These are the minerals whose composition includes silicon bound to oxygen. Considering that these two elements alone make up 75% of the Earth’s crust, it is not surprising that this family is the most abundant (25% of known minerals) and one of the most complex. The chemical formulas of silicates are often quite close, and to classify them, structural data had to be introduced. The starting point is a structural unit formed of one atom of silicon surrounded by 4 atoms of oxygen and whose positions are represented by a geometric shape of 4 faces in isosceles triangles: this is what we call the “SiO4 tetrahedron”. In silicates, the presence of foreign ions modifies the arrangement of the bonds between the tetrahedrons. Based on the how the tetrahedrons are arranged among themselves, we can define several sub-families. Numerous foreign anions can be present: the hydroxyl ion (OH-), halides (F-) and (Cl-), sulfates (SO42-) and carbonates (CO32-), etc.
Silica is a silicon oxide. It should normally be classed with oxides, but can also be classed among the silicates as silica is formed entirely from tetrahedrons that are bound according to a dozen different structures depending on temperature and pressure: quartz, tridymite, cristobalite, stishovite and coesite. Only quartz exhibits spectacular specimens, the other varieties being in general microscopic. In fact, the term quartz includes two minerals of different geometrical structure: alpha quartz, stable at low temperatures, and beta quartz which remains stable at temperatures exceeding 573°C. All of the quartz crystals exhibited at the MIM have an alpha quartz structure, but the crystals initially formed at temperatures exceeding 573°C exhibit faces that tell of an ancient beta symmetry. Quartz that has formed at lower temperatures does not present these faces. Quartz is the main component of sand: small grains of sand are in fact tiny eroded crystals. It is an essential raw material for making glass, cement and silicon (the basic material of all modern electronics). Quartz has always been found in large, colorless crystals in the shape of hexagonal prisms with pointed ends (rock crystal) and has been used since the Fatimid era to carve magnificent dishware (ewers) and jewelry, either in violet (amethyst) or smoky-colored crystals.
The collection possesses a selection of very high-quality quartz from Brazil, in particular the most beautiful twinned crystal ever found, called Belovda, and resembling a Japan-Law twin but with a different angle. There is also exceptional quartz from France, Switzerland, the United States (a rare twin of smoky quartz), Russia and China; amethysts from Mexico (also a rare twin), Brazil, Namibia, etc. Quartz also appears in the form of “twisted” crystals called gwindel, well-represented in the collection.
Finally, the collection exhibits a beautiful opal from Australia: a type of hydrated silica made up of tiny joined balls of silica, either amorphous or only slightly crystallized. When the beads typically reach a diameter of between 0.2 and 0.3 microns and are well-stacked, the mineral gives an amazing multi-colored optical effect that changes according to the direction of observation.
The term neso is derived from the Greek word for isle. Nesosilicates are characterized by tetrahedrons (SiO44-) that have no bond between them, as these bonds are made with foreign ions. This is why we find the notation SiO4 in their formula. Such is the case of peridot, for example (Mg,Fe)SiO4, which gets its name from the Arabic فريدة even if this mineral is always called زبرجد in that language and is cited as such in a Hadith:
أجاب الرسول: الكوثر نهر يجري تحت عرش الله, ماؤه أشد بياضا من اللبن, وأحلى من العسل, وألين من الزبد, حصاه الزبرجد والياقوت والمرجان, حشيشه الزعفران, ترابه المسك الأذفر, قواعده تحت عرش الله عز وجل
…God’s messenger answered: “The Kawthar is a river that flows under the throne of God. Its water is whiter than milk, sweeter than honey, softer than foam; its stones are peridot, ruby and coral, its grass is saffron, its sand is fragrant musk. Its foundations are established under the throne of God Almighty.”
This sub-family accounts for around 5% of known mineral species. We find in its ranks several major mineral constituents of certain rocks. Such is the case for the olivine family, which includes forsterite, peridot and fayalite and makes up most of the rocks called peridotites. (Nowadays, it is thought that the rocks of the Earth’s mantle situated under the crust are for the most part peridotites.) The garnet family (البجادي ) includes some fifteen species, some of which are typical in certain rocks. For example, almandine garnet forms in eclogite and in mica-schist, while spessartine garnet forms in pegmatite. Zircon, which gets its name from the Arabic زرقون (signifying vermillion) and derived in turn from the Persian زرگون signifying the color of gold, is found in a great variety of rocks. It is a very stable mineral that resists all forms of erosion and Earthly transformations, to the point that some crystals are among the oldest known to exist (more than 3 billion years), thus revealing precious information on the history of our planet. Peridot, garnet and zircon are also used as prized semiprecious stones when they are sufficiently pure and colorful. And today zircon has become the essential raw material for making high-performance ceramics, special types of steel and alloys for the nuclear industry. As a general rule, nesosilicates belong to high-symmetry crystalline systems. Their hardness and a high refractive index make them beautiful stones for jewelry when they are transparent.
The collection possesses a number of specimens of high-caliber nesosilicates. An ensemble of high-quality garnets can be found, among them grossulaires from Quebec and Mali, spessartine from Pakistan as well as rather esthetic specimens from China and amazing ones from Brazil. Of those specimens that stand out for their perfection or their dimensions, we need to mention cuprosklodowskite from the Congo, one of the best ever found; one of the most beautiful euclases known in an amazing blue-green and facetted by nature like a carved precious stone; a rare green zircon twin from Sri-Lanka, partially transparent; a large zircon from Australia that is 732 million years old; and one of the most beautiful green sphenes from Brazil (the word sphene comes from the Greek for “corner”, origin of the Arabic سفينة and إسفين), etc.
The range of topazes is representative of the different qualities presented by this mineral. We can find the superb imperial topaz from Brazil that is much appreciated in jewelry; blue topaz from Russia and Brazil (of which one of the most beautiful, in an amazing blue and perfectly crystallized weighing 25,000 carats!); amazing champagne-colored topaz, perfectly limpid, from Pakistan (unfortunately, the color is unstable in daylight and therefore must be exhibited under light poor in ultra-violet rays).
The term soro comes from the Greek word for mass or cluster. In this sub-family we find SiO4 tetrahedrons bound by pairs in a common peak, thus forming Si2O7 groupings or more complex groupings of 3 or 5 tetrahedrons. Additional isolated tetrahedrons may also be present.
This sub-family represents around 3% of mineral species.
Sorosilicates offer up few specimens worthy of being exhibited in a museum. Among them we find hemimorphite (a zinc silicate), which looks a lot like smithsonite (zinc carbonate), and these two minerals are often grouped together by miners under the name calamine. Of the known minerals in this family we find epidote, a rather typical mineral in certain rocks found in fissures in alpine-type mountain ranges (the Alps, the Andes and the Himalayas) and in the form of rather beautiful and brilliant crystals. The mineral is highly coveted by collectors. The same is true of vesuvianite, which produces a few very colorful varieties, and of axinite (sometimes grouped with the cyclosilicates).
The collection possesses several highly interesting sorosilicates, both from an esthetic and a mineralogical point of view. Esthetically speaking, epidote is well represented by a classic piece from Austria and more recent, and equally esthetic, specimens from Pakistan. Vesuvianite specimens from Quebec reputed to be the most handsome are represented by very high-caliber pieces, and those from Italy by a classic piece of perfect geometric proportions. Zoisite, similar to epidote, is known through one of its varieties, tanzanite, which is one of the most fascinating minerals by virtue of its 3 colors depending on the axis of observation (blue, violet and red), and has thus become quite popular in jewelry-making. The collection possesses a series of high-quality tanzanites, including a free-form crystal that is enormous for its species. There is also a large axinite specimen from Russia. From a mineralogical point of view, the most important species is by far chevkinite, a rare species with well-defined crystals, which to date has been found in only one location in Pakistan in beautiful specimens of which the collection conserves two of the best ever found.
The term cyclo comes from the Greek work for ring. In this family, the SiO4 tetrahedrons are organized in rings, Si3O9, Si4O12 or Si6O18 (rings with 8, 9 or 12 atoms of silicon are also known but rarer).
This sub-family accounts for around 2% of mineral species.
Few cyclosilicate species are exhibited in the collection. However, these minerals are rather attractive and come in a great number of varieties. The most prestigious species is beryl, of which the most popular varieties in jewelry are blue (aquamarine), green (emerald), pink (morganite), colorless (goshenite), yellow (heliodore) or red. The opaque varieties are used as beryllium ore, a light and very resistant metal used in aeronautics and in space travel and that is an extremely rare element on Earth. There are 3 tons of beryllium for every million tons of Earth’s crust, which makes finding beautiful crystals within it nothing short of miraculous. Two of the eleven species of the tourmaline group –elbaite and liddicoatite—are known for their wide range of colors; they are sometimes qualified as “rainbow” minerals. Mineralogists and specialists in precious stones have agreed on different names for differently colored minerals: rubellite (red), verdelite (green), indicolite (blue), and achroite (colorless: paradoxically the rarest of varieties). The other tourmaline species generally produce dark or black stones like that of schorl, which is by far the most frequent species in this group.
The collection possesses an ensemble of cyclosilicates of international fame: beryl (one of the most beautiful collections of aquamarine with spectacular pieces from Pakistan, including a large bouquet of some fifty crystals of rare beauty; some of the most handsome morganites from Brazil and Afghanistan; a series of emeralds from Colombia, too light in color to be carved, which saved them from the chisel, but nonetheless in spectacular green, etc.). The collection also exhibits one of the best series of a recent and very rare species related to the beryl group: pezzottaite. Tourmaline is also very well represented: the most beautiful rebullites from Brazil; verdelite and indicolite from Afghanistan; liddicoatite from Madagascar, with complete crystals but also in fine, polished pieces showing the incredible geometry at the heart of these crystals that are only found in few deposits on that island.
In addition to beryl and tourmaline, there are other spectacular cyclosilicates like dioptase and benitoite. Dioptase’s deep green color makes it one of the most sought-after minerals. The collection possesses one of the most beautiful specimens to have ever come out of the famous Tsumeb mine in Namibia, with a brilliant contrast between the green of the crystals and the white of the rock on which they developed. Sheets of benitoite crystal associated with neptunite on white rock are considered to be the best ever known for this species and have been found in only one location in the United States.
The term ino is derived from the Greek word for fiber. In this family the tetrahedrons organize themselves in chains of SiO3, [SiO3]2, [SiO3]3 or more (the absolute record for natural minerals is the chain [SiO3]12). Tetrahedrons can also organize themselves in ribbons resulting from condensation of two chains. The most frequent ribbon is Si4O11. There are also more complex arrangements in multiple chains, columns, tubes, etc.
This sub-family represents around 4.5% of mineral species. It consists of two groups of minerals essential in the composition of rocks: pyroxenes or tetrahedrons arranged in chains (more than 20 known species), and amphiboles or tetrahedrons organized in ribbons (more than 65 species known). These minerals can be so abundant that certain rocks have been baptized pyroxenites and amphibolites. All the minerals in these two groups have fibrous varieties, collectively called asbestos. Asbestos exhibits amazing mechanical and thermal properties long exploited in industry until it was discovered that it seriously damaged the lungs of people who had breathed in the fibers.
The most beautiful examples of inosilicates in this collection are kunzites from Pakistan and Brazil. (Kunzite is the pink and transparent variety of a pyroxene that is normally stony, called spudomene, a lithium ore used in modern batteries.) The collection possesses several high-caliber pieces, one of which in association with a magnificent morganite from Afghanistan. Other excellent examples of inosilicates are arfvedsonite from Malawi; ferro-actinolite from Pakistan (from a one-time discovery that yielded few samples); a beautiful green diopside from Tanzania; and less well-known species such as serandite from Quebec, inesite from South Africa and China and hedenbergite from Sweden.
The term phyllo comes from the Greek work for leaf. Indeed, in this sub-family the tetrahedrons are organized in leaves, or sheets, for which the anionic group can be written as Si4O10. It includes important minerals such as those from the mica, clay and serpentine groups.
This sub-family represents around 6.5% of mineral species.
Micas --more than 40 known species— are essential minerals in certain rocks. Some of them, such as muscovite, can be found in abundance in well-formed crystals that can reach or exceed a meter in size and are often used in industry as insulators of heat or electricity. Because of this, they are not very popular with collectors, which is a mistake because the truly beautiful specimens are quite rare. Clays are in general composed of microscopic crystals, making them a poor choice for a collection focused on mineral esthetics. The serpentine group is a vast family of minerals that make up the soft and often green rocks used for making
تنّور type ovens or cheap sculptures. The leaves in this last family are not always flat: they can be undulating (like in sheet metal) or folded back like rods, in which case we get asbestos. However, we do find rarer and much sought-after minerals in this family as well as other beautiful varieties like chrysocolla, cavansite and apophyllite.
The collection possesses some exceptional examples of phyllosilicates. One of the rarest is pyrosmalite from Sweden in perfectly hexagonal prisms. In the realm of mineral esthetics, we should mention one of the best chrysocolla specimens from the Congo. Apophyllites are very well represented with specimens from the rich deposits in India. Green-colored samples are particularly popular, the most spectacular consisting of two balls of green crystal on a white gangue of stilbite. A cavansite specimen in deep blue contrasting with white stilbite also comes from India.
The term tecto comes from the Greek word for structure or framework. In this family all the SiO4 tetrahedrons are linked to one another by common oxygen bonds in such a way that they constitute a framework of tetrahedrons throughout the space. A great many tectosilicates are also aluminosilicates, meaning that a more or less large part of the silicon atoms have been replace by aluminum atoms. (We remember that aluminum is the third most common element in the Earth’s crust after oxygen and silicon, these three elements together making up 82% of the crust.) Thus, we find in their chemical formula symbols of the type (Si,Al)O2. As aluminum is a 3+ ion and silicon a 4+ ion, the imbalance in the charges is compensated for by other metal ions.
This sub-family accounts for around 4% of mineral species. It includes, among others, the feldspar, feldspathoid and zeolite groups. Feldspars form a family of 16 species, some of which are the most important constituents of the principal magmatic rocks of the Earth’s crust, like granite and basalt for example. Extremely abundant, feldspar crystals often take on a stony aspect that renders them less attractive. In industry they constitute the raw material for making porcelain. The feldspathoid family sets itself apart by a percentage of aluminum that is more elevated than in feldspar. In this group we find some colorful minerals that are much sought-after such as lazurite (the main component in lapis lazuli), afghanite and sodalite. Zeolites form a very large family with more than 80 species. Their name comes from Greek for “stone that boils” because they lose their water of constitution when heated. They are fragile minerals that form quite easily when a rock (often basaltic) is “washed” by hot water. The great basaltic expanses like those in Iceland and the Deccan peninsula of India are rich in geodes covered in zeolite crystals. In industry, natural or artificial zeolites are used as filters to purify and soften water, and as support for catalysts, etc., thanks to their internal structure. As a general rule, the hardness of tectosilicates is mediocre and their density is relatively low.
The collection exhibits a representative sampling of feldspar: transparent orthoclase from Madagascar, one of the best found; amazing amazonite (blue-colored microline) with smoky quartz from Colorado (USA); an enormous blue-green microline from Brazil; and hyalophane from Bosnia. Feldspathoids are also represented by lazurite and afghanite from Afghanistan. One can also discover minerals not belonging to the families cited above, such as some very handsome danburites from Mexico and an excellent helvite series from China.