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Garnets

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Magnesioaxinite

The rarest species of gem Axinite is Magnesioaxinite - Ca2MgAl2BSi4O15OH. This is the only Axinite species that is allochromatic (colored only by impurities), and it was first identified in 1975 with the analysis of a pale blue gem from Tanzania (Jobbins, E. et al. 1975). Examples of Magnesioaxinite tend to be light in color, and all but two of the samples we tested fluoresce orange under longwave UV light. Magnesioaxinites that are near the pure endmember are weaky magnetic due to very low concentrations of iron and manganese, but examples with higher manganese content such as the pale-yellow gem below can show a strong magnetic response with floatation. The refractive index range (RI 1.656-1.678) of Magnesioaxinite is also lower than other Axinite species.

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Magnesioaxinte & Tinzenite and Questions of Color

  Yellow Magnesioaxinite, 0.81ct, Afghanistan
SI 267, Strong

Measured magnetic susceptibility values of Magnesioaxinite gems and crystals near the pure endmember are at the minimum measurable level at less than SI 20, indicating that a very small but detectable amount of manganese (> 0.13 wt% MnO) is present. No diamagnetic samples (SI < 0) were found. Our estimated magnetic susceptibility range for the Magnesioaxinite species is SI < 0 -275. Axinites with values higher than SI 275 would be primarily Manganaxinite in composition. Among samples in our study, we found little or no miscibility with Ferroaxinite within the Magnesioaxinite species, and no Magnesioaxinites with brown body color were found. Evidence of mixing between Magnesioaxinite and Ferroaxinite was found only within the Ferroaxinite species.

Since brown color from iron is mostly absent, other colors such as pink, lavender, blue, yellow, and orange become more apparent in Magnesioaxinite. These colors are due to small amounts of transition metal impurities such as vanadium and manganese (Mn2+ & Mn3+). Magnesioaxinite can also be completely colorless.

The colorless Axinite gem shown below has the lowest measurable magnetic susceptibility (SI 17) of any Axinite in our study. A complete lack of color, in addition to low magnetic susceptibility and low refractive index (RI 1.656-1.665), are definitive indicators that this gem is Magnesioaxinite.

   Colorless Magnesioaxinite. 0.7ct, Tanzania
SI 17, Weak

The absence of iron (Fe2+), which inhibits fluorescence, permits Magnesioaxinite gems to fluoresce strongly under longwave UV light and weakly under shortwave UV light. As with Manganaxinite, the bright orange color of fluorescence is due to trace amounts of manganese (Mn2+). The absence of a red color component in the fluorescence of this gem suggests chromium content is absent or too low to contribute red fluorescence.

Colorless Magnesioaxinte Fluoresces Strongly

Color zoning is evident in some Magnesioxinites such as the crystal shown below, which has pale pink body color and pale brownish orange color zoning. The pink color is produced by trivalent manganese (Mn3+). This crystal is near the pure Magnesioaxinite endmember in composition and shows a very weak magnetic response when floated. It also shows strong orange fluorescence under longwave UV light.

When sufficient blue color from vanadium is present, Magnesioaxinite can have pale blue or pale lilac (pinkish blue) color, as shown in the photo below (left). Lilac Magnesioaxinites with blue color can show distinct color change to pink in incandescent light, as pictured below (right). Unlike the pink crystal pictured above, the composition of the crystal below is somewhat removed from the pure Magnesioaxinite endmember. It is strongly magnetic (SI 139) due to greater mixing with Manganaxinite. The higher manganese content is unrelated to the blue color. This Magnesioaxinite crystal was incorrectly sold as Manganaxinite.

   Lilac Magnesioaxinite, 5.6ct, 17 mm Tall, Tanzania
SI 139, Strong

The composition of another lilac Magnesioaxinite crystal shown below is even further from the pure endmember, with significantly greater Manganaxinite content. Higher magnetic susceptibility (SI 234) and a strong magnetic response when floated reveal that this Magnesioaxinite is approaching the composition boundary between Magnesioaxinite and Manganaxinite. Magnesioaxinites with pale lilac color can look similar to lavender Manganaxinites and can show similar orange fluorescence, but differences in magnetic susceptibility separate the two species.

Solid blue body color is very rare in Axinite, and we were unable to find any examples for this study. The crystal pictured below is a solid violet-blue Magnesioaxinite from Tanzania, as reported by a trusted source. Although we were unable to test this crystal and verify its identity, we include the photo here to show the potential range of body color found in Magnesioaxinite.

  Blue Magnesioaxinite, 3.1ct, Tanzania
Photo Courtesy of Unlimite-Gems.com

Orange is another unusual color found in Magnesioaxinite. The pear-shaped gem pictured below has bright orange body color produced by a very small amount of manganese. We know the composition of this Magnesioaxinite gem is near the pure endmember because it is very weakly magnetic (SI < 20).

  Orange Magnesioaxinite, 0.7ct, Tanzania
SI < 20, Weak

Magnesioaxinites with orange color have absorption spectra that are distinct from brown/yellow Axinites or lavender/pink Axinites. The absorption spectrum below shows broad peaks at 465nm and 740nm, which are almost certainly attributable to manganese (Mn2+). Transmission of light occurs primarily in the orange region of the visible spectrum near 620nm.

The rough Magnesioaxinite crystal shown below has solid orange body color that is nearly identical to the gem above. But the chemical composition of the orange rough deviates significantly away from the pure endmember. This crystal has a measured magnetic susceptibility of SI 204 and shows strong magnetic attraction when floated. We can infer that the higher magnetic susceptibility is due to significantly higher manganese content from mixing with Manganaxinte. Mixing with Ferroaxinite is not indicated, as this crystal has no brown color and shows strong UV fluorescence.

Orange Magnesioaxinite, 2.25ct, Tanzania
SI 204, Strong

Another example of strongly magnetic Magnesioaxinite is shown below. This pale-yellow gem has high magnetic susceptibility (SI 272), and like the orange crystal above, its composition does not approach the pure Magnesioaxinite endmember. The predominant yellow color indicates manganese (Mn2+) from Manganaxinite. The measured magnetic susceptibility and lack of a Drag response verify the gem is primarily Magnesioaxinite in composition, but borders on Manganaxinite composition. Unlike all other Magnesioaxinites tested in our study, yellow Magnesioaxinites don’t fluoresce, so we know that more than a trace amount of iron (Fe2+) from Ferroaxinite must also be present to inhibit fluorescence from manganese.

  Pale Yellow Magnesioaxinite. 0.43ct, Pakistan
SI 272, Strong

Tinzenite 

Tinzenite - Ca2Mn42+ Al4[B2Si8O30](OH)2 - is a yellow to orange idiochromatic species of Axinite that is found in only a few locations worldwide, and it is rarely faceted as a gemstone. When Tinzenite was first discovered in 1923, it was not recognized as an Axinite species at all and was assigned a different mineral name based on the location of its discovery near the town of Tinzen, Switzerland (Lumpkin & Ribbe, 1979).

Like Manganaxinite, manganese is the defining transition metal in Tinzenite. The difference is that Tinzenite has a much higher concentration of manganese than Manganaxinite, and as manganese replaces calcium, the proportion of calcium is lower than in Manganaxinite. We found that the unusually large translucent 10ct Tinzenite gem shown below is 3 ½ times more magnetic than most Manganaxinites due to the higher manganese content. The measured magnetic susceptibility (SI 1879) of this gem is equivalent to that of many Garnets.

The pure intense orange color suggests that this Tinzenite could be near the pure Tinzenite endmember in composition (> 20 wt% MnO), but we were unable to test other samples of Tinzenite for comparison. The absorption spectrum shown below suggests the color is largely influenced by trivalent manganese (Mn3+ peak at 565nm). Yellow color from bivalent manganese (Mn2+) plus red color from trivalent manganese (Mn3+) can produce orange color. Specimens of manganese-rich Tinzenite can be either yellow or orange (Milton, C. et al.,1953).

  Translucent Tinzenite, 10.8ct, Val Graveglia, Italy
SI 1879, Drag

Due to large size and weight, the orange gem above shows only a Drag response, but Tinzenites with similar magnetic susceptibility can be expected to show a Pickup response when samples are under approximately 6ct. in weight. The high concentration of manganese (Mn2+) also raises the refractive index of Tinzenite to around RI 1.69-1.70, which is higher than the RI of other Axinite species. These identifying characteristics separate Tinzenite from other Axinites.

Tinzenite is in continuous solid solution with Manganaxinite, and many intermediate examples may exist (Grew, E., 2018), although such intermediates are rarely encountered as gemstones. The large orangey-brown gem below appears to be one such intermediate, and the absorption spectrum for this gem indicates manganese and iron content. The brown color component is likely due to a minor amount of iron (Fe2+), presumably from mixing with Ferroaxinite.

   Manganaxinite-Tinzenite Intermediate, 9.2ct, Pakistan
SI 998, Drag

The gem above is over-dark and heavily included, yet transparent. It shows a Drag response and has a magnetic susceptibility of SI 998, which is less than our orange Tinzenite gem, but much higher than either Ferroaxinite or Manganaxinite. The abnormally high magnetic susceptibility informs us that this gem may be a Tinzenite-Manganaxinite intermediate, with a chemical composition that is a bit closer to Manganaxinite than to Tinzenite. The refractive index and specific gravity are slightly higher than what we have measured for Manganaxinite, but lower than for Tinzenite. Neither this intermediate gem nor the orange Tinzenite gem above fluoresce under UV light.

            Lavender Manganaxinite                                                 Pink Manganaxinite

Neither vanadium (V3+) nor trivalent manganese (Mn3+) is represented in the chemical formula for Manganaxinite, and these coloring agents therefore act as impurities. In the absence of V3+ and Mn3+, lavender and pink Manganaxinite would be colorless. By definition, allochromatic gems are colored entirely by impurities and colorless when pure. No completely colorless Manganaxinites have so far been reported, but we pose an interesting question: Are lavender and pink Manganaxinites allochromatic?

Yellow Color

A related question is: Is bivalent manganese (Mn2+) responsible for all yellow color in gem Axinite, and if so, why is it inconsistent in creating yellow color? Mn2+ sometimes fails to create color even when in high concentration, yet at other times it is associated with yellow color. We did not find any yellow color in Manganaxinites that were near the pure endmember in composition, suggesting that yellow color could instead be linked to lower concentrations of manganese in gem Axinite, as we find in yellow-brown Axinites that contain a mixture of manganese and iron.

Or is it possible that an additional chromophore such as intervalence charge transfer is involved in inducing yellow color? We know that in yellow Tourmaline, intervalence charge transfer involving manganese and titanium (Mn2+-Ti4+) contributes to yellow color in addition to manganese (Mn2+) (Rossman & Mattson, 1986). Does something similar occur in Axinite? Or could REE’s (rare earth elements) that are present in Manganaxinite (Zagorsky, V. et al., 2016) act as sensitizers in concert with Mn2+ to trigger yellow color?

  Pink Magnesioaxinite, 6.3ct, Tanzania, 21 mm Tall
SI < 20, Weak

  Absorption Spectrum for Orange Magnesioaxinite

  Yellow Manganaxinite

Or might trivalent iron (Fe3+), or IVCT (intervalence charge transfer) involving trivalent iron (Fe3+), be a source of yellow color in Axinites as it is in yellow Grossular and yellow Andradite (Topazolite) Garnets? Trivalent iron (Fe3+) primarily occurs at octahedral sites in Axinite, as it does in yellow Grossulars and Andradites (Andreozzi, G. et al., 2004 & Sugitani, Y. et al., 1974). And the highest concentrations of Fe3+ in Axinite are to be found in yellow-brown Axinites that contain a mixture of Manganaxinite and Ferroaxinite.

We present these potential inducers of color only as theoretical scenarios to address the question of inconsistent yellow color in Axinite. To date, none of these color mechanisms have been proposed or documented to cause color in Axinite.

Orange Color

We can also ask: Why is manganese in Axinite sometimes associated with orange color rather than yellow color? Is orange color simply the result of yellow color from Mn2+ combining with pink or red color from Mn3+? Yellow + pink or red = orange. And if so, does orange color appear in Axinite only when Mn3+ is present in sufficient concentration to add a pink/red color component? This appears to be the case with orange Tinzenite, where the optical absorption spectrum suggests that orange color is strongly influenced by Mn3+. Is this also true for the Magnesioaxinite species, with orange color being triggered by available Mn3+?

    Orange Tinzenite Magnified

Or perhaps manganese isn’t the only chromophore that produces orange color in Axinite? A possibility to consider is that intervalence charge transfer is involved. A recent study of orange Spessartine Garnet (Zhu, M. & Guo, Y., 2023) found that iron to iron (Fe2+-Fe3+) charge transfer contributes to orange color in conjunction with manganese (Mn2+). Could such a charge transfer process also occur in Axinite? Evidence of intervalence charge transfer is investigated primarily by optical and Xray absorption spectroscopy, and to date it has not been described or suggested as a cause of orange color in Axinite.

Color Intensity

Yet another enigma of Axinite color is that the intensity of yellow and orange color associated with manganese (Mn2+) is not proportional to the concentration of manganese. How is it that the extremely low concentration of manganese oxide found in the orange Magnesioaxinite gem shown below (left), which likely approaches 0.13 wt% MnO, can create orange color that is equally as intense as orange color found in the Magnesioaxinite crystal shown below (right), which contains a much higher concentration of manganese oxide?

  These Two Orange Magnesioaxinites with Similar Color Intensity
Have Markedly Different Concentrations of Mn2+

Likewise, strong orange color can occur in both Magnesioaxinite and Tinzenite, yet the concentration of Mn2+ in Tinzenite is vastly higher. This curious phenomenon is not unique to Axinite. We also see it in orange Hessonite Garnet and orange Spessartine Garnet, two species of Garnet that can have very similar orange body color from manganese even though the concentration of manganese is wildly different between the two species. Such examples suggest that intervalence charge transfer like that already noted for orange Spessartine might be involved, as IVCT requires only tiny amounts of transition metals to create intense color.

In a similar fashion, there is no direct relationship between the concentration of manganese (Mn2+) and the color intensity of orange longwave UV fluorescence activated by Mn2+. Only a few parts per million of Mn2+ are needed to activate visible orange fluorescence under longwave UV light (Haberman et al., 1997). We find that orange fluorescence in a completely colorless Magnesioaxinite appears just as strong as orange fluorescence in a lavender Manganaxinite that may contain as much as 100 times more manganese (Mn2+).

  Colorless Magnesioaxinite (lft) Fluoresces Just as Strongly
as Lavender Manganaxinite (rt)

But when the concentration of Mn2+ reaches the much higher level found in our orange Tinzenite, it produces no UV fluorescence at all. Is the lack of orange fluorescence in Tinzenite due to concentration quenching by Mn2+? Again, we find a similar circumstance in Garnet. Moderate orange fluorescence from manganese (Mn2+) is seen in pale and colorless Grossular Garnets that may contain only trace amounts of manganese, but fluorescence is absent in orange Spessartine Garnet, where the concentration of manganese oxide (MnO) can reach as high as 41% (Deer, Howie & Zussman, 1962).

To our knowledge, none of our questions regarding gem color or gem fluorescence in Axinite have been specifically addressed or answered in published research. This concludes our report on Axinite. You can find a complete list of Axinite references on the Resources and Links page near the bottom of the page.

Lilac Magnesioaxinite, 21.45ct, 22 mm Tall, Tanzania, Strong Orange Fluorescence
SI 234, Strong

  Tinzenite Absorption Spectrum Peaks at 562nm Indicating Mn3+

Questions of Color

Over the course of our study of magnetism and color in Axinite, we were puzzled by number of ambiguities about color in relation to manganese. Color theory is a complex field of physical chemistry, and information about causes of color within Axinite and many other gem minerals is incomplete. It is our hope that the answers to questions presented below will at some point be addressed by researchers who study gem color.
Idiochromatic?

Gemologists consider Manganaxinite to be an idiochromatic (self-colored) gem species, with bivalent manganese (Mn2+) as the coloring agent inherent in its chemical formula. But as we have shown, bivalent manganese (Mn2+) is a weak chromophore that can at times produce little or no color.

Two good examples are lavender Manganaxinites and pink Manganaxinites, in which Mn2+ fails to produce color even though it is present in high concentration. All visible color in lavender Manganaxinite appears to be the result of trace impurities of vanadium (blue color) plus tiny amounts of trivalent manganese (Mn3+, pink color). In pink Manganaxintes that have no blue color component, trivalent manganese (Mn3+) is the sole coloring agent. In the same way that Mn3+ generates pink color in Tourmaline, it is believed that Mn3+ in Axinite is converted from Mn2+ during crystal formation due to gamma radiation from radioactive decay (Reinitz & Rossman, 1988).

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