minerals explained iii—rock forming non-silicates · forming minerals, the silicates. in...

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Minerals explained III—Rock forming non-silicatesCraig BarrieMinerals Explained editor

Overview

Parts I and II of the Minerals Explained online event took the opportunity to discuss the most abundant rock forming minerals, the silicates. In contrast, this release will concentrate on minerals that are predominantly rock formers, but that are non-silicates (i.e. carbonates, sulphates and halides). Although the title ‘rock form-ing minerals’ might be a bit misleading, as in actuality only the carbonate minerals (i.e. calcite and dolomite) discussed can really be called rock-formers, the sulphates and halides (i.e. gypsum, barite, fluorite) tend to occur as discrete concentrations.

As would be expected, considering the abundance of silicon and oxygen in the Earth’s crust, non-silicate minerals constitute less than 10% of the total. Although much rarer than the silicate minerals, non-silicates tend to be far more important to human society. This is because all of the major metals and elements used in everyday human society are generally sourced from non-silicates. The most common non-silicate minerals are the carbonates, oxides and sulphides, while sulphates, halides and phosphates are also prominent but much less common in terms of their abundance. Furthermore, there are some minerals that occur in a pure, native form as elements, primarily although not exclusively, metals, with some of the best known including gold, silver and copper. The latter mineral groupings, oxides, sulphides and native elements, will all be discussed in the next two releases of the Minerals Explained online event, while mineral examples of the carbonates, sulphates and halides will be discussed in this release.

Carbonates, in the simplest of terms, include all of the minerals in the Earth’s crust that contain an ion of carbonate (CO

32–) within their crystal lattice. These carbonate minerals are the primary constituent of only a

handful of sedimentary (i.e. limestones, chalks and special travertines and tufas) and metamorphic (i.e. marble) rock types. The precipitation of carbonate rocks at the Earth’s surface is generally favoured in warm, shallow continental shelves, rather than deeper cold-water conditions. This preference arises from the fact that cold wa-ters generally favour carbonate dissolution, and so precipitation does not occur. The carbonate compensation depth (CCD), which is the depth below which dissolution occurs, in the ocean varies both laterally and spatially but currently ranges from around depths of 4–6 km. From an environmental standpoint the formation of carbonate minerals (i.e. calcite, dolomite, magnesite) can be considered a sink for atmospheric CO

2. Two of the

most common carbonate minerals are discussed in this release: calcite (CaCO3) and dolomite (CaMg(CO

3)2)

Halides are those minerals that have a chemical composition in which a halide element (i.e. F, Cl, Br, I) is a major component. Halide minerals form in a range of environments, with halite (NaCl), or ‘rock salt’ (Fig. 1A) being one of the major evaporite minerals (which is a series including not only halides but also sulphates and borates), while atacamite (Cu

2Cl(OH)

3) is a rare Cu mineral that is formed as an alteration product of ores both

at depth and in arid climates (Fig. 1B). The halides are a relatively small, although economically significant, class of minerals, with halite and fluorite being the most abundant, well known, and also useful. The sulphate minerals are one of the larger mineral groupings, which simply put, includes all of those minerals that contain an ion of sulphate (SO

42–) within their crystal lattice. Sulphate minerals can be both hydrous (i.e. gypsum:

CaSO4·2H

2O) and anhydrous (i.e. baryte: BaSO

4) and they form in a range of environments from evaporites

at Earth’s surface to sulphide ore deposits at depth. The best known sulphate minerals are probably barite and gypsum both for their highly distinctive characteristics and their widespread nature, indeed it is for those reasons that these two minerals which will be discussed in this release

Mineral details

Calcite has the chemical formula CaCO3 and is by far the most stable polymorph of this chemical composition.

The other CaCO3 polymorphs: aragonite (Fig. 2A) and vaterite (Fig. 2B) are thermodynamically unstable at

the Earth’s surface and will, given the opportunity, and more importantly the correct conditions, transform to calcite. Aragonite is essentially a metastable mineral at Earth’s surface, as it forms both by organic and inorganic processes, being the primary constituent of corals and mollusc shells as well as occurring in some aragonite-rich speleotherms (Fig. 3A) Due to its metastable nature there is no direct evidence for aragonite in the geological record prior to the Carboniferous period. Calcite is orthorhombic and it can form in a wide array of colours from pinks and reds through to blues and greens but is probably best known for being creamy-white


and slightly translucent (Fig. 3B). The crystal habit of calcite can range from rhombohedra to prisms as well as tabular crystals, although the rhombohedral form is actually relatively rare in naturally formed crystals. Calcite is a very soft mineral, only 3 on Mohs scale of hardness, meaning it can be scratched easily with a knife. This property, together with the fact that it will dissolve readily in acid, makes it an easy mineral to identify. Dissolution in weak acid, including moderately acidic rain, means that calcite-rich rocks (i.e. limestones and marbles) are inherently susceptible to weathering. This can result in the formation of limestone cave systems and karst topography in the natural world and the weathering or even destruction of marble pillars and buildings in the human one!

Did you know high-grade optical calcite was used in World War II for gun sights, specifically: bomb sights and anti-aircraft weaponry?

The Dolomite group of minerals specifically refers to a group of carbonate minerals which all have the general formula Ca, X (CO

3)2 where X is any one of a number of elements (i.e. Mg, Fe, Zn, Pb, Mn, etc.) The mineral

dolomite (Fig. 4A), which is the most well known member of this group, has the formula CaMg(CO3)2. The

term dolomite can also be used to describe a carbonate rock composed entirely of the mineral dolomite, also known as a dolostone (Fig. 4B). Dolomite is a trigonal mineral that has a more restricted range of colours than calcite, generally occurring as either pinks to whites and greys. Although appearing very similar to calcite, dolomite is slightly harder, ~3.5–4 on Mohs scale, and it only very weakly dissolves in acid at the rock face (if at all at cold temperatures). This lack of ‘fizzing’ is due to the differing crystal structure between calcite and dolomite; however, if you powder the sample or heat the acid—90 °C will do it—then dolomite will dissolve in a similar rigorous manner to calcite. Although dolomite can be precipitated at the Earth’s surface at the present day, it is very rare and generally restricted to unique environmental conditions. This is in stark contrast to the presence of vast quantities of dolomite in the geological record, with the debate still raging as to how conditions varied previously (i.e. much warmer oceans?) compared to the present day. A solid–solution series exists between dolomite and the other common dolomite group member ankerite (Ca(Fe,Mg, Mn)(CO

3)2).

Ankerite is a common constituent in Fe-rich ore deposits (along with the Fe-carbonate siderite) and often occurs as gangue material in many of the world’s sulphide deposits (including Greens Creek, Alaska). Although ankerite can appear white or grey in colour it characteristically forms yellowish-brown crystals (Fig. 5).

Did you know the mineral dolomite is named after the French geologist Deodat de Dolomieu? Imprisoned in Italy during the Napoleonic wars he was only released at the express demand of Napoleon after the annexing of the entire country!

Baryte is a mineral with the chemical formula BaSO4, and also can refer to a grouping of sulphate minerals,

specifically the anhydrous ones. The baryte group of minerals includes celestine (SrSO4), anglesite (PbSO

4) and

anhydrite (CaSO4). Although the mineral baryte can form in a variety of colours it occurs typically in white

(Fig. 6A) and pink (Fig. 6B) varieties. By far the most striking characteristic of baryte is its density (4.48 g/cm3) generally very high for a non-metallic mineral, giving hand specimens a weighty feel. This density, along with the insoluble nature of the mineral, is what gives it many of its industrial applications. Baryte is by far the most common source of barium, although other barium minerals do form naturally (i.e. alstonite (BaCa(CO

3)2) and benitoite (BaTiSi

3O

9)). Baryte often forms as gangue material in ore deposits, including the

Zn-Pb hydrothermal ores of Ireland (i.e. Navan, Galmoy, Lisheen), sedimentary ores and rarely in volcanic massive sulphide (VMS) ores. The crystal habit of baryte can be highly variable, ranging from tabular and prismatic varieties to the descriptive ‘cockscomb’ texture and more often than not as massive aggregates. Baryte is primarily used, industrially, as a weighting agent in drilling fluids in the oil exploration industry to stop overpressure blowouts but is also used in a range of other applications including to increase the weight of paper. The mineral celestine (Fig. 7) or celestite (SrSO

4) shares a solid-solution series with baryte and is the

primary source of worldwide strontium. Unlike baryte, celestine tends to be slightly blueish in colour and is much rarer in occurrence.

Did you know the baryte group mineral anglesite (PbSO4) was named after the Parys Copper Mine in Anglesey, Wales where it was first discovered in 1832?

Gypsum is a hydrous sulphate mineral with the chemical formula (CaSO4·2H

2O) and is primarily an evaporitic

mineral, although the hydration of anhydrite also gives rise to gypsum. In terms of hardness, gypsum is one of the softest natural minerals, having a value of 2 on Mohs scale, only just above talc (Mg

3Si

4O

10(OH)

2) with

a value of 1. Gypsum occurs in a number of crystal varieties, with the most common being selenite (Fig. 8A, 8B), alabaster (Fig. 8B) and ‘desert rose’ (Fig. 8C), this latter texture is common in both gypsum and baryte minerals! The differing forms of gypsum are partly related to the habit and appearance of the crystals and the environment in which they form.

The size of gypsum crystals can range from the micrometre scale to over 10 m for individual crystals with


the best example of this larger growth coming from the caves of the Naica Mine of Chihuahua, Mexico. Due to its evaporitic nature, gypsum is by far most commonly associated and intercalated with sedimentary rocks and strata. Gypsum is also a common feature of convergent plate margins, often forming a ‘décollement’ also known as a gliding plane or basal detachment fault. This feature was first described, and is probably still best known, in the Jura Mountains just north of the Western Alps. Gypsum can also be formed via the oxidation of sulphide minerals (i.e. pyrite, FeS

2) in the presence of calcium carbonate (CaCO

3) to generate CaSO

4. This

can be problematic in the building industry where the presence of both FeS2 and CaCO

3 in sufficient quantities

together can result in gypsum formation and the development of fissures and cracking within building walls and foundations.

Did you know the unique conditions of the White Sands National Monument in New Mexico, USA has resulted in the formation of 710 km2 of gypsum sand, enough to supply the construction industry with gypsum for 1000 years?

Fluorite also known as flurospar, is one of the halide group of minerals with the chemical formula CaF2. Fluorite

can form in a wide array of colours with virtually every hue possible (Fig. 9A). Some varieties of fluorite also fluoresce in ultraviolet light (Fig. 9B, C), although not all do. Indeed, the term fluorescence is adapted from this very nature in fluorite minerals. Fluorite is an isometric crystal, primarily forming in the cubic habit although other habits can occur, albeit they are much rarer. Fluorite is a relatively widespread non-silicate mineral, generally being found as gangue in hydrothermal ore deposits but can crop up as a component mineral (both major and minor) in some igneous and sedimentary rocks (primarily dolostones and limestones). The largest known single crystal of fluorite was a cube measuring 2.12 m and weighing in at a colossal 16 tonnes.

Did you know fluorite has been the state mineral of Illinois since 1965, when at the time Illinois was the largest producer of fluorite in the USA?


Fig. 1. A. A cluster of intergrown, yet separate, cubes of halite measuring up to 3.2 cm across; locality: Wieliczka Mine, Poland. B. A plate covered with lustrous atacamite crystal pinwheels; Locality: La Farola Mine, Chile.

Fig. 2. A. Cluster of pink aragonite crystals. B. A large sample of vaterite crystals up to 2 cm in size; locality: San Vito Quarry, Naples Province, Italy.

Fig. 3. A. A fine specimen of inorganically precipitated aragonite, the blue colour is a result of the incorporation of Cu (size: 9.4 × 6.9 × 6.5 cm); locality: Wenshan Mine, Yunnan Province, China. B. Matrix plate covered with glassy, clear quartz crystals up to 3 cm in size; locality: Wyndham Mines, Cumbria, UK.


Fig. 4. A. Pinkish-white dolomite crystals, up to 4 cm in length, held in a matrix of limonite; locality: Ojuela Mine, Mexico. B. Brecciated dolostone in a limestone matrix of Lower Jurassic (Lias) age (geological hammer is 28 cm long); locality: Slope of Devinska, Slovakia.

Fig. 5. Brownish-orange crystals of ankerite intergrown with quartz (size of view: 2 cm); locality: Huaron Mines, Cerro de Pasco, Peru.

Fig. 6. A. Specimen of white baryte crystals (size of view: 33 × 32 × 18 cm); locality: Mibladen Mine, Meknès-Tafilalet Region, Morocco. B. A rosette of bladed, light pink baryte on a quartz matrix (size of view: 3.4 × 2.5 × 2 cm); locality: Weldon Mine, Arizona, USA.


Fig. 7. A dramatic specimen of lustrous, water-clear, topaz-shaped, blue celestine crystals up to 5.5 cm in size on a massive celestine matrix; locality: Sakoany Mine, Madagascar.

Fig. 8. A. A large selenite crystal from the Crystal Cave of Kentucky. The striking, rounded, yellow termination area rests atop the colourless body (size: 25.8 × 2.1 × 1.8 cm); locality: Crystal Cave, Kentucky, USA. B. Two, water-clear fishtail-twinned selenite crystals growing from a matrix of snow-white alabaster (size: 4.6 × 4.4 × 3.0 cm); locality: Alabaster Quarries, Zaragoza, Spain. C. A large specimen of ‘gypsum’ in the form of a ‘desert rose’ (size: 15.8 × 12.4 × 10.5 cm); locality: Oklahoma, USA.


Fig. 9. A. Cubes of fluorite showing a range of colour hues, within individual crystals. B. Fluorite octahedron crystals showing fluorescence when exposed to UV light (size: 17.8 x 10.4 x 2.5 cm); locality: Navidad Mine, Durango, Mexico. C. Fluorite under white (left) and shortwave UV-light (right) from Weardale, UK.


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