- Published: 14 March 2016 14 March 2016
by John Pearce
Microminerals are very small mineral specimens that require some magnification and illumination to appreciate them and see the crystals in detail. Not much magnification is needed, a 10 times or 20 times hand lens could do it, but a stereomicroscope with a magnification of 10 to 60 times would be even better. Small minerals with crystals of 1 to 2mm can barely be seen with the naked eye, but can look wonderful under a microscope (see Fig. 1). Sheer magic!
- Published: 18 September 2015 18 September 2015
I have already covered the exotic Desert Roses that are crystals of Gypsum that have incorporated sand grains into their flower like structure, see issue 59, Spring 2013.
The mineral Gypsum is both a practical mineral and a beautiful one, it often forms as slender wonderful crystals in deserts and mines, we all use this mineral in our every day lives in everything from toothpaste to plaster on the walls of our homes and offices.
Gypsum and related minerals
This is an evaporite class of mineral that is mined the world over on a vast scale, often in massive underground mines in ancient evaporite deposits. It is frequently in these mines that we find the wonderful crystal clusters, forming amongst other massive evaporate beds in which Gypsum can be found. Sometimes the crystals form as very beautiful, delicate, slender needles, they grow this way due to the ease with which Selenite dissolves in water. The word Gypsum actually comes from the Greek gypsos which means plaster and because it was well known from the Quarries of Montmatre in Paris it became known as Plaster of Paris. After mining, the material would be dehydrated, heated, crushed and the resulting powder would be mixed with water and after rehydration could be used for casting and carving. The first recorded use of plaster in this way was by a 4th century Greek sculptor called Lysippus of Sicyon who made the first plaster cast of a human face.
On a practical level Gypsum CaSO4 2H2O is a Calcium Sulphate dihydrate, meaning it has two molecules of water in its structure, it crystallises in the Monoclinic system. It is very soft at only 2.5 on the Mohs scale and it can be scratched with the fingernail.
The name Selenite comes from the Greek selene (moon) and lithos (stone) not to be confused with the moonstone that is a variety of feldspar used in jewellery. The mineral can form as Curved Rams Horn crystals, as fishtail or swallowtail twins, duck bill twins or exhibiting spear or arrow heads in the crystals.
Amongst the earliest recoded uses of Gypsum were as large cleaved fragments of clear Gypsum used in many homes to protect pictures of the Virgin Mary when glass was far to expensive and difficult to form as sheets. At that time the glass industry was in its early days and sheets of clear Gypsum were used to protect pictures of the sacred Virgin Mary and thus it gained the name of Marienglass, protecting the sacred images of the mother of Jesus thathung in most devout Catholic homes.
The massive variety of layered Gypsum can exhibit on cleaved surfaces a distinctive sheen of a Satin like material, the very silky sheen originates from the fine parallel fibres and gives the cleaved material a wonderful inner glow on the fibrous surface, this can be seen in one of the pictures associated with this article, this variety is given the apt name of Satin Spa.
This is another name given to the massive form of Gypsum when no layering can be seen, this is anhydrous Calcium Sulphate, CaSO4, and this mineral belongs to the orthorhombic class of crystal system and it has a hardness of 3.5. When this massive form is exposed to circulating ground waters then the Anhydrite absorbs the water and rehydrates to turn into fibrous Gypsum, as this happens, it also expands in the process and this distorts and crumples pre existing layering that is typical seen in many associated evaporites.
continued on page 20
Gypsum is the name used when referring to massive layered deposits, the very beautiful crystals of translucent Gypsum are usually given the name Selenite; this mineral comes in a variety of shapes and sizes and can be found amongst the evaporite deposits from mines such as Poland’s famous salt mines.
Good crystals are also frequently found in desert environments in evaporite deposits formed in the vicinity of dried up lakes and pools in the deserts such as in Australia.
A recent beautiful find is the Golden translucent Selenite crystals from the now abandoned mine of Salinas de Otuma, Paracas, Pisco; Ica, Peru, crystals seem to have an inner glow and are highly prized by collectors.
Selenite can be found in a variety of colours from Black, Brown, Orange, White, Yellow and Green depending on small amounts of additional elements, some crystals even have several colours in the same specimen.
Many of you will have seen the images of the spectacular giant crystals found in a cave in the Naica Mine, Chihuahua, Mexico; this is a spectacular large cave found in the mine and preserved in a pristine environment, here the vital temperatures and humidity are kept at consistent levels to preserve this find. One giant crystal weighs 55 tons and at 11 metres long is calculated to be 500,000 years old.
In a totally different occurrence for Gypsum, when deep water clay deposits are found near to the surface, normal weathering leaches out Sulphur from included Iron Pyrite and this then mixes with Calcium from adjacent fossil shells, the combination of elements produces perfectly shaped transparent to translucent Selenite Crystals that grow in the soft clay.
A good example of this is foundin the Cretaceous Gault Clays of the English Coast, exposures of this type of mineral growth can be found near to Dover Harbour, good Ammonite fossils can also be found here with very fine Selenite crystals growing on the fossil surface.
Gypsums practical uses
The main use for the mineral Gypsum is as plaster and plasterboards used in the building industry. Also related evaporite minerals such as Polyhallite are used and this has the addition of Potassium and Magnesium in its structure, it is used in the Agricultural and Chemical industries. This evaporite is referred to as Potassium Alum and has both Potassium and Aluminium in its makeup, this is extracted together with other related evaporite minerals at the Boulby Mine in Yorkshire.
- Published: 18 September 2015 18 September 2015
By John Betterton
Thorium, Th, is a relatively rare metal in the Earth’s crust with an average abundance of roughly 5.6 ppm. World production currently stands at around 5000 tonnes per year with Brazil, India, Turkey, Australia and the USA the major produces. The monazite group of phosphate minerals are the current source of the world’s supply of this element. Most of the older uses for this element have been discontinued due to its radioactivity and is still used gas tungsten arc welding. Other uses are in heat-resistant ceramics, quality lenses, catalysts, themionic emitting devices, scientific equipment and as a possible source of nuclear fuel. Most of its minerals can be found among the oxides and silicate groups.
Thorianite, ThO2, is a common accessory mineral found in granitic pegmatites, carbonatites and serpentinites and in various detritial heavy mineral concentrates. It is associated with thorite, zircon, ilmenite, cassiterite, allanite group minerals, diopside, spinel and phlogopite. The mineral occurs as rough cubes and as rounded grains. Twinned crystals are frequent. It has a poor cleavage and an uneven to subconchoidal fracture. The mineral is brittle with a hardness of 6.5 to 7 when unaltered. Specific gravity is high at 9.9 and is radioactive in nature. It is black, brownish black, dark grey, to reddish brown in colour and may have a bronzy tarnish in some examples. The lustre is variable from submetallic when fresh, altering to resinous or horn-like with a grey to greenish grey streak. Thorianite is opaque and transparent only in thin fragments. Dark colourations, high specific gravity, radioactivity and mineralogical environment contribute to its identify. This species is widespread with numerous known locations in Madagascar, South Africa, Russia, USA, Canada, Brazil, China and in many other countries.
Thorutite, (Th,U,Ca)Ti2((O,OH)6, is a monoclinic species that is always found in the metamict state and possesses a conchoidal fracture with a hardness of 5 to 6. The specific gravity is around 5.6. It occurs as short prismatic crystals up to about 2 cm in size. This mineral is translucent with a black colour that is brown on thin edges. The streak is pale brown with a resinous lustre. Thorutite occurs in veins syenite with microcline, thorite, zircon, calcite, barite and galena. The above cited physical and chemical properties together with x-rays aid in identification. About eight separate locations are know for this uncommon thorium-bearing species and include those in Australia, Brazil, India, Kyrgyzstan, Namibia, Russia, Sri Lanka and the USA.
Tuliokite, Na6BaTh(CO3)6.6H2O, is an unusual mineral compositionally and occurs in hydrothermal veins in syenite pegmatites associated with nepheline, sidorenkite, vinogradovite, villiaumite, microcline, pirssonite, shortite, trona, thermonatrite, natron, natrolite and aegirine. Tuliokite crystallises in the hexagonal system and forms prismatic and rhombohedral crystals up to about 4 mm in size. It is pale to dark grey in colour with a white streak and vitreous lustre.
This mineral is radioactive, brittle and has a hardness of 3 to 4. Specific gravity is 3.1. The bizarre chemistry, radioactivity and restricted number of locations known are enough for its recognition. The single site for this species is the Kirovskii apatite mine, Kukisvumchorr Mt, Khibiny Massif, Russia.
Cheralite, (Ca,Ce)(Th,Ce)(PO4)2, is to be found in carbonatites and disseminated kaolinized pegmatites and is accompanied by tourmaline supergroup members, chrysoberyl, zircon, quartz and huttonite. The mineral is monoclinic, frequently metamict in character and is usually found as subhedral to anhedral massive aggregates. It is translucent and pale to dark green, honey-brown to yellow-brown in colour with a white streak. The lustre is variable from resinous, greasy to vitreous. It possesses several distinct to difficult cleavages, and a poor parting. The fracture is uneven and is brittle. Its hardness is 5 and the specific gravity is 5.2. This species is radioactive. The above cited properties and chemistry help towards characterisation.Specimens can be found in several countries including the USA, Germany, Italy, Finland, the Czech Republic, Austria, Australia and Argentina. Other lesser sites are known for this species.
Thorium molybdates were unknown to exist until 2014, and now we have two species described below from the same locality. X-rays and chemical analysis are best for identification for these two minerals.
Ichusaite, Th(MoO4)2.3H2O, is the first thorium molybdate mineral species to be discovered and crystallises in the monoclinic system. Its habit is as aggregates of thin tabular crystals. The mineral is colourless with a pearly to adamantine lustre. The streak is white and transparent.
A perfect cleavage is present. The calculated specific gravity is 4.2. Ichusaite is found intimately with nuragheite within vugs of quartz veins associated with muscovite and xenotime-(Y).
It has been found as a probable alteration product of molybdenite ore under basic conditions at the Mo-Bi deposit at Su Seinargiu, Cagliari, Sardinia, Italy.
Nuragheite, Th(MoO4)2.H2O, is the second thorium molybdate mineral to be reported in nature. It is orthorhombic and forms minute thin tabular crystals.
This mineral is colourless with a pearly to adamantine lustre. It is brittle with a perfect cleavage. Other data is not known. Nuragheite occurs with muscovite, xenotime-(Y) and ichusaite.
This species occurs as a probable alteration product of molybdenite ore under basic conditions in the Mo-Bi deposit at Su Seinargiu, Cagliari, Sardinia, Italy.
Ekanite, Ca2ThSi8O20, crystallises in the tetragonal system and forms poorly developed pyramidal crystals to 1 cm in size and are usually striated. Several different crystal habits are known along with massive and clustered aggregates.
Distinct to indistinct cleavages are present and the fracture is irregular in form.
This mineral is brittle and radioactive. It is
transparent to translucent, and is colourless, light to dark green, yellow-green, yellowish-brown, straw yellow or dark red in colour.
The streak is white and the lustre is vitreous. Its hardness is 4.5 to 5 and the specific gravity is 2.9. Chemistry, x-rays and physical properties are used for characterisation. Ekanite occurs in various geological environments including syenitic rocks, volcanic ejecta and as a detrital species. Fluorite, garnet supergroup members, quartz, microcline, clinopyoxenes, apatite supergroup members, plagioclase, hematite, zircon and titanite etc commonly accompany it. Specimens of this gorgeous mineral have been found in Sir Lanka, Burma, Canada, Italy, Germany, Portugal and USA.
Huttonite, ThSiO4, can be found in beach sands with scheelite, cassiterite, uranothorite, ilmenite, gold, cheralite, augite, and ningyoite. It occurs as flattened anhedral grains that are colourless, cream to pale yellow green. It is transparent to translucent and fluoresces dull white with a pink tinge under SW UV light. Distinct to poor cleavages are present with a conchoidal fracture.
The hardness is around 4.5. and the specific gravity is high at 7.1. It is radioactive in nature. The above physical properties together with x-rays are helpful to distinguish from the dimorph thorite.
A wide variety of locations are known for huttonite with New Zealand, Australia, Germany, Finland, Poland, Italy, Czech Republic, Sweden, USA, Russia among the best recognized.
Thorite, ThSiO4, is the dimorph of huttonite and crystallises in the tetragonal system but is commonly metamict. It generally is found as square
prisms or as pseudo-octahedral crystals to 8 cm in size. Massive and compact aggregates also exist. The mineral is nearly opaque and is yellow-orange to brownish yellow in colour with a light orange to dark brown streak. Its lustre is vitreous, resinous to greasy and is transparent in thin splinters. A distinct cleavage is present and the fracture is conchoidal in form. It is a brittle mineral with a hardness of 4.5 to 6.
The specific gravity is high between 6.6 and 7.3.
Thorite is paramagnetic and radioactive in nature.
The above physical properties and x-ray analysis are used to identify it from huttonite.
It occurs in as an accessory speciespecies in fesic rocks and associated pegmatites with zircon, monazite, gadolinite, fergusonite group minerals and uraninite etc. Good samples have been obtained from the following nations: Norway, Finland, Germany, Sweden, Austria, Czech Republic, France, Italy, Madagascar, Argentina, USA, Canada, Australia, Brazil, China, Japan, Russia, Kazakhstan, Namibia and South Africa.
The element silicon and its numerous and abundant minerals will be described in the next series of future articles starting with the silica minerals.
- Published: 27 March 2015 27 March 2015
By John Betterton
Sulfur, S, occurs at about 260 ppm in the Earth’s crust and its various minerals are very diverse and some are abundant.
World production currently stands at exceeds 70,000,000 tonnes per year. Most of it is recovered from fossil fuels from leading nations such as the USA, Canada, Russia, China, Saudi Arabia etc.
The industrial uses for sulfur are vast and range from vulcanisation of rubber, sulphuric acid, sulfur trioxide, numerous other chemical and pharmaceutical compounds, fertilizers, rust removal, explosives, fuels, paints, bleach papers, detergents, many organic compounds etc.