Oval-cut, 3.60-ct axinite, 10.6 x 8.4 x 5.8 mm, purple brown, Pakistan. © ARK Rare Gems. Used with permission.
When faceted, the members of the axinite mineral group are usually intensely trichroic, with considerable brilliance and rich brown and purple colors dominating. Although very rare, these gems could make magnificent jewelry stones.
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| Data | Value |
|---|---|
| Name | Axinite |
| Varieties | Ferro-axinite, Magnesio-axinite, Manganaxinite |
| Crystallography | Triclinic. Distinctive wedge-shaped crystals; also tabular |
| Refractive Index | 1.656-1.704 |
| Colors | Violet-brown, colorless, reddish purple, pink, orange, yellowish (Mn), greenish, pale violet to reddish (Mn), violetish blue, blue (Mg). |
| Luster | Vitreous. |
| Hardness | 6.5 - 7, variable with direction |
| Wearability | Very Good |
| Fracture | Uneven to conchoidal |
| Specific Gravity | 3.26-3.36; magnesioaxinite = 3.18 |
| Birefringence | 0.010-0.012 |
| Cleavage | Good 1 direction |
| Dispersion | 0.018-0.020 (Large) |
| Heat Sensitivity | Slight |
| Luminescence | Red in SW (Franklin, New Jersey). Dull red in SW, orange-red in LW (Tanzania: magnesioaxinite). |
| Luminescence Present | Yes |
| Luminescence Type | Fluorescent, UV-Long, UV-Short |
| Enhancements | None |
| Transparency | Translucent to transparent. |
| Absorption Spectrum | Narrow line at 5120, broad lines at 4920 and 4660, also at 4150. Sometimes lines visible at 5320, 4440, and 4150 (latter may be strong). |
| Phenomena | Color change (Very Rare). |
| Formula |
(Ca, Mn, Fe, Mg)3Al2BSi4O15(OH).
|
| Pleochroism |
Intense in all colored varieties: cinnamon brown/violet-blue/olive green, yellow, or colorless.
|
| Optics | α = 1.674 - 1.693; β = 1.681 - 1.701; γ = 1.684 - 1.704. Magnesioaxinite: α = 1.656; β = 1.660; γ = 1.668 Biaxial (-), 2V = 63-80° or more. However, may turn (+) if high in Mg. |
| Optic Sign | Biaxial +, Biaxial - |
| Etymology | From the Greek axine for “axe,” alluding to the common wedge shape of its crystals. Ferroaxinite, magnesioaxinite, and manganaxinite refer to the chemical composition of these varieties. Tinzenite refers to the type locality of this variety, Tinzen, Switzerland. |
| Occurrence | Axinite is found in areas of contact metamorphism and metasomatism. |
| Inclusions | Usually included. |
Axinite: Baja California, Mexico (~ 2.0 cut, rough 1½ inches long). Photo © Joel E. Arem, PhD, FGA. Used with permission.
Axinites form a mineral group, which includes iron (Fe) dominant ferroaxinite, magnesium (Mg) dominant magnesioaxinite, and manganese (Mn) dominant manganaxinite. Tinzenite is an intermediate member between ferroaxinite and manganaxinite.
Most gem-quality axinites are ferroaxinites, but you will likely encounter specimens called simply “axinites.” However, specific gems should ideally be referred to by their composition-specific names or with designations such as “axinite-(Fe).”
Greenish manganaxinite crystals on acicular johannsenite crystals. Field of view 7 mm. Iron Cap Mine, Landsman Camp (Landsman group), Aravaipa, Santa Teresa Mts, Aravaipa District, Graham County, Arizona. Photo and specimen Leon Hupperichs. Licensed under CC By-SA 3.0.
These strongly pleochroic gems can show three colors, depending on the viewing angle, and can have significant brilliance, too. (See the “Pleochroism” section in the Information Table above). With a hardness of 6.5 to 7, axinites of all varieties have “Very Good” wearability grades. This means they can withstand the rigors of use in most jewelry settings. However, these stones have anisotropic hardness. It may vary within a single specimen depending on the gem’s orientation. In addition, they have somewhat brittle tenacity.
Axinites are almost never completely free of clarity flaws, such as feathers and veils. Nevertheless, because of their rarity, these exquisite gems would still command the interest of both gem collectors and jewelry enthusiasts.
Most gem-quality specimens of the axinite group are iron-dominant ferroaxinites, like the rough and cut set shown here. 2.2 x 1.7 x 0.3 cm (crystal), 1.95-ct (gem), Puiva, Saranpaul, Tyumenskaya Oblast’, Prepolar Ural, Russia. © Rob Lavinsky, www.iRocks.com. Used with permission.
Axinites are both piezoelectric and pyroelectric. They generate an electrical charge under physical pressure and when heated, respectively.
Although typically biaxial negative, an axinite’s optic character may become biaxial positive as its Mg content increases.
Unusual violetish blue and reddish purple color zoning has been documented in ferroaxinites from Pakistan.
Rare magnesioaxinites have demonstrated a slight color change effect. Specimens from Tanzania have shown a pink color under incandescent light and a blue color under fluorescent light. Other changes have also been noted.
This faceted magnesioaxinite shows a very slight color change, from violetish purple in daylight to pink in incandescent light. Diamond step cut, 0.53 cts, 7.1 x 5.7 mm, Merelani, Tanzania. © The Gem Trader. Used with permission.
Axinite and another rare gemstone, andalusite, have overlapping ranges of body and trichroic colors as well as hardness. However, axinite’s refractive index (RI) range and specific gravity (SG) range exceed those of andalusite.
This pear-cut axinite from Pakistan shows this gem’s strong pleochroism. When you view the gem through the crown, you’ll see a honey brown color. When you view the gem through the pavilion, however, you’ll see a lilac hue. 3.27 cts, 12.5 x 8.5 mm. © Rob Lavinsky, www.iRocks.com. Used with permission.
No known synthetic axinites.
No known gemstone treatments.
Bright orange tinzenites on clear quartz crystals, 3.2 x 2.2 x 1.5 cm, Molinello Mine, Graveglia Valley, Ne, Genova Province, Liguria, Italy. © Rob Lavinsky, www.iRocks.com. Used with permission.
Axinites occur in many localities across the globe, but gem-quality material is rare. Notable sources include the following:
Clove brown axinite crystals, 2.5 x 2.0 x 1.6 cm. Obira mine, Ono-gun, Oita prefecture, Kyushu Region, Japan. © Rob Lavinsky, www.iRocks.com. Used with permission.
Axinites with clean clarity over 5 carats are difficult to find and worthy of museum display. Faceted specimens of any type over 10 carats are rare. Material from Baja California will yield gems to about 25 carats. However, if clean, these stones will cut less than 5 carats.
Although resistant to scratching due to their hardness, axinites have good cleavage and brittle tenacity. Avoid jewelry settings where these gems may receive blows.
Although axinites have very good wearability, their inclusions may pose risks if these gems are cleaned in mechanical systems. Have a gemologist examine them, identify any inclusions, and recommend a cleaning method. Since axinites also have some heat sensitivity, avoid boiling or steam cleaning. A soft brush, mild detergent, and warm water can serve as a safe alternative. Consult our gemstone jewelry cleaning guide for more recommendations.
Faceted axinite, 1.02 cts, Brazil. Photo by DonGuennie. Licensed under CC By-SA 4.0.
A gemstone’s optic sign may be listed as either + or -. Learn whether impurities in a specific stone or the choice of axes to measure RI cause the change.…
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