Sample of Haüyne from Mayen, Eifel Mountains, Rhineland-Palatinate, Germany. Attribution: Rob Lavinsky, iRocks.com

Lazurite is a major component of Lapis Lazuli (or stone of azure) which can be differentiated from Sodalite by the presence of small inclusions of pyrite (FeS₂) giving it an attractive appearance.

Lapis Lazuli
Image: Wikipedia.

It is said that the best Lapis Lazuli has been mined in the Badakhshan province of Afghanistan for a period of c.6,000 years, with other sources located in South America, Russia and India. It was employed as an aphrodisiac by the Romans, and as a treatment for a variety of ailments.

From around the 6^th century A.D. Lapis Lazuli was ground for use as the pigment ultramarine. The brilliant blue permanent colour became much-prized during The Renaissance in Europe when it complemented the purest red vermillion and gold of egg-tempera religious paintings and illuminated manuscripts. Following a laborious extraction process in Afghanistan, the expensive pigment would be imported through the port of Venice. In 1508, the artist Dürer complained that 100 florins (approximately US$20,000) purchased barely a pound of ultramarine, making it more expensive than gold at the time.

Bacchus and Ariadne by Titian uses ultramarine for the robes.
Image: Wikipedia.

Fossil wood is not usually found associated with the rest of the tree (leaves and roots) and identification can be difficult, in these cases the specimens are given a special botanical name. These usually feature the term xylon, along with the plant type it is assumed to be part of, to show that the identification is not bases on whole specimen. (e.g. Arucarioxylon – linked to the Arucaria (Monkey Puzzle) or related genus)

A well-preserved specimen of fossil wood recovered from the Redcar Mudstone Formation near Staithes.

This example of a piece of fossilised wood was found on the beach near Staithes. It has been preserved through a process known as permineralization. The original piece of wood was buried amongst sediment deposited millions of years ago. Over time the minerals from the rock soaked into the wood and replaced the original organic material, whilst keeping the structure of the wood. (in this case it was part of the Redcar Mudstone Formation which was laid down approximately 190 million years ago).

In the specimen shown above you can still clearly see the growth rings hundreds of millions of years after the tree, which grew on land, was transported by rivers to the ancient Tethys Sea where it eventually became incorporated into the sea floor sediment.

©2011 Tees Valley RIGS Group.

In situ fossils in the Redcar Mudstone Formation on Redcar Scar.

The ancient seas that covered Britain millions of years ago are now continually revealing the extinct organisms that once roamed the oceans.

Living within the Tees Valley area provides us with many opportunities and locations to acquire a whole host of these interesting objects. The abundance and diversity of fossils throughout our region is wide and varied. From the famous Jurassic ammonites to the alluring fossil gastropods (sea snails).

Once collected most fossils will need preparing in some way to enhance their natural beauty, this can take many hours even weeks to complete and requires patience and a good eye for detail.

A selection of fossil preparation equipment.

When starting out on preparation most hobbyist collectors will find the use of a rotary tool kit with interchangeable heads immensely versatile. Along with an electric engraver, steel probes and craft knives, these can be helpful for carefully picking away and removing the surrounding sediments. Fortunately the tools just mentioned do not take up much space and can be purchased at very little expense to the user.

A selection of prepared fossils and pertinent literature.

As the collector becomes more proficient in preparation they may eventually want to turn to more robust specialist equipment. There are many tools and supplies on the market to choose from, and one recommendation has to be the pneumatic fossil preparation pen – using compressed air to vibrate a tungsten tip at high speed these can remove the hardest of matrices accurately and effectively in a fraction of the time of other methods. Although professional equipment can prove somewhat expensive the end results almost always outweigh any of the costs.

We would like to thank RIGS Group member Scott Bradley for providing this month’s article.

Please Note: Fossil collecting must be done responsibly to preserve key beds and specimens for the enjoyment of others. Please feel free to collect loose specimens from the beaches, but leave in-situ specimens for the enjoyment of those who follow in your footsteps. Click here to see an informal fossil collecting code.

PLEASE NOTE: Tees Valley RIGS Group cannot be held responsible for the content of external sites.

©2011 Tees Valley RIGS Group.

1. References

Frosterley Marble is a dark grey to black limestone which has been used as an ornamental stone locally and internationally in churches and buildings such as Durham Cathedral.

Frosterley Marble is not a true marble as it is not a metamorphic rock (true marble is limestone which has recrystallised during thermal or regional metamorphism). Stonemasons use the term marble for some limestones which can take a high polish.

Much of the Frosterley Marble has been worked from quarries around Frosterly, in Weardale, County Durham. Frosterley Marble was formed during the Lower Carboniferous Period (325 million years ago) when the northern Pennines area was closer to the equator. The area alternated between a deltaic and tropical marine depositional environment. The Great Limestone Member was deposited in a marine environment and is in places, 22m thick. The Frosterley Marble forms a fossil rich band in this rock.

Frosterly Marble containing numerous fossil corals.
Image: Carole Rushall.

This photograph shows a boulder of the marble which contains abundant fossil specimens of the coral Dibunophyllum bipartitum. Unlike many corals which live as colonies, Dibunophyllum bipartitum was a solitary coral (it is now extinct). The solitary coral organism had a curved cone shaped calcareous skeleton in which lived a soft-bodied polyp whose tentacles captured organic matter from the sea water. When the organism died, the skeleton settled into the limy ooze on the sea floor which eventually formed limestone.

You can download an illustrated leaflet about Frosterley Marble produced by the North Pennines AONB Partnership by clicking here…

References

Stone, P., Millward, D., Young, B., Merritt, J.W., Clarke, S.M., McCormac, M. & Lawrence, D.J.D. (2010). British Regional Geology: Northern England (Fifth edition). Keyworth, Nottingham: British Geological Survey.

Scrutton, C. (ed) (2004). Northumbrian Rocks and Landscape : A Field Guide. (Second edition). Yorkshire Geological Society.

Our thanks go to Carole Rushall for providing this month’s article.

PLEASE NOTE: Tees Valley RIGS Group cannot be held responsible for the content of external websites.

©2011 Tees Valley RIGS Group.

Derived from the Latin Argilla – meaning ‘clay’, this group of rocks primarily comprise particles of the finest grade, including clay- and silt-sized clasts up to ¹/₁₆ mm in diameter. They may be divided into subclasses of shales, mudstones and siltstones.

Both shale and mudstone are composed of the finest particles of sediment less than ¹/₂₅₆ mm in diameter, and can be distinguished by the way in which they cleave. Shale is generally finely-laminated and fissile, able to be split easily along its bedding planes, mudstone on the other hand has no preferred axis of cleavage and tends to exhibit a ‘blocky’ fracture¹. Siltstone follows similar principles of cleavage but comprises grains between ¹/₂₅₆ mm and ¹/₁₆ mm in diameter.

Shales of the Whitby Mudstone Formation (grey) form the foreshore and lower cliff beneath Middle Jurassic sandstone (yellow) at Rosedale Wyke. The remains of Kettleness alum quarries form the headland in the background.

Clay minerals (alumino-silicates) make up the bulk of such rocks and may include kaolinite, illite, chlorite and montmorillonite-smectite. Argillites are rarely pure but include a mixture of minerals. For example the Alum Shale Member of the locally exposed Whitby Mudstone Formation contains all four of the above mentioned clay minerals plus pyrite (FeS₂), quartz (SiO₂), siderite (FeCO₃), calcite (CaCO₃), collophane (apatite), goethite (FeO(OH)), gypsum CaSO₄ • 2(H₂O), jarosite (KFe³⁺₃(OH)₆(SO₄)₂), mica, feldspar, zircon and anatase. The latter three minerals in only minor amounts.

Shales and mudstones may also frequently contain inclusions in the form of calcium carbonate, siderite or other minerals. These features form after deposition of the originating sediment during the process of lithifaction. They grow in-situ when minerals distributed through the body of the deposit are drawn toward a single point through ionic transportation. Often a shell fragment or fossil will provide a nucleating point around which the inclusion develops as the accreting mineral is drawn from the surrounding sediment. In the image below showing the Jet Rock Member at Rosedale Wyke, bedding can be seen to pass around the outside of weathered calcium carbonate nodules.

Laminations in the Jet Rock Member of the Whitby Mudstone Formation passing around weathered limestone nodules.

Argillites are all sedimentary in origin, their components being either water or wind-borne. They are the products of fairly low energy environments such as deep sea floor, tidal flats, lakes and (in the case of loessite – lithified wind-borne rock dust from a number of sources) continental environs. They may occur in a variety of colours ranging from the dark red-brown, blue-grey, or tea green Triassic deposits to the light brown or black Bituminous Shale and Jet Rock Members of the Lower Jurassic containing hydrocarbons.

Red-Brown Permo-Triassic mudstone as seen at Seaton Carew.

The fine-grained nature of argillites make them ideal for the preservation of detailed fossil specimens. One example of such excellent preservation is the enigmatic suite of remains discovered in the Burgess Shale of Canada by Charles Doolittle Walcott (1850-1927) in the early 20th century². These Middle Cambrian fossils were reappraised in the 1970s and found to represent the remains of creatures with a number of body plans previously unknown to science such as Marrella a kind of extinct crustacean.

Marrella - An extinct crustacean from the Burgess Shale of Canada with no modern day relatives.

Historically, argillites have been exploited locally for a number of reasons. Millions of tons of Lower Jurassic (Toarcian) Alum Shale were quarried and processed to serve the local alum trade at over twenty sites in and around the Tees Valley. At Ravengill, near Commondale, Middle Jurassic (Aalenian) mudstone was quarried and milled for the brick and tile trade.

When sedimentary argillites become altered (or metamorphosed) by heat and/or pressure to form rocks such as slate, hornfels, etc. the resulting fine-grained metamorphic rocks tend to be referred to as Pelites.

Notes

^[1] The ironstone miners of Cleveland had their own terminology for many kinds of rock and tended to refer to mudstone units inter-bedded with ironstone seams as shale.

^[2] If the anecdote concerning this discovery is to be believed, although C.D Walcott recovered the fossils from this lagerstatten, it was actually his horse which drew his attention to their presence.

PLEASE NOTE: TVRIGS Group cannot be held responsible for the content of external sites.

©2011 TVRIGS Group.

Trilobite of the species Paradoxides.

They are one of the earliest creatures to have evolved hard-parts, presumably to protect against predators and, as such, are well-represented in the fossil record. The oldest rocks in the Tees Valley, the Carboniferous and Permian strata, may hold fossilised examples of these remarkable creatures. Trilobites have three parts to their bodies: the head or cephalon, body or thorax and tail or pygidium. They were marine animals which lived on the sea floor at a variety of depths.

Trilobites are so named for the three longitudinal lobes: 1 – left pleural lobe; 2 – axial lobe; 3 – right pleural lobe. The trilobite body can also be divided into three major sections (tagmata): 4 – cephalon; 5 – thorax; 6 – pygidium.

Trilobites survived until the end of the Palaeozoic Era, marked by the Permo-Triassic Mass Extinction which occurred around 250 million years ago.

This trilobite is Walliserops trifrucatus from Djebrl Oufaten in Morocco.

TVRIGS Group cannot be responsible for the content of external sites.

Red variety of Chiastolite Slate showing randomly orientated crystals. The cross sections show the typical characteristic black spot as part of the cruciform structure, e.g. on the extreme centre right. The cross sections are about 1.5mm square.
Image: John Waring

The Skiddaw Group consists of a succession of marine sediments varying in grain size from fine grained sandstones to mudstones as well as greywackes, all of which were laid down during Ordovician times about 480 – 470 million years ago in the narrowing Iapetus Ocean which then separated “Scotland” from “England”. As the ocean narrowed, the sediments underwent folding during end Silurian times. This led to the sediments undergoing low grade metamorphism.

Grey variety of Chiastolite Slate again showing randomly orientated crystals. Paler grey roundish spots are early stage cordierite crystals which are more fully developed nearer the contact with the granite (inner hornfels zone).
Image: John Waring

Towards the end of the Caledonian Orogeny, about 394 Ma during Devonian times, the Skiddaw Granite was intruded into the slates. The resulting thermal as well as low grade metamorphism led to the formation of different minerals depending on the composition of the host rocks as well as their distance from the intrusion. Chiastolite (or andalusite) formed in the more argillaceous sediments. As seen in the photographs, the random orientation of the crystals indicates that their formation was due mainly to heat and not to pressure.

The mineral andalusite, an aluminium silicate (Al₂ SiO₅) gets its name from where it was first found, viz. Andalusia in Spain. The name chiastolite is derived from the Greek chiastos, meaning “cross”.

Cross section of chiastolite photographed under a polarising microscope. The carbon inclusions can just be seen radiating towards the corners of the crystal forming the typical cruciform pattern.
Image: John Waring

Further Reading:
Rastall R.H. 1910. The Skiddaw Granite and its metamorphism. Quart. Jl. Geol.Soc. London., Vol. 66, 116-141
Hitchen, C.S., 1934. The Skiddaw Granite and its residual products. Quart. Jl. Geol.Soc. London., Vol. 90, 158-200
Eastwood et al. 1968 Geology of the Country around Cockermouth & Caldbeck. pp. 119 – 125 pub. B.G.S.
Roger Mason. 1986. Petrology of the Metamorphic Rocks. pp. 61-68, pub. George Allen & Unwin

This article written by John Waring.

1. Background
2. Occurrence and Properties
3. Conclusions

Water, the Hub of Life.
Water is its mater and matrix, mother and medium.
Water is the most extraordinary substance!
Practically all its properties are anomalous, which enabled life
to use it as building material for its machinery.
Life is water dancing to the tune of solids.

- Albert Szent-Gyorgyi – (1972)

Welcome to this, the first offering in the TVRIGS Rock of the Month series of articles for 2011. The subject of this particular contribution may appear a little odd to some readers. After all, applying the designation ‘rock’ to (of all things) water seems preposterous. The latter is a liquid and surely not a mineral in any way, shape or form. To assist in disentangling this this dilemma, we must delve back into the mists of time and briefly examine the history of mineral classification.

Background

Ideas concerning the classification of Nature’s multifarious components can in no way be viewed an innovative notion. An ability to place objects into categories would appear to have naturally evolved and provides humankind with a way in which to understand the environment and interact with it. Humankind’s earliest stone-tool using forbears must have been able to distinguish suitable source materials such as flint, obsidian and quartzite from less workable rock. Cave artists too must have been able to distinguish and prepare pigment from mineral sources. History’s earliest geologists (therefore) appear to have existed long before any direct means of recording their ideas had arisen.

A selection of stone tools from the United States.
(Source: CommonsWikimedia – Public domain)

The first hints at formal classification emerge in Classical times through the writings of the Ancient Greek and Roman civilisations where we must turn to the likes of Theophrastus (371 – 287 BC) and Pliny the Elder (AD 23 – 79). Their early systems of classification, along with the preceding Egyptian civilisation, tended to be based upon a mineral’s practical uses, giving categories such as gemstone, pigment, metalliferous ore and so on.

During the Middle Ages, we look to the Arabic scholars. Jabir Ibn Hayyaan (a.k.a Geber; 721 – 803) was one of the earliest to perform methodical experiments and devised a system based upon external characters. For example fusibility (melting point), malleability and fracture. Here we see hints of a rough basis for the modern system of field identification. This work was added to by, amongst others, Avicenna (Ibn Sina; 890 – 1037) and scholars of the Renaissance Era in Europe such as Georgius Agricola (1494 – 1555) who began using physical characteristics like colour, streak, hardness etc. in their classifications.

Georgius Agricola (1494 – 1555)
(Source:CommonsWikimedia-Public domain)

Through all of this time one breed of scholar, those with a hand in philosophy, ancient practices and experimentation, became known as Alchemists. A dictum followed by these early chemists is Solve et coagula, which translates as ‘dissolve and coagulate’ or ‘separate and join together’. It refers to the deconstruction and reconstruction of matter, and points to the experiments which alchemists were famous for conducting as they sought a number of substances deemed to have mystical properties. Perhaps the most notorious of these searches regales under the title chrysopoeia, or the quest to transmute base metal into gold, other notable searches include the creation of a panacea or elixir of longevity, a universal solvent dubbed alkahest, and (last but by no means least) to achieve ultimate wisdom of the universe. Despite their high ideals no single way of classifying minerals emerged from their alchemical labours. For any of the preceding systems of classification water proved not to be a problem as it was deemed a fundamental element along with air, earth and fire.

A strong belief in scientific rationality dawned around the eighteenth century which saw commencement of modern science. During this period Linnaeus (1707-1778) developed a much broader Systema Natura (System of Nature) first published in 1735, in which he divided Nature into a three-fold system of Regnum Animalia, Regnum Vegetibales and Regnum Lapideum – or the kingdoms of Animal, Vegetable and Mineral. The Linnaean system for minerals was based on three further subdivisions namely Petrae (rocks), Minerae (minerals) and Fossilia (fossils).

Page showing part of Regnum Lapideum from Linnaeus' Systema Natura (1735)
(Public domain)

The nineteenth century saw the emergence of professional geologists as the discipline gained a scientific footing. By the 1820s, Friedrich Mohs (1773 – 1839) developed a practical scale of hardness based upon the ability of one mineral to scratch another. The scale is non-linear, and familiar minerals define the integer values (1 – 10), with common substances like glass and the human fingernail providing additional benchmarks convenient to measuring hardness according to the Mohs’ scale.

As new disciplines making up the edifice of science emerged into the twentieth century ideas concerning the nature of matter were revolutionised. As a result, a swathe of more fundamental classifications emerged based upon the chemistry and atomic structure of minerals, Dana & Strunz classifications being two modern examples. One respected definition of the term mineral is:

A mineral is a body produced by the processes of inorganic nature, having usually a definite chemical composition and, if formed under favourable conditions, a certain characteristic atomic structure which is expressed in its crystalline form and other physical properties.

- Dana & Ford – (1932)

A side-effect of narrowing this definition is that some substances formerly residing within the mineral class are forced out. The stipulations concerning a mineral being inorganically-derived and crystalline solid, exclude the likes of coal, jet and amber (organic and non-crystalline), obsidian and opal (amorphous) and mineral oil (organic liquid) to name but a few. Although these no longer satisfy the definition mineral, they are ‘mineral-like’ and have attracted the name mineraloids:

A mineraloid is a mineral-like substance that does not demonstrate crystallinity. Mineraloids possess chemical compositions that vary beyond the generally accepted ranges for specific minerals.

-Webster’s Online Dictionary -

Opal - a mineraloid.
(Photo by: Rob Lavinsky, iRocks.com)

Amongst the mineraloid class we can also include lechatelierite (nearly pure silica glass often produced through lightning strike), limonite (mixture of oxides), tektites (meteoritic silica glass) and petroleum (liquid). Any remaining substances which fall neither into the mineral or mineraloid classes (e.g. natural gas) are dubbed nonmineral. One definition being:

A nonmineral (mineralogy) is a substance found in a natural environment that does not satisfy the definition of a mineral and is not even a mineraloid. Many non-minerals are mined and have industrial or other uses similar to minerals, such as jewellery.

- Webster’s Online Dictionary -

So ends our, somewhat potted, history of mineral classification. Now let’s take a look at water.

Occurrence and Properties

How inappropriate to call this planet Earth when clearly it is Ocean.

- Arthur C. Clarke – (Nature, 1990)

Familiar and ubiquitous, water can be described as one of the planet’s most important resources accounting for some 75% of the Earth’s surface area. The US Geological Survey estimates that the planet fosters a volume of water amounting to 1.386 billion km³, of which a full 97% (or 1.34442 billion km³) resides within the oceans alone.

Pie chart showing proportions of salt and fresh water on Earth.

This leaves a meagre 3% of the total (or 41,580,000km³) as fresh water, though not all of this is readily accessible. The majority of fresh water, 2.1% of the total (or 29,106,000km³), is sequestered in its solid form as ice-caps, pack ice, glaciers and the like. Leaving only 0.9% (or 12,474,000km³) to account for all lakes, rivers, snow-cover, groundwater and atmospheric vapour. So less than 1% of all Earth’s water reserves can be described as ‘readily accessible’ sources of water.

It has perhaps the most widely recognised chemical formula (H₂O) of any other compound. A symbolic representation which informs us that each water molecule comprises two hydrogen atoms bound to a single oxygen atom. This particular arrangement imparts slight polarity to the molecule which enables it to bind easily with a great number of other different molecules. So good is water at this that it has been described as a universal solvent (alchemists rejoice!) after its ability to dissolve more substances than any other known solvent. The ease with which water achieves this feat is the reason why the oceans are salty, and even so-called fresh water is seldom pure but carries a dissolved mineral load.

Observations of non-metallic liquids during cooling reveal that practically every example thus tested experiences an increase in density as their critical freezing-point is reached. This makes complete sense when one considers that at lower temperatures the freedom of the molecules becomes more restricted. They are forced to occupy a smaller volume in order to bond with one another. Water provides the only known counter-example to this rule by virtue of the fact that H₂O molecules are forced farther apart as weak hydrogen bonds are formed. The result being that in its solid phase water occupies a volume around 9% greater than its liquid precursor, and hence floats. It achieves its greatest density at a temperature of 4°C.

The term ice can be used to describe, amongst other things, any one of fifteen known crystalline forms of water, many of which adopt different crystal systems. The most common form here on Earth is known as ice I_h and occurs when liquid water is cooled below 0°C at a pressure of one atmosphere (1 atm). Solid water can also occur in five different amorphous (non-crystalline) forms, these have been named; Amorphous Solid Water (ASW), Low-Density Amorphous ice (LDA), High-Density Amorphous ice (HDA), Very High-Density Amorphous ice (VHDA) and Hyper-quenched Glassy Water (HGW).

Water dripping from a melting icicle.
(Image: cliff.rigg)

Under pressures greater than one atmosphere most liquids freeze at a higher temperature than normal. Similarly, they freeze at lower temperatures under lower pressures. Water (however) acts in precisely the opposite manner, freezing at temperatures below 0°C under pressures greater than one atmosphere, and vice versa.

Within the realms of Earth Systems Science water is a fundamental player. It contributes the water cycle to the array of systems under which Nature operates. In its liquid state it is an agent of both erosion and transport, in addition to providing an environment within which by far the greatest proportion of the planet’s sediments accrue. It plays a role in the geochemistry of rock formation. Its propensity to expand on freezing is an important mechanism in the breakdown (weathering) of existing rock, through the process of freeze-thaw action. In the form of glaciers, continental ice-sheets and the like, it is a tremendously potent agent of erosion, transportation and re-deposition. Water has the power to raze great mountain ranges to flat plains, and yet retains the finesse to mediate in fragile bio- and geochemical reactions.

In addition, water is responsible for the redistribution of heat energy across the planet through the action of the thermo-haline conveyor. It is used in the definition of a number of units of measurement. An hypothetical cube of water one centimetre on a side at 4°C has a mass of one gramme. It is also employed as a basis in the calculation of specific gravity (SG), defined as being the the ratio of a body’s mass compared to that of an equivalent volume of water. Pyrite, for example, has an SG of 5.0, meaning that a cube of pyrite one centimetre on a side has a mass of 5 grammes. Water also defines the range of temperature on the Celsius scale, with 0°C marking water’s freezing-point and 100°C its boiling point.

There is much more, but I feel that to go on could be viewed as labouring the point somewhat. I feel therefore that it may be time to assess what we have discussed.

Conclusions

Between earth and earth’s atmosphere, the amount of water remains constant;
there is never a drop more, never a drop less.
This is a story of circular infinity, of a planet birthing itself.

- Linda Hogan – (Northern Lights, Autumn 1990)

So, what conclusions can we draw following our brief sojourn into the history of mineral classification and the occurrence and properties of water. Is water a mineral? Well, the answer appears to be sometimes! In its liquid form it best fits the definition of mineraloid. However, in any of its fifteen solid crystalline forms it qualifies as being a fully-fledged mineral, and is (in fact) treated by geologists as another kind of rock. In its solid amorphous states listed above, water reverts to being a mineraloid. Lastly, in its gaseous form, as water vapour, it strictly becomes a nonmineral.

©2011 Andy Cooper

1. General Description
2. Jewellery
3. Folklore and Medicine
4. Pyrite in the Tees Valley

For this month’s article we are going to take a look at a commonly occurring mineral having a long history of association with humankind. Fool’s Gold is a common name used to describe a number of different minerals including weathered biotite mica, though most frequently the name is attached to iron pyrite [FeS₂], technically known as iron di-sulphide.

General Description

The name pyrite derives from the Greek purites lithos, meaning “stone of fire“, after its tendency to spark when struck against steel, one of several minerals to do so. For this reason it was used in wheel-lock firearms from around the year 1500, before the invention of flint-lock style weapons.

Cubic crystals of pyrite in their matrix.
Image courtesy of Ra'ike.

This mineral is brassy-yellow in colour and has a metallic lustre, thereby outwardly resembling gold [Au] and accounting for its common name. Like any attractive mineral it has accumulated a number of other names. For example, when associated with coal deposits pyrite is dubbed brass, brazzle or brazil.

It occurs as cubic, octahedral, orthorhombic, tetrahedral or pyritohedral crystals, and not infrequently in irregular aggregates of various sizes. Twinning, is common with the orthorhombic variety producing ‘cockscomb’ or ‘spearhead’ arrangements. Pyrite is a very common sulphide mineral and can be found in sedimentary, metamorphic, intrusive, eruptive and hydrothermal deposits commonly associated with other sulphides and oxides. It occurs in mudrock as fine-grained impregnations, detached crystals and aggregates, also in sandstone, coals and schistose rocks. Pyrite can act as a replacement mineral occasionally producing beautiful lustrous fossil specimens. It weathers to produce iron sulphate or the hydrated iron oxide limonite [FeO(OH)]. Other accessory minerals which associate with pyrite include minor amounts of copper, nickel, tin, cobalt and silver.

Sparkling crystals of pyrite demonstrating replacement in this ammonite.
Image courtesy of Didier Descouens.

Pyrite may (very-rarely) be associated with actual gold. Auriferous pyrite (also dubbed arsenopyrite or mispickel) contains arsenic which is able to form a substitution couple with gold within the crystal lattice. This type of deposit can be economically viable and it is worked in both Rossland, British Columbia and Carlin, Nevada, where the pyrite contains up to 0.37wt% gold making it a valuable ore. Carlin’s motto is;

the town originally being founded as a camp-site during the gold-rush of the 1840s.

Native Gold Nuggets
Image courtesy of Aramgutang

In spite of its accusatory common name however, pyrite and gold are rather easy to tell apart, even without the advantage of sight. Pyrite has a density of around 5gm/cm³ but gold a density of between 15 and 19.3 gm/cm³. This means that gold is much heavier in the hand. Pyrite has a hardness of 6 or 6.5 on Moh’s scale, is brittle and difficult to scratch, whereas gold is much softer having a hardness of only 2 or 3 on the same scale and is malleable. If any doubts remains then when a sample is tested on unglazed porcelain, pyrite leaves a black streak whereas gold has a yellow streak.

Modern uses of pyrite are primarily ornamental and industrial, with the sulphide being mined and processed on a large scale to produce both sulphuric acid [H₂SO₄] and ferrous sulphate [FeSO₄] at sites including Rio Tinto (Spain), Sulitjelma (Norway) and Mount Lyell (Tasmania).

A few related minerals may themselves be mistaken for pyrite amongst which we can include;

Chalcopyrite [CuFeS₂] or copper iron di-sulphide is a related mineral outwardly resembling pyrite, and sometimes, along with Bornite [Cu₅FeS₄], dubbed Peacock Ore due to its iridescent surface. It (too) is generally brassy-yellow in colour, but conforms to the tetragonal crystal habit, is slightly softer than pyrite (only 3.5 to 4 on Moh’s scale) and has a green-black streak. Chalcopyrite can be an important ore of copper.

Metallic, brassy-gold twinned crystals of Chalcopyrite.
Image courtesy of Rob Lavinsky

Pyrrhotite is an iron sulphide of variable composition and, like chalcopyrite, is softer than pyrite. It is a darker bronze colour, weakly magnetic and has a greyish black streak.

Jewellery

Pyrite crystals have been found associated with early (Palaeolithic) human collections of precious objects, and the mineral has been used by many civilisations since that time for a variety of purposes including as jewellery. From Ancient Mesopotamian Materials and Industries by P.R.S. Moorey, we learn that;

The Ancient Greeks also used pyrite to manufacture pins, earrings, brooches and mirrors, and it continues to be employed in jewellery-making across the world to the present day.

Marcasite, is a polymorph of iron pyrite. Polymorphs are minerals with identical chemical formulae but which conform to different crystal systems. In this case, marcasite [FeS₂] conforms to the orthorhombic system rather than the cubic or octahedral systems found in pyrite. It is known as white iron pyrite and, as its name suggests, is lighter in colour than pyrite and more brittle due to its different habit. Oddly, marcasite is unsuitable for the making of jewellery as it reacts more readily under humid conditions to form sulphuric acid and iron (II) sulphate, together producing a white powdery deposit of the mineral melanterite [FeSO₄·7H₂O] in a process known to geologists as pyrite decay. Pyrite decay can be slowed by ensuring specimens of marcasite are kept at less than sixty percent humidity. Despite this failing, gem quality iron pyrite is still referred to as marcasite in the jewellery world and the material is making a comeback. This is from an article concerning a seminar held in India reported in the jewellery trade publication Diamond World:

Cubic Crystallisation by Ornella Iannuzzi. Pyrite set in black & gold rhodium
Image by Simon Armitt

On The Rock by Ornella Iannuzzi. Pyrite and matrix set in vermeil
Image by Simon Armitt

Magnum Opus in Crucible by Ornella Iannuzzi. Pyritohedral Pyrite set in silver
Image by Simon Chapman

Folklore and Medicine

The civilisations of Meso-America (Aztec, Inca, Mayan, etc.) were renowned for polishing large slabs of pyrite to employ as mirrors which could be used in scrying, or crystal gazing, the magical practice of divining the course of future events by the power contained within crystals.

Native North American tribes also employed pyrite as mirrors, in ceremonies and for medicinal purposes, and believed that by peering into polished specimens they could see into a person’s soul.

The Ancient Chinese deemed that pyrite would guard against crocodile attack. In addition, they viewed the Earth as being a golden cube, a form which pyrite reproduces perfectly.

A new-age movement with a belief in the innate power of crystals, like the alchemists and magicians who preceded them, imbue pyrite with a number of useful qualities. The following is taken from a web page on The Metaphysical and Healing Properties of Minerals:

Pyrite is also thought to enhance the flow of energy between right and left brain hemispheres and protect against any number of infections and illnesses. It assists one in seeing behind the façades of others and promotes an understanding of that which lies beneath words and actions. It can be used to stimulate the powers of the intellect, enhancing memory and providing for recall of relevant information, when required. Pyrite also encourages and sustains the flawless ideal of health, intellect, and emotional well-being. It symbolized the warmth and lasting presence of the sun and promotes the recall of beautiful memories of love and friendship.

Medicinally, pyrite became known as the Healer’s Stone, and it is thought to offer physical aid in treating a wide range of afflictions including infections, viruses and fevers, blood disorders, it increases blood flow to the brain and improves circulatory system, increases memory, bone and cellular formation, helps fatigue, lung problems, digestive tract problems, relieves anxiety and stress.

Pyrite in the Tees Valley

Locally, pyrite may be found in several rock units of Jurassic age, most commonly within Lower Jurassic (Lias Group) strata. The Redcar Mudstone Formation contains a unit dubbed the Pyritous Shale, within which pyrite is commonly found as a replacement mineral in fossil specimens.

The Whitby Mudstone Formation (formerly Upper Lias) is perhaps most noted for the presence of pyrite locally. The formation is divided into three broad informal units, the Grey Shale, Bituminous Shale, and Alum Shale Members. Aggregates of several centimetres, and octahedral crystals can be found locally within the Bituminous Shale around Port Mulgrave, Runswick, Kettleness, and Sandsend. In spite of the presence of much pyrite in the scar nearby, it seems that the village of Goldsborough derives its name from Norse roots, viz.; “the burgh (fortified manor or hill) belonging to Golda”.

Octahedral pyrite (outlined) in the scar at Rosedale Wyke, Port Mulgrave, North Yorkshire.

The Alum Shale Member also contains pyrite, though it is generally less visible being present in the form of framboidal aggregates. These comprise many tiny spherical masses of pyrite particles produced through the action of pyrite-producing bacteria which ‘fed’ on iron and ferrous sulphate ions in the unconsolidated sediment. Pyrite’s presence was essential to the success of the local alum trade, assisting in the alum making process. During calcination (heating) of the quarried shale, iron di-sulphide is broken down via an exothermic reaction which increases the overall heat involved. Some of the sulphur is lost as sulphur dioxide, the remainder takes part in reactions which eventually form sulphuric acid and ferrous sulphate. Sulphuric acid, in turn, breaks down the alumino-silicates within the shale, rendering them soluble and available to recrystallise into alum.


The Cleveland Ironstone Formation locally contains pyrite. In the East Cleveland area, around Skelton, a thin bed of pyritous material, dubbed the Sulphur Band, caps the Main Seam. This was occasionally recovered and sent to Teesside to be used by the growing chemical industry in the production of synthetic sulphuric acid.

Following abandonment of the ore-field, in the 1960s, pumping of the ironstone mines was curtailed causing them to gradually fill with water. Contact with water encourages the breakdown of both siderite (FeCO₃) and pyrite, the mine-water becomes saturated and on exposure to air precipitates ochreous iron oxide imparting a bright orange-red colour. This has caused severe problems in the area, not least at Eston and New Marske. The beck running through Skinningrove was formerly heavily polluted. So much so that the area became known locally as Red-River Valley for many years until filtration to remove the pollutant was employed. Saltburn Beck, running through Saltburn Gill, has been suffering from mine-water pollution for many years, since ochreous water burst out of workings connected with the former Longacres Mine near Skelton. Ochreous iron oxide has smothered life in the stream which will (however) recover naturally once the pollution is successfully dealt with.

The source of mine-water pollution above Saltburn Gill, Cleveland.

Ant trapped in amber. Picture by Mila Zinkova.

The Ancient Greeks named amber electrum, from to its ability to attract small particles when rods of this material were charged by rubbing them against wool. It was also looked upon as being the tears of the Sun, was highly prized and thought to possess incredible powers.

Pliny the Elder (AD23 – 79) was the first to record the presence of insects trapped within amber. He deduced from this that amber must once have been mobile and so named it succinum, or gum-stone.

In the past, the word amber was also used to refer to ambergris, a prized oil of a similar colour to some amber, which is produced by the Sperm Whale, and was first used with respect to fossil resin c.1400.

Amber forms when trees become incorporated into sediments which later become more deeply-buried. The increase in temperature and pressure associated with this turns the resin first into a substance called copal. If these conditions are maintained for extended periods volatile terpenes are driven off to leave amber.

Box made of Amber. Picture by Dariusz Beigacz.

It comes in a variety of colours. Baltic amber is generally a shade of yellow-orange-brown and translucent, though blue and red varieties are also found. It exists in rocks across the globe, primarily in strata of Cretaceous age or younger.

The Baltic area is the World’s greatest source of amber. Dredging, diving and collection by hand are all employed here to collect this semi-precious material. Mining, both open-cast and underground methods using bell-pits or galleries, is also carried out, especially in The Dominican Republic.

Amber is soft, easily worked and takes a high polish making it invaluable in the production of jewellery. As a folk medicine, amber was referred to as ‘the balm of Europe’ and used in the treatment of any number of ailments during the Middle Ages.

Other myths involved its presumed association with the lynx, an animal formerly revered in Europe for its keen eyesight and almost supernatural ability to avoid contact with humans. Amber was deemed to be the petrified urine of these mystical animals and referred to as Lyncurium.