Lets Talk Gemstones

By Edna B. Anthony, Gemologist

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The Silicates

An Introduction

In the earlier articles written on andalusite, kyanite, and sillimanite, (polymorphic forms of the aluminosilicate group of the Nesosilicates), I made the observation that few people were aware of the relationships between so many gemstones. Only now am I beginning to grasp the complexity of some of these relationships. I find it much easier to remember important information if I fully understand the underlying structure wherein these relationships exist. In my gemology training, we did not have the time to explore these structures, nor comprehend how a gemstone fitted into the overall picture.
 
My acquisition of an old textbook, The Manual of Mineralogy, after J. D. Dana, 19th Edition, published by John Wiley & Sons, authored by Cornelius S. Hurlbut, Jr. and Cornelus Klein has been one of the best investments I have made. Its systematic organization and concise explanations will influence every article that I write. For this reason, I will begin with its basic definition of a mineral. "A mineral is a naturally occurring homogeneous solid with a definite (but not generally fixed) chemical composition and an ordered atomic arrangement." The orderly arrangement of the atoms of interacting elements create the multitude of earth's minerals.
The Manual of Mineralogy states that "--- the earth's crust can be regarded as a packing of oxygen ions with interstitial metal ions, such as Si4+, Al3+, Fe2+, Ca2+, Na+, K+, etc. The dominate minerals of the crust are thus shown to be silicates, oxides, and other compounds, such as carbonates." For the purposes of this article, the discussion herein will be limited to the silicates.

Silicates comprise about a quarter of the known minerals and almost 40% of the common ones. The basic unit of structure of all silicate crystals is the tetrahedron. There are four oxygen atoms, one located at each apex of a regular tetrahedron. A single silicon atom is located at the center of the tetrahedron. This silicon atom has a valence charge of 4, meaning that it is looking to acquire four electrons through sharing with other atoms to complete its outermost energy shell, known as the valence shell.

An oxygen atom has two electrons in its outermost shell that are available to bond with the silicon atom. If four oxygen atoms surround one silicon atom, where each oxygen atom offers one electron, then the silicon atom's outermost shell will be complete and stable. The resulting arrangement comprises a silicate molecule. One electron remains, allowing those oxygen atoms to search for another silicon atom to share an electron and form another tetrahedron. Tetrahedrons are linked together through oxygen bonds.

The arrangements of links between the basic tetrahedral units determines the classification of the silicate. When the tetrahedra are not linked together, as each exists in isolation, the material is classified as a Nesosilicate. If groups of two tetrahedra are linked together, the material is then classified as a Sorosilicate. If all of the tetrahedra link back onto each other to form a closed ring, then the material becomes a Cyclosilicate. These arrangements represent three of the classes of silicates.

The linking of the tetrahedra and the incorporation of other elements, such as aluminum, zinc, magnesium, iron, beryllium, calcium, manganese, sodium, and titanium, into these structures, create the multitude of mineral species of the six classes of silicates. Only relatively few of these species are suitable for use as gemstones, but a greater number are cut or faceted for collectors. Their crystals are often of great beauty and highly prized.

Nesosilicates or Orthosilicates

Isolated tetrahedra are classified as Nesosilicates. These SiO4 tetrahedra, connected to each other only by ionic bonds from interstitial cat-ions of other elements, are called Orthosilicates or Nesosilicates. (This sentence is a paraphrase of information found in the aforementioned Manual of Mineralogy.)
 
Here, the tetrahedra are held together by other charged atoms that exist in the voids between the silicon and the oxygen atoms in the host tetrahedral lattice. Ions are atoms that have either gained electrons, called an an-ion, or lost electrons, called a cat-ion. Whenever an electrically charged atom is near another such charged atom, there will be forces of attraction or repulsion between those atoms. In the case involving the isolated tetrahedra, the charged atoms of the crystal voids or interstitial ions subsequently attract each other. This sets in motion the associated tetrahedra to attract together. The metal ions of aluminum, zinc, magnesium, iron, beryllium, calcium, manganese, sodium, and titanium comprise the ions in the voids.
The nesosilicates include datolite, sphene, (titanite), zircon, phenakite, the aluminosilicates, olivine (peridot), garnet, and the humite groups.

Datolite [CaB(SiO4)(OH)] contains SiO4 and B(O,OH)4 tetrahedra. The sphene structure involves SiO4 tetrahedra with CaO7 and TiO6 polyhedra. Zirconium is the connecting element in zircon [ZrSiO4]. Aluminum takes the role in the aluminosilicate group (staurolite, topaz, and the polymorphic Al2SiO5 gems andalusite, kyanite, and sillimanite). Beryllium in phenakite [Be2SiO4] and zinc in willemite [Zn2SiO4] are "connectors" in the phenakite group. The gemstone peridot is a member of the forsterite [Mg2SiO4] to fayalite [Fe2SiO4] solid solution series (olivine), where magnesium replaces iron.

It should be noted that in a lower temperature H2O saturated environment, the minerals of this series have frequently altered to serpentine minerals of the phyllosilicate class. Garnets, which also undergo alteration to serpentine minerals, include the subgroups known as pyralspite, ugrandite, and hydrogrossularite. These several species yield a wide range of gemstone varieties. The four species of the humite group, humite, clinohumite, norbergite, and chondrodite, are less known as gemstones.

The garnet, olivine, and humite groups involve additional elements in their composition, as do topaz and staurolite of the aluminosilicate group, and are the more complicated nesosilicates.

Sorosilicates

Double SiO4 tetrahedra that share a single apical oxygen atom to create the Si2O7 groups distinguish the Sorosilicates. Here, two silicon atoms and seven oxygen atoms are present, where both tetrahedra share one oxygen atom. Most of the more than seventy known sorosilicate minerals are rare. Hemimorphite, lawsonite, idocrase (vesuvianite), and the epidote group yield gemstone material.
 
Independent SiO4 tetrahedra, as well as the double tetrahedra, are found in the epidote structure. This group includes epidote, allanite, piedmontite, hancockite, clinozoisite, and zoisite (tanzanite, thulite, and green zoisite). Calcium, aluminum, zinc, iron, manganese, magnesiumand, sometimes rare earths, are often incorporated in the sorosilicate structures. Hydrogen, in the forms of OH and H2O, appears in some of the minerals.

Cyclosilicates or Ring Silicates

Cyclosilicates or SiO4 tetrahedra joined into rings can take three configurations. Benitoite [BaTiSi3O9] is the gem mineral of the simplest ring configuration. The axinite group has a more complicated form. The most complex ring, Si6O18, constitutes the framework for dioptase, the beryls (aquamarine, emerald, golden beryl, goshenite, morganite, and red beryl), the polymorphs indialite and cordierite [(Mg,Fe)2Al4Si5O18], and the extremely complex tourmaline group. The nine species of tourmaline (elbaite, liddicoatite, dravite, chromdravite, ferridravite, tsilaisite, buergerite, uvite, and schorl) all differ widely in chemical composition. However, they all share, essentially, the same crystal structure.

Inosilicates

The Inosilicates consist of two related structures. When single tetrahedra share an oxygen and a link together to form a single chain, this forms the basic structure for the pyroxenes. The sodium pyroxene group is composed of jadeite, (NaAlSi2O6), aegirine (NaFe3+Si2O6), and spodumene (LiAlSi2O6), where lithium replaces sodium. The enstatite-orthoferrosilite series is a solid solution series wherein increasing amounts of iron replace magnesium. This group also includes enstatite [MgSiO3], bronzite, hypersthene, ferrohypersthene, eulite, and orthoferrosilite [FeSiO3]. The silky sheen of bastite (a pseudomorph of serpentine after bronzite, named for its composition including barium, silicon, and titanium) makes it a suitable gem material for cabochons.
 
Iron is also the element that replaces the magnesium in the diopside [CaMgSi2O6] to hedenbergite [CaFeSi2O6] series, along with its intermediate members, salite and ferrosalite. Various elements, including manganese, zinc and chrome, are all incorporated into the structures. The star and the transparent diopsides are used as gemstones. Chrome diopside is better known than the color varieties alalite, baikalite, malacolite, and violane (purple). Dr. Joel Arem's Color Encyclopedia of Gemstones tells us that alalite is the local name for the fine green crystals from the Ala, Piedmont region in Italy.
Deposits of the green minerals baikalite, malacolite, and chrome diopside are found in Slyudyanka near Lake Baikal in Russia. Slight variations of the single chain structure create the pyroxenoid group. The pectolite to serandite series, where manganese replaces calcium, furnishes cabachon material of various colors that include the lovely blue variety of larimar, found in the Dominican Republic. Very small crystals of pectolite from Asbestos, Canada are the only known facetable gemstone material from this series.

Rhodonite [MnSiO3 + Ca] is a second member of this pyroxenoid group. Rare transparent crystals from Australia and Japan are faceted, but most of the pink to rose-red material is massive to translucent and usually used for beads and carving.

A third pyroxenoid is the abundant rock-forming mineral, wollastonite. Again, the only source of facetable crystals is Asbestos, Canada. Catseyes are cut from fibrous material, but fashioning the stone is complicated, due to its pronounced tendency to cleave.

Double chaining, the second inosilicate structure, produces the amphibole group. Here, side by side single chains of tetrahedra are linked together by a shared oxygen atom. The most familiar gem material from this group is in the tremolite to ferroactinolite series. The compact variety nephrite, [Ca2(MgFe)5(Si4O11)2(OH)2], with its unique "felted" structure, is often confused with the single chain structured jadeite.

The intermediate member, actinolite, is seldom faceted. However, fine catseyes are obtained from the chatoyant fibrous material. In the partial series of glaucophane to riebeckite, the familiar tigereye gemstone is created by the psuedomorphic replacement of crocidolite, the asbestiform variety of riebeckite, by quartz.

Phyllosilicates

Various arrangements of layers of six-fold rings of hydroxyl [OH] bearing tetrahedra create the brucite, antigorite, and pyrophyllite structural forms of the serpentine, mica, clay mineral, and also the clorite groups of the Phyllosilicates. The phyllosilicates form in lower temperature environments than pyroxenes and amphiboles, frequently replacing previously formed minerals through hydrothermal alteration. The four species of the serpentine group (antigorite, chrysotile, clinochrysotile, and lizardite) yield several varieties with the same composition [Mg3Si2O5(OH)4+Ni] but show differing properties.
 
Among the varieties used for ornamental objects, cabochons, and some faceted gemstones are bowenite, williamsite, antigorite, lizardite, ricolite, satelite, pseudophite (stryian jade), verde antique, and connemara marble. A mica group mineral, lepidolite, is used for ornamental objects, such as book ends and paperweights. Perfect cleavage and varying hardness within these crystals make faceting the rare transparent crystals found in Brazil extremely difficult. Of the clay mineral group, pyrophyllite, {Al2Si4O10(OH)2}, used for carvings and cabachons, is a main constituent of the Chinese material known as agalmatolite.
The aforementioned Manual of Mineralogy states that the clorite group is "characterized by its green color, micaceous habit and cleavage, and by the fact that the folia are not elastic." (With the exception of kammererite, ironically, a delicate cranberry red material, I was unable to find any gemstone material specifically identified as a member of the clorite group).

The Manual of Mineralogy does note that apophyllite [KCa4Si8O20(F,OH).8H2O], prehnite [Ca2Al2Si3O10(OH)2+Fe], and chrysocolla [(Cu,Al)2H2Si2O5(OH)4.nH20] are all closely related to and very difficult to distinguish from the clorite group. In apophyllite, "the sheets of tetrahedra are composed of four-fold and eight-fold rings", rather than six-fold rings, "linked by Ca, K, and F ions." Another complicated arrangement of layers of aluminum tetrahedra, as well as the silicon tetrahedra, form the structure of prehnite.

Chrysocolla is a most interesting material for several reasons. It is a gelatinous precipitate that contains an impaired crystalline structure of layers of Si4O10. Thus, its generally amorphous nature calls into question its classification as a mineral. When the gel includes sufficient silica, however, the very fragile substance becomes hard chrysocolla-saturated quartz, very suitable for gemstone use. Its name is derived from its resemblance to the substance, chrysos (gold) kolla (glue), used by the ancient Greeks in soldering metals, much as borax is used today.

Tectosilicates

Most of the silicate gemstone varieties are Tectosilicates. In tectosilicates, a very stable three dimensional framework is created by the sharing of all of the oxygen ions of the SiO4 tetrahedron with adjacent tetrahedra. Four groups, SiO2, feldspar, feldspathoid, zeolite, and the scapolite series, all provide gemstone materials. The SiO2 group includes tridymite, cristobalite, the numerous cryptocrystalline and crystalline forms of quartz, and the amorphous opal. Quartz has the lowest symmetry and the most compact structure of the three SiO2 polymorphs. The related opal has an orderly arrangement of SiO2 spheres. Many varieties of the potassium (alkali) feldspars, the plagioclase feldspars, and danburite of the feldspar group have been and are still used as gems.
 
Again, a quote from the Manual of Mineralogy best describes the complex nature of this group. The feldspar structure "consists of an infinite network of SiO4, as well as AlO4 tetrahedra" with "concomitant housing of Na+ (or K+ or Ca2+) in available voids. In the plagioclase structures, the amount of tetrahedral Al varies in proportion to the relative amounts of Ca2+ and Na+; but not as to maintain electrical neutrality; the more Ca2+, the greater the amount of Al3+". The feldspathoid group exhibits a chemically similar anhydrous framework, containing about two-thirds the amount of silica as alkali feldspar, often incorporating unusual anions, such as S, Cl, CO3, or SO4. The zeolite group consists of very open frameworks of AlO4 and SiO4 tetrahedra.

According to Dr. Joel Arem, very few faceted chabazite [CaAl2Si4O12 . 6H2O} gems of this group exist. Suitable crystals are extremely rare, and its softness makes it inappropriate for wear. Calcium replaces sodium in the solid solution series natrolite, mesolite, to scolecite. This series is the source of most of the gemstone material. Again, its softness, distinct cleavage, and lackluster appearance make its rarity its greatest appeal. A more complete discussion of the numerous varieties of tectosilicate gemstones will be conducted in future articles devoted to the specific groups of the tectosilicates.

Meanwhile, my sense of continuity prompts me to return to writing of the cyclosilicate, "ring", class of the silicates. The previous articles on benitoite and the tourmaline group began the discussions of gemstone material from this class. Articles on the beryl and axinite groups, the polymorphic cordierite and indialite, and dioptase will complete the coverage. I must admit, too, that Nancy Attaway's experience with the orange rough that was purported to be beryl material from Brazil has piqued my curiosity. Wish me luck!