14.4: Halide Minerals

Halides
halite NaCl
sylvite KCl
chlorargyrite AgCl
atacamite Cu₂Cl(OH)₃
fluorite CaF₂
cryolite Na₃AlF₆

Fluorine and other halogens generally form nearly pure ionic bonds of moderate strength. Consequently, they bond with alkali and alkali earth elements to make chlorides, fluorides, bromides, and other salts referred to collectively as halides. Due to the generally large cation size and the nature of the bonding, halide structures tend to be simple with high symmetry. At high temperatures, some halides exhibit mutual solubility, but under normal Earth-surface conditions most are usually close to end-member composition. Halite and fluorite are the most common halide minerals, but others may be locally abundant.

For more general information about halides, see Section 7.4.5 in Chapter 7.

Halite NaCl

Origin of Name
From the Greek word halos, meaning “salt.”

Hand Specimen Identification
Its softness (H = 2.5), transparency or white color, salty taste, and cubic cleavage identify halite. Classic specimens are clear transparent white cubes, like those seen below in Figure 14.300. Commonly, however, the cubes are translucent (instead of transparent) and develop in clusters (Figure 14.301). Halite may be deliquescent (absorb water and turn into a liquid under extreme humidity.

Halite comes in just about any color; Figures 14.302 and 14.303 show pink and green varieties. The pink halite crystals in Figure 14.322 are hopper crystals, which means that their edges grew faster than the centers of faces.

Halite is similar to sylvite, but is quite easily distinguished by its less bitter taste. It can also be confused with other soft clear or white minerals such as cryolite.

Figure 14.300: Cubic halite crystals, 1-2 cm across

Figure 14.301: Cluster of halite crystals from Saskatchewan; the specimen is 16.8 cm tall

Figure 14.302: Pink halite hopper crystals on nahcolite, from Searles Lake, California; FOV is 9 cm across

Figure 14.303: Green halite, From Silesia, Poland, 10.4 cm tall

Physical Properties

hardness	2.5
specific gravity	2.16
cleavage/fracture	perfect cubic {100}/conchoidal
luster/transparency	vitreous/transparent to translucent
color	colorless and transparent but, if impure, may be shades of red, blue, purple, or other colors
streak	white

Properties in Thin Section
Halite has low relief, perfect cubic cleavage, and is colorless in thin section. Isotropic, n = 1.5446.

Crystallography
Halite is a cubic mineral, a = 5.6404, Z = 4; space group \(F\dfrac{4}{m}\overline{3}\dfrac{2}{m}\); point group \(\dfrac{4}{m}\overline{3}\dfrac{2}{m}\).

Habit
Cubic crystals, sometimes granular, sometimes massive, and sometimes in clusters, characterize halite. Cubic cleavage is pronounced.

Structure and Composition
Closest packed Na⁺ and Cl^– alternate in a face-centered cubic arrangement. Both ions are in perfect octahedral coordination. Sylvite (KCl), galena (PbS), periclase (MgO) and several other minerals are isostructural with halite.

Figure 14.304: Deposits of halite on the shores of the Dead Sea, Israel

Occurrence and Associations
Halite, a rock-forming mineral, occurs in salt flats, in sedimentary beds, in salt domes, and as deposits from volcanic gasses. Figure 14.304 shows halite deposited along the shores of the Dead Sea. Halite is, by far, the most common evaporite mineral. Associated minerals include many other salts, gypsum, calcite, sylvite, anhydrite, sulfur, and clay.

Related Minerals
Galena, PbS; alabandite, MnS; periclase, MgO; sylvite, KCl; carobbiite, KF; and chlorargyrite, AgCl are all isostructural with halite.

Sylvite KCl

Origin of Name
Named after Francois Sylvius de le Boe, a 17th century Dutch chemist.

Figure 14.305: Sylvite from near Carlsbad, New Mexico; FOV is 5.5 cm across

Figure 14.306: Orangish sylvite with lesser amounts of clear halite; FOV is 2.6 cm across

Hand Specimen Identification
Sylvite shares most properties with halite. It is soft (H = 2), generally white or clear, and displays well-developed cubic cleavage. It is distinguished from halite by a more bitter taste. Sylvite may be deliquescent (absorb water and turn into a liquid under extreme humidity.

Figure 14.305 is a photo of a sylvite sample that can only be distinguished from halite by its taste. Figure 14.306 shows orange sylvite with clear halite, a common combination. This association and orange color identify the mineral.

Physical Properties

hardness	2
specific gravity	1.99
cleavage/fracture	perfect cubic {100}/uneven
luster/transparency	vitreous/transparent to translucent
color	colorless, but may be white, or shades of yellow, blue, or red caused by impurities
streak	white

Properties in Thin Section
Sylvite has low relief, perfect cubic cleavage, and is colorless in thin section. Isotropic, n = 1.490.

Crystallography
Sylvite is cubic, a = 6.29, Z = 4; space group \(F\dfrac{4}{m}\overline{3}\dfrac{2}{m}\); point group \(\dfrac{4}{m}\overline{3}\dfrac{2}{m}\).

Habit
Sylvite crystals are typically cubes, often with modifying octahedra. Massive or granular aggregates are typical.

Structure and Composition
Halite (NaCl), sylvite (KCl), galena (PbS), periclase (MgO) and several other minerals are isostructural. In sylvite, K⁺ and Cl^– are arranged in a face centered cubic manner. Only very minor solid solution exists between the two salts.

Occurrence and Associations
Sylvite is rarer than halite but has the same origin, associates, and occurrences (see halite occurrences).

Related Minerals
Sylvite and halite are isostructural with a number of other minerals including galena (PbS) and periclase (MgO). Associated potassium salts include carnallite, KMgCl₃•6H₂O; kainite, KMg(Cl,SO₄)•nH₂O; and polyhalite, K₂Ca₂Mg(SO₄)₄•2H₂O.

Chlorargyrite AgCl

Origin of Name
From the Greek words chlor and argyros, meaning “green” and “silver.”

Figure 14.307: Chlorargyrite crystals in a geode from Saxony, Germany; FOV is 5 mm across

Hand Specimen Identification
A waxy or resinous appearance and typical light green, gray, or brown color characterize chlorargyrite. It is soft (H = 2.5) and has relatively high specific gravity (5.55). Tarnished appearance and occurrence with other silver minerals aid identification. It is generally very fine-grained, however, sometimes making identification difficult.

Physical Properties

hardness	2.5
specific gravity	5.55
cleavage/fracture	poor {100}/subconchoidal
luster/transparency	adamantine, resinous or waxy/translucent, rarely transparent
color	colorless if unoxidized, otherwise pale green, pearl-gray, or brown
streak	white

Properties in Thin Section
Chlorargyrite is isotropic, n = 2.071.

Crystallography
Chlorargyrite is cubic, a = 5.55, Z = 4; space group \(F\dfrac{4}{m}\overline{3}\dfrac{2}{m}\); point group \(\dfrac{4}{m}\overline{3}\dfrac{2}{m}\).

Habit
Rare chlorargyrite crystals are cubic. Chlorargyrite is usually massive or columnar.

Structure and Composition
Chlorargyrite is isostructural with halite (NaCl), sylvite (KCl), galena (PbS), periclase (MgO) and several other minerals. Br, F, and I may substitute for Cl in chlorargyrite. Hg or Fe may be present in small amounts.

Occurrence and Associations
Chlorargyrite is a secondary silver mineral found in the oxidized zones of silver deposits. Associated minerals are many, including native silver.

Varieties
Embolite is a Br-rich variety of chlorargyrite.

Related Minerals
Chlorargyrite forms complete solid solutions with bromargyrite, AgBr. It is isostructural with a number of other minerals, including halite (NaCl), sylvite (KCl), galena (PbS), periclase (MgO) and several other minerals. Other related minerals include iodargyrite, AgI.

Atacamite Cu₂Cl(OH)₃

Origin of Name
From Atacama, a province in Chile.

Figure 14.308: Atacamite needles from the Mt. Gunson Copper Mine, Australia; FOV is about 3 mm across

Figure 14.309: Specimen containing mostly atacamite, from Copiapo, Chile; the specimen is 6.5 cm across

Hand Specimen Identification
Atacamite occurs in various hues. It typically has a bright emerald-green, olive-green, or blackish-green color. It may form as fine-grained granular crystal aggregates or in clusters of prismatic crystals, sometimes radiating. Occurrence with other secondary copper minerals aids identification.

Physical Properties

hardness	3 to 3.5
specific gravity	3.76
cleavage/fracture	perfect {010}, fair {101}
luster/transparency	adamantine/transparent to translucent
color	various shades of green
streak	green

Properties in Thin Section
Atacamite is biaxial (-), α = 1.831 , β = 1.861, γ = 1.880, δ = 0.049, 2V = 75°.

Crystallography
Atacamite is orthorhombic, a = 6.02, b = 9.15, c = 6.85, Z = 4; space group \(P\dfrac{2_1}{n}\dfrac{2_1}{m}\dfrac{2_1}{a}\); point group \(\dfrac{2}{m}\dfrac{2}{m}\dfrac{2}{m}\).

Habit
Slender, striated prisms or needles are typical for atacamite. Massive, granular, or fibrous aggregates are common.

Structure and Composition
Atacamite contains two kinds of octahedra: CuCl(OH)₅ and CuCl₂(OH)₄. Mn commonly substitutes for Cu.

Occurrence and Associations
Atacamite occurs in the oxidized zones of copper deposits and in sands. It is associated with malachite, cuprite, and a number of rare secondary copper minerals.

Fluorite CaF₂

Origin of Name
From the Latin word fluere, meaning “to flow,” referring to the ease with which it melts.

Figure 14.310: A typical mass of purple fluorite cubes; the specimen is 4.9 cm across A typical mass of purple fluorite cubes; the specimen is 4.9 cm across

Figure 14.311: Fluorite octahedra

Figure 14.312: Blue fluorite

Hand Specimen Identification
Fluorite comes in just about any color, and may be clear. If it is purple, identification is generally straightforward. It is relatively soft (H = 4) and forms distinctive cubic (Figure 14.310) or, less commonly, octahedral (Figure 14.311) crystals. It always displays excellent cleavage. When uncolored, fluorite is sometimes confused with calcite or quartz, but may be distinguished by hardness and habit.

Purple fluorite is most common; Figures 14.310 and 14.311 show examples. Figure 4.3 (Chapter 4) is another example; it contains purple fluorite on top of white calcite and brown scheelite. Figure 14.312 shows rarer blue crystals, and Figure 9.91 is a photo of green fluorite with tan calcite and gray galena.

Physical Properties

hardness	4
specific gravity	3.18
cleavage/fracture	perfect octahedral {111}/ conchoidal and splintery
luster/transparency	vitreous/transparent to translucent
color	colorless to light green, blue-green, yellow, or purple; more rarely can also be white, brown, or rose
streak	white

Properties in Thin Section
Fluorite is usually colorless in thin section; occasionally, it appears light purple or green. Octahedral cleavage and low refractive index distinguish it from other clear isotropic minerals. Isotropic, n = 1.434.

Crystallography
Fluorite is cubic, a = 5.463, Z = 4; space group \(F\dfrac{4}{m}\overline{3}\dfrac{2}{m}\); point group \(\dfrac{4}{m}\overline{3}\dfrac{2}{m}\).

Habit
Cubic or octahedral crystals are common; octahedral cleavage fragments may appear to be crystals. Penetration twins are common. Fluorite may be massive or granular.

Structure and Composition
The cubic unit cell has Ca²⁺ ions in 8-fold coordination at the corners and in the middle of faces.
F^– ions are located in tetrahedral coordination within the cell just inside each of the four corners. Fluorite is generally close to end-member composition. Minor Y, Ce, and other rare earths may substitute for Ca. Cl, Sr, Ba, Fe, and Na may be present in small amounts.

Occurrence and Associations
Fluorite is common and widespread. It typically occurs in veins where it is associated with quartz, calcite, galena, barite, and a number of other minerals. In some carbonate-hosted ore deposits, fluorite is a replacement or fracture filling mineral associated with pyrrhotite, galena, and pyrite. It is also found as an accessory mineral in limestones and in igneous and metamorphic rocks.

Related Minerals
Fluorite is isostructural with thorianite, ThO₂; cerianite, (Ce,Th)O₂; and uraninite, UO₂. It is closely related to sellaite, MgF₂, and frankdicksonite, BaF₂.

Cryolite Na₃AlF₆

Origin of Name
From the Greek words for “ice” and “stone,” referring to its icy appearance.

Figure 14.313: Cryolite from Mont Saint-Hilaire, Quebec; FOV is 2.4 cm across

Hand Specimen Identification
A pearly/greasy luster, white color, common coarsely granular habit, and softness (H = 2.5) help identify cryolite. It is, however, a rare mineral that is quite similar to other soft white and translucent minerals. Most cryolite specimens come from a single location in Greenland.

Figure 14.313 is a photo of cryolite from Mont Saint-Hilaire, 40 km east of Montréal. Mont Saint-Hilaire and the nearby Poudrette Quarry are known sources of many unique mineral species.

Physical Properties

hardness	2.5
specific gravity	2.97
cleavage/fracture	none/uneven; cubic parting
luster/transparency	pearly, greasy, vitreous/transparent to translucent
color	colorless to snow-white
streak	white

Properties in Thin Section
Cryolite is biaxial (+), α = 1.3385 , β = 1.3389, γ = 1.3396, δ = 0.0011, 2V = 43°.

Crystallography
Cryolite is monoclinic, a = 5.46, b = 5.60, c = 7.80, β = 90.18°, Z = 2; space group \(P\dfrac{2_1}{n}\); point group \(\dfrac{2}{m}\).

Habit
Large visible cryolite crystals are rare but may be pseudocubic. Aggregates are massive, lamellar, or columnar, often exhibiting pseudocubic parting.

Structure and Composition
In cryolite, both Na and Al are coordinated to six F. Na octahedra are distorted. F occupies a tetrahedral site, coordinated to three Na and one Al.

Occurrence and Associations
Cryolite is a rare mineral. The most significant samples are from Greenland, where it is in ore deposits hosted by granitic rocks. Associated minerals include quartz, K-feldspar, siderite, galena, sphalerite, and chalcopyrite, and less commonly other sulfides, wolframite, cassiterite, fluorite, and columbite.

Related Minerals
Cryolite has a high-temperature cubic polymorph.