Introduction

Demantoid was first presented by the Finnish mineralogist Nordenskiöld in 1864 as a green, chrome bearing variety of andradite. It was named demantoid because of its high, diamond-like dispersion and RI. Later, green andradites without any chrome was found and the use of the name has evolved. Today yellowish green, green and brownish green andradites are also named demantoid.

Since the variety name is applied based on color, the demantoid chemistry can be variable. Some of the demantoids from are grossular, whereas others have a significant uvarovite content, Yet a surprisingly large number of analysis show a composition near end-member andradite.

Some key localities

The localities presented are some of the more prolific demantoid producing localities. Some of these still produce specimens and gem rough todays, whereas others where prominent producers in the past. The localities do not only represent some of the best demantoid localities, but they also represent the different chemical compositions and geological environments where demantoid can be found.

Iran
Soghan, Arzuiyeh County, Kerman Province, Iran

Demantoid
Demantoid from the area near Bagh Borj has been known in the international gem market since 2001. There are several active operations, both in tunnels and open cuts. Individual demantoid crystals may reach up to 5 cm, but are normally much smaller. Crystals of more than 1.5 cm must be considered large for the locality. Gem crystals are normally much smaller. The color ranges from yellowish green to deep green.

Geology
The dematoid occurrences in the Kerman district is found associated with the e Late Cretaceous Haji Abad (Esfandagheh) ophiolites, consisting of metamorphosed dunites, harzburgites, gabbros and other rocks. There are several chromite (magnesio-chromite?) deposits associated with serpentinized rocks in the area. The demantoid are found fissures and veins within foliated serpentinites. The demantoids are hosted in a weathered and hydrothermally altered mafic rock, and are associated with chrysotile, antigorite, brucite, talc and chromite. Cr-bearing magnetite is often present. The demantoid is of an obvious hydrothermal origin.

Belqeys Mountain, Takab County, West Azerbaijan Province, Iran

Nouri et al.(2020) describe 5 different grossular/andradite localities in skarns in the Azerbaijan district, but only localities near Ghinarjeh and Baba Nazar yields green crystals.

Demantoid
The individual demantoid crystals from these localities can reach more than 5 cm. Even though the majority of crystals are much smaller, there are surprisingly many crystals over 2 cm, often giving nice matrix specimens. The crystals are rarely transparent, and the locality is of limited interest for the gem industry.

Geology
Demantoid is found in a metamorphic horizon consisting of mica schists interlayered with quartzites, marble horizonz and a 50-100m thich horizon of metamorphosed ophiolite ( serpentine, serpentine schist and metabasalt) belonging to the Tabak complex (Chegeni et al., 2021). The metamorphic series are intruded by granites and quartz-porphyres. Hajialioghli et al. (2007) considers the initial crystallization of the mafic rocks to be around 1000-1200ºC and 24+/-2.7 kbar. They were later exposed to a low-grade metasomatic event (lizardite/chrysotile alteration at less than 280ºC and less than 0.002 XCO2. They also record a third regional prograde event reaching peak temperatures in the range 410-540ºC with XCO2 >0.1.

Demantoid, together with major pyroxene and calcite and minor epidote, chlorite and sulfides (pyrite, chalkopyrite) is associated with intruding granites and quartz-porphyries. The timing and conditions of the formation of the demantoids are not known. The high Cr content recorded from some of these garnet ( up to 12.5% Uvarovite component) originates from chromites associated with the meta-ophiolites.

Italy
Dossi di Franscia, Franscia, Lanzada, Sondrio Province, Lombardy, Italy

Sferlùn asbestos mine, Vallone del Cengiaccio (Cengiaccio; Cengiàsc; Cengiasco), Lanzada, Sondrio Province, Lombardy, Italy

Demantoid from Val Malenco was first described in 1880, and the localities in this area produced the majority of the specimens from asbestos quarries. After the operations seized in these quarries in the 1970-ties, few specimens have been extracted.

Demantoid
The demantoid are generally yellowish green to green. Brownish crystals do also occur. Individual crystals can reach sized of more than 2 cm. Demantoid aggregates of tiny crystals also occurs. These aggregates are referred to as "asbestos seeds"


Geology
The Val Malenco ophiolitic sequence consists largely of harzburgites, with subordinate lherzolites, dunites and clinopyroxenites. The sequence were metamorphosed (serpentinized) during the alpine orogeny (350°C-400°C / 3.5 Kb Bedogne). The demantoid is found within foliated serpentinites in the eastern part of the Val Malenco ultramafic unit. The demantoid is found inside asbestos (chrysotile)–filled brittle fractures in a foliated serpentinite, where it is associated with magnetite, Cr-bearing magnetite, calcite, hydromagnesite, brucite, clinochlore, and rarely, transparent green masses of forsterite. This assemblage has hydrothermal origins and formed in a retrograde metamorphic process during the late Alpine orogenesis at temperatures below 370°C and pressures ranging from 0.5 to 1.5 kbar (Amthauer et al., 1974).

Madagascar
Antetezambato Demantoid-Topazolite Mine, Antsakoamanondro, Ambanja District, Diana, Madagascar

Demantoid
The andradite from the Antetezambato deposits range in color from yellow, via yellowish green, green, brownish green to brown. The crystals are sometimes color zoned. The yellowish green to green crystals are marketed as demantoid. The individual crystals are found in vugs and fissures in skarns and can become up to 3 cm in size. It appears that the vast majority of gem rough and garnet specimens were mined in 2009.

Geology
The demantoid is formed in sediments near the contact between intrusive and sedimentary rocks. The intrusive rocks are syenitic in composition and are considered either syenite (Razafindratsimba, 2017) or trachyte (Pezzotta, 2010,2011), whereas the sedimentary rock are considered either a lime-rich argillite( Razafindratsimba, 2017) or fine-grained sandstone interbedded with limestones (Pezzotta, 2010,2011). The demantoid is found in skarns formed along layer boundaries and fractures in the sediments, and along contacts with the intrusions. The skarn formation is caused by over-pressured pneumatolytic and hydrothermal fluids circulations generated by the intrusions. The result is a network of veins consisting of two different skarn types; a very fine-grained white/gray skarn where garnets are grossular (skarn 1) and a white/light green skarn (skarn 2) where the main garnet is andradite. The grossular typically has a thin rim of near end member andradite. The demantoid crystals are found is in fissures in skarn 2. Razafindratsimba, 2017) considers skarn 2 to be formed later than skarn 1.

Namibia
Tubussis Farm 22, Dâures Constituency, Erongo Region, Namibia

Demantoid
Demantoid crystals up to 1-2 cm are common, but crystals larger than 2 cm are rare. The color of the garnets are often green, but the color can vary quite a bit and brown crystals are not uncommon. Many of the crystals are fractured from tectonic stress.
Geology
Demantoid from this area are found in a metasedimentary sequence of biotite schist, biotite gneiss, calc-silicate rocks ad marbles. The metasedimentary sequence is intruded by granites. Contact metamorphic mineralization follow the metasedimentary bands and is strike parallel. The mineralization rarely exceeds more than a few tens of meters from the granites. Garnets occurs both in calc-silicate rocks ( often brown grossular) and in the marbles ( variable in color, but sometimes demantoid). The garnets are often fractured and contain several generations of fluid inclusions in sealed fractures. This is considered evidence tectonic and/or metamorhic events after the formation of the garnets.

Demantoid can also be found in pockets with perfect gem quality crystals, sometimes associated with calcite, prehnite, quartz and apophyllite. The color of these garnets are variable, often with color zoning. The percentage of clean, gem quality demantoid from these pockets is well above the overall production average. It is quite possible that these demantoids originates from a later hydrothermal event than the formation of the garnets embedded in the calc-silicate rocks and marbles

Pakistan
Balochistan, Pakistan

Demantoid
The demantoids from the Muslim Bagh area rarely gets larger than 1 cm. The crystals are yellowish green to green, sometimes color zoned with a yellow core and with a deeper green rim. Crystals and specimens are still produced, and are available at the gem market in Peshawar as well as on some internet sites.

Geology
The demantoid occurrences from the Muslim Bagh are sparsely described, but it is believed that they originate from ultramafic rocks, due to characteristic chrysotile inclusions. The Muslim Bagh ophiolite, mainly composed of chromite bearing peridotite, but also wehrlites and pyroxenites) The peridotites is partially to completely serpentinized (30-100%). These rocks are intruded by gabbro and mafic dyke swarms. Chrysotile veinlets are common in several of the rock types. It is not unlikely that the demantoid occurs in chrysotile veinlets in a 150km² foliated serpentinite in the Northern part of the ophiolite. The crystals often contains inclusions of magnetite and fibrous to asbestiform chrysotile. Chrysotile fibres are also often attached to the specimens, thus potentially showing a similar type of occurrence as the Val Malenco demantoids.

Russia
Poldnevskoe Demantoid Deposit (Korkodinskoe demantoid deposit), Korkodin, Ufaley District (Ufalei District), Verkhny Ufaley, Chelyabinsk Oblast, Russia

Demantoid
Demantoid were first found in placer gravel in the vicinity of Elezavetinskaya near Nizhniy Tagil in 1853 or before. It was originally thought to be chrysolite ( corresponding to forsterite), but Nordenskiöld saw it was not, and named the green pebbles demantoid. It was not until 1879, A.A. Lyosh provided a chemical analysis confirming that the chemical composition of demantoid corresponded with andradite. Demantoid was a few decades later also identified 200km further south, near Sysert, also in placers. Later demantoid has been found at several localities near the two initial localities, both in placers and in rocks. Today, most of the production comes from the southern localities.

The demantoids are rarely found as well formed crystals, but rather as rounded aggregates up to 5-6 cm in size. They often contain inclusions of chrysotile fibers, dubbed "horse-tail" inclusions. The color ranges from yellowish green to deep green.

Geology
All the localities in the Urals are associated with ultramafic rocks. Demantoid are found in fissures together with chrysotile, but the petrology and the nature of the demantoid bearing fissures are slightly different. Burlakov & Burakov (2017) describe the Korkodin deposit, located in the southern part of Korkodinsky gabbro-peridotite massif. Here demantoid is found in thin veins (normally 1-20mm thick, and rarely up to 15cm) in pyroxenites associated with serentinized dunites. The demantoid are found as individual grains (3-10mm) or intergrowths (up to 5-7cm) crystallized on the walls of the veins. Demantoid are associated with antigotite, tremolite, serpentine, magnetite and chrome-spinel. Well formed crystals are rare.



Karaseva et al. (2021) describe the Poldnevskoye deposit, some 7 km away from the Korkodin deposit. This locality is hosted in the Korkoda ultramafic massif. Demantoid are mostly found in chrysotile veins in a partly serpentinized peridotite. There are clear signs of hydrothermal/metasomatic alteration near the veins, indicating that the veins formed at relatively low PT conditions. Demantoid is found in 2-3m long veins. The demantoid are found as granular aggregates (up to 5 cm) intertwined into up to 20cm long chrysotile fibers. The individual demantoid aggregates are composed of small (less than a millimeter) grains in the central part and larger grains on the periphery. The intergranular space is filled with clinochrysotile (±lizardite?) and/or carbonate.

Chemistry

The demantoid garnets are often near end member andradite, with 97% or more of the andradite component. Data from the different localities discussed in this article show that both the grossular and uvarovite components may be significant. Chegeni et al. (2021) and Urosevic et al.(2018) show grossular dominant demantoids from Baba Nazar, Iran and Namibia respectively. Kropancev(2000) reports an uvarovite component up to 34% in demantoid from the Korkodinskoe demantoid deposit in Russia.

Demantoid Chemistry

Locality Rock type Number of analyses Adr Grs Uv Prp Sps Alm Reference Baba Nazar West Azerbaijan Province Iran Skarn 11 35.56 -60.2(48.58) 37.15-45.09(39.10) 3.86-12.57(8.95) 1.4-3.13(1.83) 0.05-0.35(0.22) 0-.06(0.65) Chegeni et al. (2021) Kerman province Peridotite 10 77.34-97.2(88.46) 0 0.95-11.77(6.69) 1.08-10.39(4.30) 0.21-.41(0.34) 0 Ahadnejad et al. (2022) Val Malenco Peridotite 41 89.38-99.40 (97.11) 0-4.92 (1.35) 0-8.44(0.82) 0-1.9 (0.5) 0-0.8 (0.04) 0 Adamo et al.(2009)& Amthauer et al.(1974)& Bedogné et al.(1999) Madagascar skarn 13 98.32-99.64 (99.38) 0-0.88(0.15) 0 0.25-1.23 (0.44) 0-0.02 (0.02) 0 Pezzotta et al.(2011) Tubussis farm Namibia Skarn 2 32.5-71.71 (52.22) 24.28-59.4 (41.84) 0.36-0.9(0.63) 3.65-6.72(5.9) Urosevic et al.(2018) Pakistan peridotite 8 95.82-98.98 (98.11) 0.2-0.9 (0.53) 0-3.06 (0.59) 0.46-0.87 (0.65) 0.05-0.12 (0.07) 0 Adamo et al. (2015) Russia Peridotite 9 44-97.19 (72.07) 0-21.5 (3.25) 0.96-34.21 (17.33) 0-0.51 (0.21) 0-0.07 (0.02) 0 Karaseva et al.(2021)& Kropancev (2000)

Coloring agents

The original description of demantoid by Nordenskiöld was a green, chrome bearing andradite. Chrome is a well chromophore, known to cause the green color in both chrome-grossulars and uvarovite. It is no big surprise that andradite can be colored green by a small chrome content. The correlation between chrome and green color has been documented from several of the localities discussed in this article.

Ahmedijan et al. (2022) finds strong Cr³⁺ absorption bands between 625 and 640nm in Demantoid from Kerman, Iran. They attribute the yellowish tinge found in some in demantoid to a Fe³⁺, which show a very strong adsorption band between 430 and 444 nm. The result is a yellowish green main transmission window between 530 and 535 nm. Adamo et al. (2009) have analyzed 8 samples from Val Malenco with colors ranging yellowish green to green. They found that demantoids void of Cr was yellowish green, a color attributed to the Fe³⁺ content of nearly end member andradite, grading towards a deeper green color with increasing Cr content The maximum Cr content in their 8 samples was 0.38 wt% Cr₂O₃ which yielded an intense green color. Also from Val Malenco, Amthauer et al. (1974) have analyzed two color zoned crystals showing the same pattern as shown by Adamo et al. (2009). Based on this, one could assume that the coloration of demantoid was relatively straight forward; near end-member andradite is yellowish green, which turns more intense green with an increasing Cr content. Demantoids from other localities shows a more complicated color pattern.




Pezzotta et al. (2011) presents data from 13 demantoid and topazolite crystals from Antetezambato, Madagascar. All the crystals have near end-member compositions ( 98.32-99.64%andradite) and LA-ICP-MS analysis show maximum 2.74ppm Cr. Also other potential chromophores, such as Mn or Ti were found in neglectable amounts. Still, the color of the analyzed samples ranges from blueish green via, green, yellowish green and brown. The color variation shown by these samples are attributed to the Fe in the samples, and both Fe²⁺ and Fe³⁺ replacing Ca and Fe³⁺ replacing Si has been proposed as responsible for the different colors observed in Cr free demantoids from Madagascar and other places. Palke & Rossmann (2017) made various tests on brownish demantoid from Russia, and concluded that it seems unlikely there is enough Fe²⁺ in any of their samples to account for their brown color. The cause of the brown color in these andradite garnets is still incompletely understood.

Conditions for formation of demantoid

Demantoid geology and coloring agent

Locality host rock Fe3+ coloring Cr coloring Comments Kerman Iran Peridotite X Relatively high Cr content Belqeys Iran Skarn X Relatively high Cr content Val Malenco Italy Peridotite X X Both chrome bearing and chrome free demantoids are known from Val Malenco Antetezambato Madagascar skarn X Near end member andradite with very variable colors Tubussis Namibia Skarn X Almost pure andradite-grossular series. Muslim Bagh Pakistan Peridotite X (X) Near end member andradite with between 0-0.92%Cr2O3 Ural Russia Peridotite X The chrome content is very variable; from 0.29% to 8.64%Cr2O3.

Near end-member andradites ( Fe³⁺ colored demantoid)

The conditions for forming near end-members demantoid are little investigated. One of the few exceptions are Amthauer et al. (1974). They studied the demantoid bearing chrysotile veins from Val Malenco, Italy and presented 19 analyses with an average andradite content of 96.91%. The chrysotile veins are considered a result of direct alteration of olivine, which was in part based on find of relict forsterite in the veins. Demantoid are thought to be confined to fissures in rocks also containing pyroxenes where the pyroxenes would be the source of the Ca necessary to form andradite. They further assume that the iron in andradite originates from the silicates, and that the hydrous fluid responsible for the alteration is strongly oxidizing. The demantoid was formed below 370°C and pressures ranging from 0.5 to 1.5 kbar, which is towards the low end of the andradite stability field (Shoji,1977, Taylor & Liou, 1978).

Demantoids from Russia, Pakistan and Iran found in association with serpentinized peridotites are also found in fissures together with fibrous chrysotile, very similar to the Italian localities described by Amthauer et al. (1974). It seems plausible that the demantoids from the other peridotite localities have formed in a similar manner under similar conditions.

The conditions forming near end member demantoids in skarns are little studied, and the conditions (pressure, temperature, chemistry, f_O, X_CO2 etc.) is not known. In Madagascar, Razafindratsimba (2017) finds two generations of skarn. An early generation (skarn1) with brownish grossular dominated garnets and a later generation (skarn2) with demantoids. Palfi (2012) describe brown garnets from the calc-silicate rocks, and both brown and green garnets embedded in marble from Namibia. The most attractive demantoids from Namibia are however found in vugs or embedded in calcite and associated with calcite, prehnite, quartz and apophyllite. These demantoids are formed later then the garnets embedded in skarns and marble and the association with prehnite indicates a low formation temperature, if the demantoid and prehnite are formed together.

It is possible to conclude that near end-member andradite from these localities are formed under different conditions than the normal brownish skarn grossular-andradite garnet, and quite possibly at lower temperatures than the earlier formed brown garnets. The actual near-end member demantoid forming conditions are however poorly known.

Information from synthesized andradites (Shoji 1977, Taylor & Liou 1978,Becker & Pollok 2002) provide generic conditions that favors the formation of andradite over grossular. The formation of andradite relative to grossular is governed by chemistry (high Fe/ Al ratio), f_O (oxidizing environment giving a high Fe³⁺/Fe²⁺ ratio), X_CO2 ( A higher CO₂ content in the hydrous fluid favors andradite over grossular, although Taylor & Liou (1978) finds that andradite require a very high hydrous content). Further studies on these localities appears possible, and better understanding of the conditions giving near Cr-free demantoids in skarn should be possible.

Cr-colored demantoid

Chrome is a strong chromophore, and any andradite with a sufficient Cr content will be green (demantoid). The demantoids from the skarns Baba Nazar, Iran is a good example. Both the associated minerals and the garnet composition (adr35.56 -60.2) are different than the near end-member demantoids, and the high chrome content (up to uv12.57) allows the presence of other coloring agents without disturbing the deep green color. Both from this locality and the Cr-bearing demantoids from peridotites elsewhere owes their Cr content from chromite altered during serpentinization of peridotites.

Summary

In order to form demantoid, conditions for forming green andradite must be present. There are two mechanisms giving green color. Demantoids either get their green color from a small Cr color content, or from Fe³⁺. The latter is the intrinsic color of andradite. For Fe³⁺ to be the dominant chromophore, other coloring agents must be near absent. Many of the Fe³⁺ colored demantoids are near end-member andradites, but a significant grossular content may be present.

For the Cr colored demantoids, both the coloring mechanism and the geological setting is relatively simple to explain: Any andradite with sufficient Cr will be green, and the chrome content comes from chromite that are altered during serpentinization of peridotites. This holds true both for demantoids found in fissures in serpentines or in skarns.

For demantoids colored by Fe³⁺ things are not straight forward. There is not always a clear relationship between composition and color. The garnets from Antetezambato, Madagascar has no clear chemical difference between green, yellow and brown garnets and the coloring mechanisms are not fully understood. The conditions forming near end-member demantoids are also not fully understood, and only Amthauer et al. (1974) has provided a theory of how near end-member demantoid has been formed in chrysotile-veins in serpentinized peridotites. No such theory has been forwarded for the demantoids from skarns.

Revision History

Revision History

Revision no date description 1.0 Feb 2023 First Published 1.1 Dec 2025 Corrected typo in Demantoid chemistry table