Mineralogy of Halides, Carbonates + Sulfates

Introduction to Halides, Carbonates, and Sulfites

Halides, carbonates, and sulfites (not "sulfides") are studied together because most of them form in similar environments and are chemically deposited from water. Being connected to halides and carbonates, sulfites will let you easily remember how to differentiate them from sulfides.

Halides, carbonates, and sulfites have many similar physical properties. Most of the minerals are light colored (colorless or white, gray, ivory, yellow, orange), transparent to translucent, with vitreous luster, low density, and low (2 for gypsum) to medium (4.5 for magnesite) hardness, making differentiation between mineral species quite challenging. Also, minerals occur in various crystal forms: form perfectly symmetrical, well-formed crystals to nodular, botryoidal, stalactitic, columnar, fibrous, granular, and massive aggregates, with a common crystal twinning.

Evaporite Minerals

Before learning each class separately, we want to pay attention to evaporites. These are salt rocks originally precipitated from a saturated surface or near-surface brine in hydrologies driven by solar evaporation (Warren, 2016). [The world salt used to mark the type of chemical compound, not only common table salt. Hydrologies are different water reservoirs like seas, lakes, rivers, and their isolated parts].

Many halides, carbonates, and sulfites are formed due to evaporation processes from water, creating expansive rock bodies with thousands of square kilometers of volume. Therefore, halides, sulfites, and carbonates are rock-forming minerals, as one mineral species can make up massive amounts of rocks. Also, these minerals are sedimentary as they were precipitated from water and not crystallized from magma, as we used to think concerning igneous (magmatic) minerals and rocks.

Around 80 mineral species can be found in evaporites, mainly represented by halides, sulfites, carbonates, and borates. However, there are only ten most common minerals of evaporites (halite, sylvite, carnalite, gypsum, anhydrite, langbeinite, polyhalite, kieserite, calcite, and dolomite). We will discuss most of them in greater detail later on.

Evaporites and Seawater Composition

To understand the chemical composition of evaporite minerals, it is helpful to see the composition of seawater first. 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 will get halite mineral with the formula NaCl. Isn't chemistry easy?

Mineral Precipitation Order

It is essential to know the simplified model of the order of mineral precipitation:

1. The first mineral to precipitate is calcite (CaCO₃). This happens when the initial water volume is twice reduced.
2. Gypsum (CaSO₄•2H₂O) and anhydrite (CaSO₄) are the following minerals to precipitate. This happens when one-fifth part of the initial water volume is left. Also, whether it will be gypsum or anhydrite depends on factors such as water temperature and salinity. Anhydrite is favorable in elevated temperature and salinity.
3. Halite (NaCl) is the third mineral to precipitate. It happens when around 1/10 of the initial water volume is left.
4. Sulfites and chlorides of Mg and K are the last minerals to precipitate.

The logical question that can arise here is why the first mineral to precipitate is not halite, as components for its formation are the most abundant. Water solubility is the main factor for it. Halite is highly soluble in water, so it takes time for all ions and water molecules to be in balance to precipitate and not dissolve.

Halides

Everybody has a representative of halide minerals at home. It is halite or table salt! You already know that mineral is easily soluble in water. It's a pity halite is soluble and soft and cannot be used in jewelry. Maybe some of you prefer corse table salt and can see cubes! Yes, this is a perfect cubic halite crystal! Even more, you can undoubtedly say table salt's main diagnostic feature… is its salty taste! Imagine that great time during the mineralogy test checking all minerals for halite :-). Let's find out more about the halide 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 high symmetry of the halide structures. You will see that 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 are important minerals for numerous industries. Halite is essential for the chemical industry for metallic Na, soda, chlorine gas, and hydrochloric acid production. Sylvite, another halide, is a starting product for high-class fertilizers. Fluorite is an essential raw material for metallurgy as a flux melting agent, glass manufacturing, and chemical industry.

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

Geology of Halides

As we already discussed, the bulk of halides form during evaporation due to precipitation from the water solution. Sometimes, halide and sylvite are sublimation products of active volcanoes. On the other hand, fluorite commonly occurs in hydrothermal veins deposited from hydrothermal solutions and as accessory phases 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. Chemically pure halides are colorless or allochromatic. All possible halide colors - blue, violet, pink, and green- result from foreign ions or mineral inclusions or due to various lattice defects.

In general, halides typically have low densities, low refractive indices, and a vitreous to dull when massive luster. 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, meaning a carbon atom is located at the center, and three oxygen atoms are in 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 the class.

Other mineral classes with oxyanions we will study later have 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, dolomite groups, and monoclinic carbonites with (OH)^- anion. The most critical carbonates we are studying in detail are calcite, magnesite, dolomite, siderite, rhodochrosite, azurite, and malachite.

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

During the presence of H⁺ ions, (CO₃)^2- ion became very unstable and decomposed into water and carbon dioxide CO₂ according to the following reaction:

2H⁺ + (CO₃)^2- → H₂O + CO₂

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

CaCO₃ + 2HCl = CaCl₂ + H₂O + CO₂

Many carbonates are rock-forming minerals. Limestone, for example, predominantly or exclusively consists of calcite. Limestone finds many practical applications in the building industry (non-hydraulic and hydraulic binders, lime mortar, Portland cement, and building and dimension stone), chemical (glass and cellulose production, as flux in metallurgy), and optical industries. Siderite FeCO₃, as iron-bearing carbonate, is a source of iron. These ores are not as rich in iron as iron oxides but still can be actively mined for iron.

Other less common carbonates like smithsonite ZnCO₃, strontianite SrCO₃, cerussite PbCO₃, witherite BaCO₃, and ankerite CaFe(CO₃)₂ are 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 crystallizing from carbonatitic magma) and can even be of biogenic origin. This means that 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 FeCO₃ 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 ores deposits. Malachite and azurite commonly occur in close association as formed by the oxidation of primary copper ores in oxidation zones.

Physical Properties of Carbonates

As we previously discussed, 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 to calcite, dolomite is hardly attacked by cold, diluted acids, such as HCl, but dissolves easily in hot acids with strong effervescence.

Because of crystal structure, carbonates usually have distinct cleavage in several directions. For example, calcite has perfect cleavage in three directions parallel to the rhombohedron form.

Another outstanding optical feature of carbonites is high birefringence (double refraction) that is visible with an unaided eye on Iceland spar variety of calcite.

Sulfites

Sulfite minerals anion 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 oxygens. Like in carbonates, the bonds between the anionic complex and the cations are predominantly heteropolar and, simply speaking, weaker.

It is important not to mix sulfides and sulfites. Both classes have sulfur in their chemical formula, but for sulfides, it is S^2-; and for sulfites, it is (SO₄)²⁻ anionic complex.

The most important and most common sulfite mineral is gypsum CaSO₄*2H₂O. It has been known for thousands of years and has had a crucial role as raw material in construction.

Other less common sulfites are anhydrite CaSO₄, baryte (sometimes spelled barite) BaSO₄, celestite SrSO₄, and anglesite PbSO₄.

Geology of Sulfites

Gypsum and anhydrite are minerals that formed due to precipitation from saltwater. Anhydrite, in this case, 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 salt lakes and salt pans' margins, often forming concentrations in muddy or marly sediments. Desert rose is a common variety of gypsum that can be formed during continental precipitations. Desert rose gypsum is rosette-like intergrowths of gypsum crystals rich in sand inclusions.

One of the most outstanding gypsum occurrences, or even natural crystal occurrences, is the fascinating gypsum cave '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 baryte, anglesite, and celestite crystallize in hydrothermal veins, in volcanogenic-sedimentary deposits, and even from the white and black smokers, hydrothermal vents on the seafloor near mid-ocean ridges. Celestite also occurs due to metasomatic exchange and anglesite as alteration products of sulfidic deposits.

Here is a small example of lead-bearing (Pb²⁺) minerals of different mineral classes we have already covered. 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 mineral classes can change depending on the geological conditions.

Physical Properties of Sulfites

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

The perfect cleavage of sulfites is a result of their crystal structure. For example, the 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 are directly linked to the planes of weak atomic bonds between sheets of stronger bonded atoms.

Diagnostic Characteristics of Halides, Carbonates, and Sulfites

Here, we would like to provide diagnostic characteristics to differentiate and identify halide, carbonate, and sulfite minerals. We are giving the most ubiquitous and economically significant ones like halite, sylvite, fluorite, calcite, magnesite, dolomite, siderite, rhodochrosite, azurite, malachite, gypsum, and celestite, emphasizing how to differentiate them from the most similarly looking minerals. The best diagnostic characteristics are highlighted in bold.

Some properties are more helpful for halides identification than others, so we are adding reactions to UV, solubility in water, taste, crystal system, birefringence, and reaction to acids. On the other hand, some properties we used as characteristic features for the differentiation of oxides and sulfides, like streak luster and diaphaneity, are no longer helpful with carbonates, halides, and sulfides.

Halide Minerals

Halite

Formula: NaCl

One of the most atypical confusion for halite we usually pay little attention to is the mineral's color we used to see in the kitchen. Halite can be vivid blue, orange, and purple. A diagnostic feature of halite is its water solubility. Geologists usually taste halite to differentiate it from sylvite. Sylvite's taste is noticeably bitter.

Sylvite

Formula: KCl

Sylvite also varies in color. Most commonly, it occurs in orange or yellowish-red color. The same as halite, sylvite is soft and soluble in water. Geologists differentiate sylvite from halite due to its bitter taste.

Fluorite

Formula: CaF₂

Fluorite occurs in a rainbow of colors. Fluorite is commonly zoned, which means colors alternate within one crystal. Color combinations are commonly green with purple, purple with white, and green with blue. Fluorite is easily recognizable due to significant color zoning and well-crystallized crystals in cubes and octahedrons. Also, fluorite glows in various colors under ultraviolet light (UV), with some specimens having phosphorescence, thermoluminescence, and triboluminescence.

Carbonates

Calcite

Formula: CaCO₃

Calcite occurs in various colors and forms, so its identification can hardly be based only on visual appearance. Calcite's most characteristic features are three directions of perfect cleavage, visible double refraction, typical twinning, and reaction to cold and diluted hydrochloric acid (HCl).

Magnesite

Formula: MgCO₃

Magnesite is less common in well-formed crystals, mainly occurring as aggregates. It can be differentiated from calcite as there is no reaction with cold hydrochloricacid. 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 is hardly attacked by 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 and grayish color, siderite occurs in orangy and yellowish hues. Also, the specific gravity of siderite is higher than that of calcite, dolomite, and magnesite.

Rhodochrosite

Formula: MnCO₃

Luckily for us, rhodochrosite has very distinct pink, rose-red, and reddish colors that help to differentiate it from other carbonates. Its typical banded structure is also helpful. However, rhodochrosite can be mistaken for other red minerals, especially rhodonite. Here, the low hardness of carbonate will be helpful. The bulk of other red minerals is much harder than carbonate rhodochrosite.

Azurite

Formula: Cu₃(CO₃)₂(OH)₂

Compared to similarly-colored lazurite, azurite commonly occurs in association with malachite, while silicate lazurite is commonly found with pyrite and calcite. Also, azurite (carbonate) is softer than lazurite (silicate).

Malachite

Formula: Cu₂CO₃(OH)₂

Malachite is also 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 other green silicates have white.

Sulfites

Gypsum

Formula: CaSO₄•2H₂O

Gypsum's distinguishing features are low hardness, crystal morphology (tabular crystals), rosettes and fibrous aggregates, and the common presence of twinning (V-shape twins). Gypsum is most commonly confused with calcite at first sight. However, remember that calcite crystals have visible doubling, but gypsum will not. An additional test to separate gypsum from calcite is the acid test. 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

1. 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/.

1. 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.

1. 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

1. 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.

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

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

1. 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

1. 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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