© Kirk Feral 2024, All Rights Reserved. These materials may be duplicated and shared for educational purposes only. No part of this website may be duplicated or distributed for profit, for commercial purposes, or for posting to another website, without the expressed written consent of the copyright holder.
Magnetism in Gemstones
An Effective Tool and Method for Gem Identification
© Kirk Feral 2009-2024
Overview
Axinite is an intriguing group of mineral species whose triclinic crystals form as thin blades with sharp edges reminiscent of axe blades, hence the name Axinite. Like Tourmaline, Axinite is a borosilicate mineral containing a significant amount of boron in its native chemistry, along with transition metals that often cause strong paramagnetism.
Axinite: Magnetism & Color
Brown is the most common color in Axinite gems, but rare examples can have a surprising variety of colors including yellow, lavender, pink, blue, green, and orange. Axinite can also be completely colorless. Strong longwave UV fluorescence is a feature of some Axinite gems, while color zoning, pleochroism and color change can be found in all species of gem Axinite.
Axinite Crystals on Matrix, 25 mm Tall, Luning, Nevada
Axinite mineral specimens are less than ideal for fabricating into gems, as transparent uncut crystals tend to be quite thin, often less than 5 mm in depth. As a gemstone, Axinite is obscure and of interest mostly to collectors of rare or unusual gems. But in recent years, mining in Tanzania of pale Axinites with unusual colors has stimulated interest in this gem mineral.
Because magnetic testing is so helpful in the identification of Axinite, we dedicate a section of our website to present results of our study of magnetism in Axinite. Much of this information is new and not published elsewhere. Depending on the species, magnetic response and measured magnetic susceptibility in Axinite can vary widely, from very Weak (SI < 20) to a Pickup response (SI 1879). However, most Axinite gems show a Drag response and have high measured magnetic susceptibility much like Peridot, around SI 500-600 (note: SI values are succeeded by 10-6).
We curated and analyzed 53 Axinites: 37 faceted gems and 16 individual uncut crystals, plus 2 parcels of homogeneous rough crystals. All but 2 samples were transparent. Most uncut crystals in our study had at least one naturally flat smooth surface suitable for measuring refractive index and magnetic susceptibility. All samples were tested for refractive index, magnetic susceptibility, magnetic response, thermal inertia (thermal conductivity), longwave and shortwave UV fluorescence, pleochroic colors, color change, absorption spectrum (using a UV-Vis-NIR spectrometer) and Chelsea filter reaction. Hydrostatic specific gravity was measured only for samples above 2 ct. in weight to ensure accuracy. In addition, we tested 6 rare and near-colorless Axinite gems at a gem show for magnetic response only. The brown Axinite pictured below is rare for its large size, depth, and flawless clarity, and it is the largest faceted Axinite tested for this study.
Ferroaxinite 10.02 ct, 13 mm X 9 mm X 6 mm, Pakistan
SI 608, Too Heavy to Drag
There are four species of Axinite, but only three of these are considered gem-quality. In gemology, the 3 gem Axinite species are usually referred to as Ferroaxinite (iron Axinite), Manganaxinite (manganese Axinite) and Magnesioaxinite (magnesium Axinite). In 2008 the International Mineralogy Association (IMA) officially re-named these three mineral species as Axinite-Fe, Axinite-Mn, and Axinite-Mg. The fourth species in the Axinite group is Tinzenite, a rare species typically found as clusters of small yellow or orange crystals colored by manganese.
Axinite gems found in the marketplace are mostly brown or yellowish-brown Ferroaxinites and Manganaxinites. A curious aspect of brown Axinites such as the 4 mm-deep rough crystal pictured below is that they can appear dark brown in reflected light, but near-colorless in transmitted light. Rotation of this crystal reveals that this change from dark to light color may be due to light extinction that varies with the orientation or angle of the stone under reflected light. The intensity of brown color also varies throughout the crystal. Due to high iron content, the crystal below has the highest measured magnetic susceptibility (SI 654) of any Ferroaxinite we tested.
Reflected Light Transmitted Light
Ferroaxinite Crystal, 12.72ct., 20 mm Tall, Pakistan, SI 654, Drag
Core to rim chemical zoning is known to be common in Axinite crystals. In Ferroaxinite, concentrations of iron tend to be higher near the core, and manganese concentrations higher at the rim. A single crystal can potentially contain the chemistry of two different Axinite species. However, we did not detect chemical zoning in our samples, as most rough crystals were measured for magnetic susceptibility at only one crystal face.
The refractive index for Axinite ranges from a low of RI 1.656 in Magnesioaxinite to a high of RI 1.704 in Tinzenite. We find that most Axinite gems fall within the range of RI 1.660-1.686. Our specific gravity measurements ranged from SG 3.14 in Magnesioaxinite to 3.38 in Tinzenite, with typical Axinites ranging from SG 3.24-3.32. We did not find specific gravity measurements to be consistently proportional to magnetic susceptibility.
Axinite is found in many locations around the world, but today the majority of faceted Ferroaxinite and Manganaxinite gems are supplied by Pakistan, with France being noteworthy secondary source. Magnesioaxinite is the rarest species of gem Axinite, and cut gems are very rare. Light-colored Magnesioaxinites, as well as light-colored Manganaxinites, originate primarily from Tanzania. They are mined in Merelani Hills, Tanzania within the same mines as Tanzanite and Tsavorite Garnet. Additional geographical sources of Axinite samples examined in our study include Afghanistan, Russia, Sri Lanka, Brazil, Italy, and Nevada (U.S.A).
Understanding Axinite through Magnetism
Axinite gems are well suited to study and identification via magnetic testing due to two transition metals inherent in their chemistry: iron and manganese, the same magnetic metals that allow us to analyze Garnet composition in detail and identify Garnets by species and variety (see Garnet p.5). The metal oxides of iron (FeO) and manganese (MnO) found within Axinite are both highly paramagnetic. Iron oxide (FeO) with 2 unpaired electrons (Fe2+) creates brown color in Axinite, and manganese oxide (MnO) with 2 unpaired electrons (Mn2+) is associated with yellow and orange color.
The third critical metal found in Axinite’s chemical makeup is magnesium, which is not a transition metal and not paramagnetic. Magnesium oxide (MgO) is diamagnetic and produces no color. Since magnesium does not cause magnetic attraction, a simple magnetic test allows us to distinguish Magnesioaxinite from Manganaxinite (Williams, B. & C., 2014) and from Ferroaxinite.
A key finding of our study is that the common misidentification of Magnesioaxinite can be solved with a simple magnetic test. As a rule, Ferroaxinite and Manganaxinite gems of average size and weight (0.5ct – 6ct) show a Drag response to a ½” N52 neodymium magnet, while most Magnesioaxinite gems and crystals do not show a direct response and instead require floatation for any magnetic response to be visible.
Ferroaxinite 1.29ct, Brazil
SI 608, Drag
As with many other mineral groups such as Garnet, Tourmaline, and Spinel, the different species within the Axinite group chemically mix with each other in solid solution series, resulting in a range of colors and magnetic susceptibilities. The quaternary diagram we drafted for this study (shown below) illustrates how the Axinite samples we tested intermix among the 4 species of Axinite.
Axinite Species Mix with Each Other in Solid Solution Series
In the above diagram, the 4 endmember Axinite species are labeled at the 4 corners of the quaternary. An estimated magnetic susceptibility value (SI) for the pure species endmember is listed under each species name based on the maximum SI value that we measured among samples we tested.
Mineralogists have measured actual magnetic mass susceptibility values for pure Axinite endmembers using a SQUID magnetometer (Filip, J. et al., 2008). However, the values that we use in the study of gemstone paramagnetism - magnetic susceptibility per unit volume in terms of SI X 10-6 - have not been determined for pure Axinite endmembers. The exception is Magnesioaxinite, which we know must have a value less than zero (SI < 0) due to the diamagnetic nature of magnesium oxide.
The diagram above shows the location of colored points at varying distances from endmember species, illustrating approximate compositions of the Axinites tested in this study. The colors of the points roughly correspond to the actual colors of the Axinite samples. The dotted lines represent the compositional boundaries between the Axinites species.
Orange-Brown Manganaxinite, 1.5ct, Pakistan
SI 525, Drag
Because any Axinite gem can be a blend of 3 species in varying proportions, accurate identification of the primary species content often requires advanced testing beyond the scope of this study. But standard gem identification tests along with magnetic susceptibility measurements, gem color and other criteria can be quite useful for identifying species and approximating the degree of mixing between species.
Point positioning in the above 2-dimensional diagram is based primarily on the single vertical axis variable of magnetic susceptibility, and no horizontal axis variable such as refractive index or specific gravity was assigned. We have high confidence in the approximate positioning of points located near endmembers and also the vertical poisoning of points that show the degree of mixing with Magnesioaxinite in the upper ternary, and with Tinzenite in the lower ternary.
Horizontal point positions along the line joining Ferroaxinite and Manganaxinite are the least accurate and represent our best guesses as to the proportion of iron to manganese within each Axinite sample based on absorption spectra, color, color intensity, and relative magnetic susceptibility.
As an example, we conclude that the composition of the gem pictured below is predominantly Ferroaxinite (brown color) with a considerable amount of Manganaxinite (yellow color), and we position its point (# 25 on the diagram below) just to the right of the boundary separating those 2 species. The relatively low magnetic susceptibility (SI 418) of this gem indicates that it also contains a significant amount of Magnesioaxinite, which raises the position of the point vertically toward Magnesioaxinite. Therefor the composition of Ferroaxinite gem # 25 is a mix of all 3 species of gem Axinite.
Ferroaxinite # 25, 0.72ct, Pakistan
SI 418, Drag
Ferroaxinite # 25 Mixes with Manganaxinite & Magnesioaxinite
An important new and unexpected finding of our study is that there is no miscibility gap between Magnesioaxinite and Manganaxinite among the samples we tested. Multiple samples of Axinite were found to have intermediate composition between Magnesoaxinite and Manganaxinite, especially within the Magnesioaxinite species. This finding is at odds with several studies of Axinite composition which performed electron microprobe analysis of Axinite samples. Those studies suggested that there is a miscibility gap or barrier between Magnesioaxinite and Manganaxinite, with minimal intermixing between the two species. They found that most samples of Magnesioaxinite were near the pure endmember in composition (Pringle & Kawachi, 1980 & Andreozzi, G. et al., 2000). However, using magnetic susceptibility as an indicator of miscibility (i.e. mixing), we were surprised to find no such miscibility gap among our samples. The discrepancy between research results could be due to inequalities among samples tested.
Separating Axinite by Species
Gem color alone is not a reliable indicator for identifying Axinite by species. Nonetheless, a complete absence of brown color rules out Ferroaxinite. Bivalent iron (Fe2+) is a strong coloring agent in Axinite, and even small amounts of iron can generate brown color. Brown Axinite gems are generally sold as Ferroaxinites, but our analyses show that some of these brown gems are likely Manganaxinites. Brown color has also been reported in iron-rich Magnesioaxinite gems from Pakistan (Zwaan J.C. et al., 2018), and a few brown Magnesioaxinite specimens have been found in other locations globally (Lauf, R., 2007), but we did not find any brown Magnesioaxinites in our study.
Ferroaxinite Crystal, 6.8ct
SI 573, Drag
Refractive index and specific gravity are not particularly effective for separating Axinite gems by species, as readings can show considerable overlap. However, Magneisoaxinite has the lowest RI and SG ranges, and Magnesioaxinites that are near the pure endmember can be separated from other Axinite species solely by their low refractive index (RI 1.656 - 1.678) and low specific gravity (SG 3.14 - 3.17).
When gems have brown body color, distinguishing between Ferroaxinite and Manganaxinite can be challenging without definitive data about chemical composition obtained from electron-microprobe analysis, EDXRF (energy dispersive Xray diffraction), SEM (scanning electron microscope) or other specialized testing methods. We find that Manganaxinites close to endmember composition have a higher refractive index than Ferroaxinites, but due to extensive intermixing between species, RI readings are generally not helpful in separating these two species when they have similar color.
Brownish Yellow Manganaxinite 6.15ct
SI 352, Too Heavy to Drag
Magnetic susceptibility measurements for Ferroaxinite and Manganaxinite also overlap, although we find that Ferroaxinite gems on average have magnetic susceptibility values that are about 10% higher than Manganaxinite gems. We cannot distinguish between iron and manganese with a magnet, and gems of both species typically show a Drag response to a magnetic wand.
UV-Vis-NIR spectrometer readings for brown Ferroaxinite and brownish yellow Manganaxinte gems also appear very similar since they both contain absorption features for iron and manganese. In the visible absorption spectrum graph shown below, we see that both the brown Ferroaxinite and yellow Manganaxinite absorb light in the blue and violet regions of the spectrum, represented by multiple small peaks from iron and manganese on the left side of the graph. On the right side of the graph, light transmission occurs in the green and red regions, which together result in brown color. Red + green = brown.
A broad absorption peak near 580nm at the yellow center of the graph is typical of the Ferroaxinites we examined. This peak represents yellow light absorption by vanadium and Mn3+. In contrast, this absorption peak at 580nm is mostly absent from Manganaxinite, and more yellow light is transmitted. We found that this minor difference in absorption spectra can be quite useful for distinguishing Manganaxinites from Ferroaxinites that have similar color.
Visible Absorption Spectra for Brown Ferroaxinite & Yellow Manganaxinite
As the above spectra demonstrate, we can use color as one criterion for separating Axinite species, as increasing yellow color suggests greater manganese (Mn2+) content due to greater mixing with Manganaxinite. UV fluorescence indicates a lack of iron (Fe2+) content derived from Ferroaxinite. Lower magnetic susceptibility and lower color intensity are signs of mixing with Magnesioaxinte and/or Manganaxinite. The yellow Manganaxinite pictured below has a high magnetic susceptibility typical for Manganaxinite, but this gem does not fluoresce due to iron (brown color) from Ferroaxinite.
Brownish Yellow Manganaxinite, 1.1ct., Pakistan
SI 555, Drag
Light-colored Axinites that have no brown color can easily be separated by species using magnetic testing. If the gem or crystal is 0.5 - 6 ct. in weight, a Drag response to a magnetic wand informs us that the Axinite must be Manganaxinite, or very close to Manganaxinite in composition. (note: to test for a Drag response of a thin uncut Axinite crystal blade of any size or weight, the broad side of the crystal must rest against the base pole of the magnetic wand).
An absence of any direct magnetic response indicates the Axinite is Magnesioaxinite, and floatation is necessary for any magnetic response to be visible. Depending on the amount of Manganaxinite in its composition, a Magnesioaxinite will show a magnetic response ranging from very Weak to Strong when floated. The crystal pictured below is near the pure Magnesioaxinite endmember in composition and has a very weak magnetic response when floated.
Lilac Magnesioaxinite, 10.35ct, 17 mm Tall, Tanzania
SI < 20, Weak
With the additional help of a Hoover magnetic susceptibility balance, magnetic measurements permit us to more precisely estimate the degree to which Magnesioaxinite intermixes with Manganaxinite or Ferroaxinite, and vice versa. The crystal above has a measured magnetic susceptibility of only SI < 20, confirming that it approaches pure endmember composition and does not mix with other Axinite species to any significant degree.
1 >
