AUGUST BIRTHSTONE-SPINEL
Due to the different elements, different spinels can have different colors, such as magnesium spinel between red, blue, green, brown or colorless; zinc spinel is dark green; iron spinel is Black and so on. Spinels are hard, glassy octahedrons or granules and blocks. They appear in igneous rocks, granitic pegmatites and metamorphic limestone.
Some transparent and beautifully colored spinels can be used as gemstones and some as iron-containing magnetic materials. More than 200 spinel varieties have been created by manual methods.
Introduction
The chemical formula is (Mg, Fe, Zn, Mn) (Al, Cr, Fe) 2O4, the composition is relatively complex, including subfamilies such as aluminum spinel and chrome spinel. Iron-containing spinel subfamilies are opaque magnetite, maghemite, and the like. Most of the spinels are aluminum spinel subfamilies, of which Mg2+ and Fe2+ can be mixed in any ratio.
Crystal system: is an equiaxed crystal system, crystal habit: often octahedral crystal form, sometimes octahedron and rhombohedral dodecahedron, cube into a polymorphic shape. It can be artificially synthesized and has a melting point of 2135 ° C and a refractoriness of about 1900 ° C.
The English name for spinel is Spinel, which means a crystal with sharp corners. It is an oxide of magnesium and aluminum, so spinel is associated with corundum. Spinel is available in a variety of colors, including red, pink, fuchsia, colorless, blue, and green. The spinel as a gemstone is almost a transparent magnesium spinel.
Spinel is a family of minerals formed in the natural world by the infiltration of molten magma into impure limestone or dolomite by contact metamorphism. Some appear in the basic magmatic rocks of aluminum-rich. Gem-quality spinel is mainly referred to as magnesium-aluminum spinel, which is a magnesium-aluminum oxide.
The crystal form is a polyhedron of octahedron and octahedron and rhombohedral dodecahedron. The colors are colorful, and there are colorless, pink, red, purple, light purple, blue purple, blue, yellow, brown, and the like. The variety of spinel is divided according to color, and there are red, orange, blue, and blue spinels. The glass is shiny and transparent. Shell-shaped fracture. Light red and red spinels emit red fluorescence under long and short-wave ultraviolet light.
history
Spinel is a long-established gemstone variety. Its name comes from two sources: one is from the Greek word "spark", which describes its red-hot color; the other is from Latin Spina, Spinels That is, the tip, the thorn, because its crystal shape is cubic crystal, has a sharp angle, so it is named.
Spinel has been a precious gem since ancient times. Because of its beauty and scarcity, it is also one of the most fascinating gemstones in the world. Because of its beautiful color, it has been mistaken for ruby since ancient times.
TimurRuby, the world's most legendary and fascinating 361-carat, and BlackPrince's Ruby, which was set in the King's crown in 1660 and weighed about 170 carats. It was not until the modern times that they were all identified as red spinels.
In 1612, the gem was attributed to the British royal family. In 1851, this jewel was exhibited together with several other smaller gems in the World Exhibition and was recorded in the official list as "Great Spinel Ruby". It was later given to Queen Victoria and kept in the Indian gallery at Buckingham Palace in London, England. The ruby tops used in the Qing Dynasty royal patriarchs and the one-piece official hats are almost entirely made of red spinel, and no real ruby products have been seen. The world's largest and most beautiful red velvet spinel, weighing 398.72 carats, was purchased by the Russian special envoy in 1676 in Beijing, China with 2,672 gold coins, and is now in the Moscow diamond library in Russia.
The world's largest spinel is produced in Myanmar and weighs 955.7 carats. It is black-red, translucent and engraved with double lion balls. The mighty lion shook his head and the shape was lively and very rare.
Crystal structure
Spinel crystal, belonging to the equiaxed crystal system, is face-centered cubic, Z=8. The basic structure is that oxygen is stacked in the (111) direction perpendicular to the ABC order. The ratio of tetrahedron to octahedron is 2:1.
The spinel structure can be regarded as the closest packing of oxygen ions to the cubic, and then the X ions occupy 1/8 of the 64 tetrahedral voids, that is, 8 A sites, and the Y ions occupy 1/2 of the 32 octahedral voids. 16 B-bits. Thus, the general formula of the spinel unit cell is X8Y16O32, which is often written as XY2O4 [1~5].
The crystal of the spinel includes a normal spinel structure and an inverse spinel structure.
Normal spinel structure: oxygen ions are tightly packed, trivalent cations occupy six coordination octahedral voids, and divalent cations occupy four coordinated tetrahedral voids. Positive spinel structure, structural formula XY2O4,
X is a divalent cation, and Y is a trivalent cation. Where X occupies the tetrahedral position and Y occupies the octahedral position.
Anti-dopeitic structure: divalent cations and half trivalent cations occupy octahedral voids, and the other half of trivalent cations occupy tetrahedral voids. The anti-spinel structure is also called an inverted spinel structure.
If all of the X cations and half of the Y cations in the structure occupy the octahedral position and the other half of the Y cations occupy the tetrahedral position, it is called the inverse spinel structure, and the structural formula Y[XY]O4. The structure of magnetite (Fe2+FeO4) is of this type.
The inversion structure can be regarded as 8 transpositions of 8 A-position ions and 16 B-position ions, that is, 8 Y2+ ions enter the tetrahedral gap (A position), and 8 Y2+ ions and 8 are left. The X4+ ion complex occupies the octahedral gap of the B position under normal conditions. In addition to the extremes of the positive and negative, there may be a mixed intermediate state distribution. Thus, the inverse distribution rate α can be used to quantify the fraction of X ions on the octahedron, thereby expanding the various spinel structural formulas as follows:
Positive type: (X) tetrahedron [Y2] octahedron O4, α = 0;
Inversion: (Y) tetrahedron [X, Y] octahedron O4, α = 1;
Mixed type: (Yα, X1-α) tetrahedron [Xα, Y2-α] octahedron O4, 0 < α < 1.
The properties of the positive and negative types and the degree of the reverse position have a large influence on the performance of the compound material. For the common 2:3 and 4:2 electricity price ratio spinel structures, it seems that the former is positive and the latter is reversed. However, in view of all species, not only a considerable number tends to be mixed, but the extent of the range cannot be determined, and there are still several varieties that do not follow this rule at all. The factors affecting this distribution are extremely complex, including the electrostatic energy of ionic bonds, the ionic radius, the spatial distribution of covalent bonds, and the crystal field. According to empirical data, the priority order of most of the divalent and trivalent ions can be discharged: Zn2+, Cd2+, Ga2+, In3+, Mn2+, Fe3+, Mn3+, Fe2+, Mg2+, Cu2+, Co2+, Ti3+, Ni2+, Cr3+. The more forward-looking, the tetrahedral interstitial, and the more inclined to the octahedral interstitial. The distribution of cations also has a major impact on the properties of spinel-type materials [1, 4].
The isomorphic substitution of the spinel chemical composition is common, and often contains iron, zinc, chromium, manganese, and the like. The octahedral crystal form is very common; it is also often a double crystal with the octahedral surface as the twin plane and the joint surface, called the spinel law twin crystal. Spinels are formed by contact metamorphism or by magma crystallization and are produced in basic, ultrabasic igneous rocks. The transparent and brightly colored spinel is a high-grade gem material.
nature
Optical properties
(1) Color: There are red, orange red, pink, purple, colorless, yellow, orange, brown, blue, green, purple and other colors.
(2) Gloss and transparency: glass to yam diamond, transparent to translucent.
(3) Photographic characteristics: homogeneous body.
(4) Refractive index: 1.718 (+0.017, -0.008), zinc spinel is 1.805, iron spinel is 1.835, chrome spinel can be as high as 2.00, no birefringence.
(5) Dispersion: 0.02
(6) Luminescence: red and orange spinel are weak to strong red and orange fluorescence under long-wave ultraviolet light, no weak red and orange fluorescence under short-wave; yellow spinel weak to medium-intensity brown under long-wave ultraviolet light Yellow, short brown to yellow brown; green spinel long-wave ultraviolet light without orange-orange red fluorescence. Colorless spinel has no fluorescence.
(7) Absorption spectrum: red and pink spinel mountain chrome coloration, with a broad absorption band around the 550nn in the yellow-green region, absorption in the purple region, and multiple fluorescent lines in the red region, which are described as organ pipes. The blue spinel is colored by iron or even by the drill. The main absorption band is in the blue region, and the absorption band at 458 nm is the strongest, and there are several weak bands such as 478 nm. The absorption spectrum of zinc spinel is similar to that of blue spinel, but it is weaker. Synthetic blue spinel: Cobalt-induced color, green, yellow and orange-yellow areas have three strong absorption bands, and the green area has the narrowest absorption band. 630 nm, 580 nm, 543 nm, with the widest absorption band at 580 nm.
(8) Special optical effects: starlight effect (four-shot starlight, six-shot starlight) is rare, color change effect.
(9) Crystallization characteristics: crystals are often octahedral crystals and abrasive pebbles, sometimes octahedrons and rhombohedral dodecahedrons and cubic polymorphs, with characteristic spinel law twins, ie {111} is a twin junction The contact twin crystal is formed.
(10) Under the polarizer: the color changes with the composition, colorless, light rose color (magnesium spinel), dark green (iron spinel), light gray white (zinc spinel). Spinel is a homogeneous body, but zinc spinel can have optical anomalies.
Mechanical properties
(1) cleavage: cleavage is not developed, common shell-like fracture.
(2) Hardness: Mohs hardness 8.
(3) Density: 3.57-3.90, generally 3.60, and the Zn-containing variety (zinc spinel) can reach 4.60.
Thermal properties
Spinel can be heat treated to improve color. The blue color turns green at 900 ° C and turns yellow when heated to 1200 ° C. This color change effect is stable. There are also reports of heating the red spinel to remove the brown component to obtain a pure red color.
Internal inclusion
1, solid inclusions: common octahedral spinel inclusions, arranged separately, in rows or in the form of fingerprints. Sometimes see octahedral negative crystals, which are partially filled with calcite and dolomite, followed by inclusions such as flake graphite, columnar apatite, and quartz. In the spinel produced in Myanmar, there are fine misty inclusions and blade-shaped vermiculite inclusions, which form a starlight effect when dense.
2, liquid inclusions: common liquid inclusions in open cracks. There may be fingerprint-like inclusions formed by tension cracks around the octahedral crystal inclusions.
3. Growth phenomenon: It can be seen that the growth zone and double crystal grain developed along the octahedral crystal plane are most easily observed under oil immersion under orthogonal polarized light.
classification
colour
Spinel often divides gemstones by color and special optical effects. Common varieties are:
(1) Red spinel: mainly containing trace coloring element Cr3+ and red in various shades. Among them, pure red is the most precious gemstone in spinel. This variety used to be mistaken for ruby. The famous ruby "Black Prince Ruby" and "Tiemuer Ruby" in the British crown, until the modern times, was identified as a spinel. Among them, the red to deep red spinel is a popular red gemstone variety, and the light pink to dark red spinel is similar to garnet.
(2) Blue spinel: it contains Fe2+ and Zn2+ and is blue. Most blue spinels are from dark blue to purple blue, or greenish blue.
(3) Orange spinel: a variety of spinel from orange to orange.
(4) Colorless spinel: pure and colorless is very rare. Most natural colorless spinels are more or less pinkish.
(5) Green spinel: generally caused by Fe2+, the color is dark, and some are basically black. The real black spinel is found in Montesangma, Thailand red sapphire mine.
(6) Discoloration spinel: very rare. In daylight, it is blue, under artificial light (incandescent), it is purple.
(7) Starlight spinel: This kind of spinel is generally dark brown red, dark purple to black, and the number is small. The spinel can have multiple sets of needle-shaped inclusions inside, which makes it have four or six stars. Mainly found in Sri Lanka.
Spinel is a mineral composed of magnesium-aluminum oxide. Because it contains elements such as magnesium, iron, zinc, manganese, etc., it can also be classified according to these, such as aluminum spinel, iron spinel, zinc spinel, manganese tip. Spar, chrome spinel, etc. Due to the different elements, different spinels can have different colors, such as magnesium spinel between red, blue, green, brown or colorless; zinc spinel is dark green; iron spinel is Black and so on. Spinels are hard, glassy octahedrons or granules and blocks. They appear in igneous rocks, granitic pegmatites and metamorphic limestone. Some transparent and beautifully colored spinels can be used as gemstones and some as iron-containing magnetic materials.
Origin and occurrence
Spinels are often found in schist, serpentinite and related rocks, and most gem-quality spinels are found in alluvial fans. It can be produced in marble (skarn type), symbiotic with rubies, sapphires, etc., and can also be produced in sand mines.
The main producing areas of spinel are Myanmar Mogok, Sri Lanka, Kenya, Nigeria, Tanzania, Takjistan, Vietnam, the United States and Afghanistan.
Identification
Due to its rich color, spinel is similar to many gemstones, especially in ruby, sapphire, garnet, beryl, zircon, glass, and artificial garnet.
Morphological difference
The difference between spinel and similar gemstones and artificial spinels. Red spinel is very similar to ruby. The difference is that ruby has dichroism, uneven color, and silky inclusions. Spinel is a homogeneous body, has no dichroism, uniform color, and the solid inclusion body is octahedron.
Blue, gray-blue, blue-violet, green spinel and sapphire are easy to mix, the difference is: sapphire dichroism is obvious, the ribbon is straight, with silky inclusions and twin faces. The density, refractive index, and polarization of the two gemstones are different.
The artificial spinel is rich in color, uniform, with few inclusions, occasionally curved growth lines, high refractive index, generally about 1.728±0.003, synthetic red spinel is 1.722~1.725, imitation lapis lazuli is 1.725, synthetic discoloration The spinel is 1.73. The red artificial spinel is multi-colored like ruby red, and the blue spinel is more brilliant blue.
Natural spinels can also be distinguished from artificial spinels based on the characteristics of the inner inclusions.
Optical difference
It is not difficult to distinguish them by conventional methods such as polarizing mirrors, spectroscopes, and magnification observations, as well as refractive index and density. The common variety of spinels is red, paying attention to the distinction between rubies and red garnets, especially magnesium garnet.
Spinel is a homogeneous body with no birefringence, while ruby is a one-axis crystal negative. The spinel has a lower refractive index than ruby, and its absorption spectrum does not have three absorption lines common to rubies in the blue region.
Both spinel and garnet are homogenous, and there is abnormal extinction under polarizer, but the refractive index of spinel is significantly lower than that of garnet, and the absorption spectrum is also very different. In addition, there are single or octahedral inclusions arranged in a row or in a row in the spinel, and a round inclusion is often found in the magnesium aluminum garnet.
Evaluation and purchase
The quality of spinel is mainly evaluated in terms of color, transparency, clarity, cut and size, of which color is the most important. The color is best in deep red, followed by purple, orange, light red and blue, requiring pure and vivid colors. The higher the transparency, the less the flaw, the better the quality. The best color of the spinel is dark red, followed by purple, orange, light red and blue. The color is required to be pure and vivid.
The transparency of the spinel affects the color and luster, and is affected by the clarity. The clarity of the spinel is generally less preferred. The inclusion of many inclusions or strong deformation of the crystal structure will affect the transparency of the spinel. The higher the transparency, the better the quality. Most spinels are relatively clean, and if the spinel is flawed, the price is lower.
Spinel cutting is also a factor influencing its price. High-quality spinel often occurs in faceted cuts, and requires a correct cutting ratio, preferably with emerald cuts. When the spinel is cut, it is not necessary to consider the directionality too much. The larger the cut, the better, and the fine polishing is required. For size, spinels exceeding 10 ct are less, so the price per carat is higher than the average spinel.
Color, transparency and weight are the main basis for the evaluation and purchase of spinel. High-quality spinel requires good color, high transparency, good clarity, cut ratio and polishing degree. Spinel has a variety of colors, usually contains more inclusions, layered distribution, and good transparency. The red spinel is the most popular, bright red, high transparency, and its weight is great. Spinels with a star effect are also more expensive. Crimson, red, brilliant blue, and green spinels are also preferred.