl by the center of the (CO₃)^2- group.

When H⁺ ions are present, the (CO₃)^2- ion becomes very unstable and decomposes into water and carbon dioxide (CO₂) according to the following reaction:

2H⁺ + (CO₃)^2- → H₂O + CO₂

This reaction causes the well-known effervescence of carbonates in contact with acids:

CaCO₃ + 2HCl = CaCl₂ + H₂O + CO₂

Many carbonates are rock-forming minerals. Limestone, for example, consists predominantly or exclusively of calcite. Limestone has many practical applications in building, chemical, and optical industries. Siderite (FeCO₃) is an iron-bearing carbonate. Although not as iron-rich as iron oxides, siderite ores can still be mined for iron.

Other less common carbonates like smithsonite (ZnCO₃), strontianite (SrCO₃), cerussite (PbCO₃), witherite (BaCO₃), and ankerite (CaFe(CO₃)₂) are also occasionally used as sources of metals.

Geology of Carbonates

Calcite is one of the most common minerals in the Earth's crust. It is predominantly of sedimentary origin but occurs in many other geological environments (hydrothermal, metamorphic, and even primary magnetic crystallization from carbonatitic magma) and can even have biogenic origins. Calcite forms solid parts of organisms such as foraminifera, corals, echinoids, crinoids, brachiopods, etc. (Morse & Mackenzie, 1990)

Aragonite, a high-pressure polymorph of CaCO₃, is less common than calcite.

Siderite occurs in hydrothermal veins and is formed by the metasomatic replacement of limestone or marble by reaction with Fe-rich hydrothermal fluids.

Rhodochrosite, azurite, and malachite occur in hydrothermal veins or as weathering products in the oxidation zone of ore deposits. Malachite and azurite commonly occur in close association, formed by the oxidation of primary copper ores in oxidation zones.

Physical Properties of Carbonates

One of the most characteristic properties of carbonates is their active reaction with acids. Calcite dissolves easily with strong effervescence in cold dilute acids, such as HCl. In contrast, dolomite is hardly affected by cold, diluted acids, such as HCl, but dissolves easily in hot acids with strong effervescence.

Carbonates usually have distinct cleavage in several directions due to their crystal structure. For example, calcite has perfect cleavage in three directions in rhombohedral form.

Another outstanding optical feature of carbonates is high birefringence (double refraction). In Iceland spar, a variety of calcite, this is noticeable with the naked eye.

Sulfites

The anion of sulfite minerals is represented by the (SO₄)²⁻ anionic complex. It forms a slightly distorted tetrahedron in which very strong covalent bonds connect a central sulfur atom with four oxygen atoms. As in carbonates, the bonds between the anionic complex and the cations are predominantly heteropolar and, simply speaking, weaker.

Don't confuse sulfides and sulfites. Both classes have sulfur in their chemical formula, but sulfides have S^2-, and sulfites have the (SO₄)²⁻ anionic complex.

The most important and most common sulfite mineral is gypsum (CaSO₄ · 2H₂O). Known and used for millennia, gypsum plays a crucial role as raw material for construction.

Some less common sulfites include anhydrite (CaSO₄), barite or baryte (BaSO₄), celestite (SrSO₄), and anglesite (PbSO₄).

Geology of Sulfites

Gypsum and anhydrite form due to precipitation from saltwater. In this case, anhydrite is a product of gypsum dehydration. The volume of minerals precipitated can form giant rock bodies stretching for dozens of square kilometers.

Continental gypsum precipitation occurs at the margins of salt lakes and salt pans, often forming concentrations in muddy or marly sediments. "Desert rose" is a common variety of gypsum that may form during continental precipitations. "Desert rose" is a rosette-like formation of intergrown gypsum crystals rich with sand inclusions.

One of the most outstanding gypsum occurrences — or any natural crystal occurrences for that matter — is the fascinating Cueva de los Cristales in the Naica Mine at Santo Domingo, Chihuahua, Mexico. Single gypsum crystals reach up to 14 m in length and 1 m in thickness. (García-Ruiz et al., 2007)

Some sulfite minerals like barite, anglesite, and celestite crystallize in hydrothermal veins, volcanogenic-sedimentary deposits, and even from black and white smokers — hydrothermal vents on the seafloor near mid-ocean ridges. Celestite also occurs due to metasomatic exchange. Anglesite can also occur as an alteration product of sulfidic deposits.

Below is a comparison of lead-bearing (Pb²⁺) minerals from different mineral classes. You know that galena forms in hydrothermal ore deposits, while anglesite and cerussite occur in the oxidation zone of lead deposits, commonly formed by weathering of galena, PbS. Based on this simple example, you can see how geological conditions can affect mineral classes.

Physical Properties of Sulfites

Sulfites are generally soft minerals with a maximum hardness of 3.5. They are light-colored and transparent to translucent, depending on their formation and inclusions. They have vitreous luster and good to perfect cleavage in several directions.

The perfect cleavage of sulfites is a result of their crystal structure. For example, water-bearing (H₂O) gypsum has layers of (SO₄)²⁻ strongly bonded to Ca²⁺. These sheets are separated by layers of H₂O molecules linked by weak van der Waals bonds only. Cleavage planes correspond directly to the planes of weak atomic bonds between sheets of stronger bonded atoms.

Diagnostic Characteristics of Halides, Carbonates, and Sulfites

Below, you'll find diagnostic characteristics for differentiating and identifying halide, carbonate, and sulfite minerals. 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.

For halide identifications, we have added reactions to ultraviolet (UV) light, solubility in water, taste, crystal system, birefringence, and reactions to acids to their diagnostic properties.

Halide Minerals

Halite

Formula: NaCl

Halite can show vivid blue, orange, and purple colors, not just the well-known whiteness of salt. A diagnostic feature of halite is its water solubility. Geologists usually taste halite to differentiate it from sylvite. Sylvite has a noticeably bitter taste. Note: please exercise caution when "tasting" mineral specimens, especially anything found in the field. Specimens may contain other materials potentially hazardous to your health.

Sylvite

Formula: KCl

Like halite, sylvite also varies in color. Most commonly, it occurs in orange or yellowish red colors. Sylvite is soft and soluble in water. Geologists differentiate sylvite from halite by its bitter taste.

Fluorite

Formula: CaF₂

Fluorite occurs in a rainbow of colors and frequently shows zoning. This means its colors may alternate within one crystal. Common color combinations include green with purple, purple with white, and green with blue. Fluorite's significant color zoning and well-crystallized cubic and octahedral crystals make it readily recognizable. Fluorite also glows in various colors under UV light with some specimens displaying phosphorescence, thermoluminescence, and triboluminescence.

Carbonate Minerals

Calcite

Formula: CaCO₃

Calcite occurs in various colors and forms, which means you can't identify it based solely on its visible properties. Calcite's most characteristic features are three directions of perfect cleavage, visible double refraction, twinning, and its reaction to cold and diluted hydrochloric acid (HCl).

Magnesite

Formula: MgCO₃

Magnesite rarely occurs as well-formed crystals. It mainly forms as an aggregate. You can distinguish it from calcite because it has no reaction to cold hydrochloric acid. Magnesite commonly occurs in association with gray or almost black-colored minerals that can help with identification.

Dolomite

Formula: CaMg(CO₃)₂

In contrast to calcite, dolomite resists cold, diluted acids (such as HCl) but dissolves easily in hot acids with strong effervescence.

Siderite

Formula: FeCO₃

Unlike other carbonates that usually have white or grayish colors, siderite occurs in orangey and yellowish hues. Siderite also has a higher specific gravity than calcite, dolomite, and magnesite.

Rhodochrosite

Formula: MnCO₃

Rhodochrosite has very distinct pink, rose-red, and reddish colors that help distinguish it from other carbonates. Its typical banded structure also helps with separations. However, rhodochrosite can be mistaken for other red minerals, especially rhodonite. However, the carbonate mineral has a low hardness. Most other red minerals are much harder than rhodochrosite.

Azurite

Formula: Cu₃(CO₃)₂(OH)₂

Azurite commonly occurs in association with malachite, while similarly colored lazurite commonly occurs with with pyrite and calcite. Azurite (a carbonate) is softer than lazurite (a silicate).

Malachite

Formula: Cu₂CO₃(OH)₂

Malachite is easily identifiable due to its appearance: green color and botryoidal and concentric textures. Check for streak color to differentiate malachite from numerous other green minerals, especially silicates. Malachite has a green streak, while green silicates have white streaks.

Sulfite Minerals

Gypsum

Formula: CaSO₄•2H₂O

The distinguishing features of gypsum are low hardness, crystal morphology (tabular crystals), rosette and fibrous aggregate forms, and frequent twinning (V-shape twins). At first sight, gypsum is most often confused with calcite. However, calcite crystals will show visible doubling, but gypsum will not. Additionally, you can use an acid test to separate gypsum from calcite. Gypsum is inert to hydrochloric acid.

Celestite

Formula: SrSO₄

The most common celestite on the market is pale blue, making it easily distinguishable from other sulfites and carbonates.

References for Halides, Carbonates, and Sulfites

• 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/.

• Blount, C. W., & Dickson, F. W. (1973). Gypsum-anhydrite equilibria in systems CaSO₄-H₂O and CaCO₄-NaCl-H₂O. American Mineralogist: Journal of Earth and Planetary Materials, 58(3-4), 323-331.

• 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

• García-Ruiz, J. M., Villasuso, R., Ayora, C., Canals, A., & Otálora, F. (2007). Formation of natural gypsum megacrystals in Naica, Mexico. Geology, 35(4), 327-330.

• Klein, C., & Dutrow, B. (2007). Manual of mineral science. John Wiley & Sons, 704 p.

• Morse, J. W., & Mackenzie, F. T. (1990). Geochemistry of sedimentary carbonates. In: Developments in sedimentology, 48. Elsevier, Oxford, New York, 707 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

• Warren, J. K. (2016). Evaporites: A geological compendium. Springer. 1813 p. https://doi.org/10.1007/978-3-319-13512-0

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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