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Formula: Ca3Fe3+2(SiO4)3
Nesosilicate (insular SiO4 groups), garnet group
Varieties
Demantoid is a green variety of andradite
Hydroandradite is a variety of andradite with some (SiO4) replaced by (OH)
Melanite is a black, titanium-bearing variety of andradite
Topazolite is a variety of andradite resembling topaz in colour and transparency
Crystal System: Isometric
Specific gravity: 3.8 to 3.9 measured, 3.859 calculated
Hardness: 6½ to 7
Streak: White
Colour: Yellow, greenish yellow to emerald-green, dark green; brown, brownish red,
brownish yellow; greyish black, or black
Solubility: Andradite is slightly soluble in hydrochloric acid
Common impurities: Ti,Cr,Al,Mg
Environments:
Pegmatites
Carbonatites
Metamorphic environments (typical)
Hydrothermal environments (typical)
Meteorites (rare)
Andradite typically occurs in contact or thermally metamorphosed
impure calcium-rich sediments and particularly in skarn deposits, associated with
hedenbergite and magnetite, or with
diopside, vesuvianite,
calcite and clintonite (Lauf p111)
and maybe
danburite. Andradite is also common in calc-silicate
hornfels and marble
(Lauf p112). The varieties demantoid and topazolite are formed primarily in
serpentinite and
chlorite schist
(Lauf p115).
Localities
At Tange Ashin, Kengara, Afghanistan, andradite has been found with chlorite
(Lauf p116).
The Two Mile and Three Mile deposits, Paddy's River, Paddys River District, Australian Capital Territory, Australia,
are skarn deposits at the contact between
granodiorite and volcanic rocks.
Andradite is a primary mineral occurring as crystals up
to 2.5 cm across, associated with actinolite and sometimes with
talc
(AJM 22.1.33).
The Ma On Shan Mine, Ma On Shan, Sha Tin District, New Territories, Hong Kong, China, is an abandoned
iron mine, with
both underground and open cast workings. The iron ores contain
magnetite as the ore mineral and occur predominantly as masses of all sizes
enclosed in a large skarn body formed by contact metasomatism of
dolomitic limestone at the
margins of a granite intrusion. In parts of the underground workings
magnetite is also found in
marble in contact with the
granite. The skarn rocks
consist mainly of tremolite,
actinolite, diopside and
garnet.
Andradite is common in the skarn and
iron ore. It usually occurs as scattered small grains intimately associated with
tremolite-actinolite,
diopside, biotite,
fluorite and magnetite
(Hong Kong Minerals (1991). Peng, C J. Hong Kong Urban Council)
At the Shijiang Shan-Shalonggou mining area, Inner Mongolia, China, the mineral deposits occur predominantly in
veins of hydrothermal origin in skarn. Andradite is a typical
skarn mineral that in this deposit is associated with
clinochlore, olshanskyite
and other boron minerals. The samples suggest a late hydrothermal alteration of
the andradite that in turn crystallised to yield secondary
crystals on borates
(R&M 96.5.398).
Andradite from the Shijiangshan Mine -
Image
At Serifos, Greece, andradite has been found on hedenbergite
(Lauf p113).
Andradite from Serifos - Image
At the Kurar quarry, Malad, Ward 38, Mumbai, Mumbai District, Maharashtra, India, just a few specimens of andradite to 0.5 mm
have been found with prehnite, julgoldite and
ilvaite
(Minrec 34.1.30).
In Kerman province, Iran, demantoid is found in serpentinite
(Lauf p115).
Andradite from Kerman Province - Image
At Belqeys Mountain, Takab County, West Azerbaijan Province, Iran, skarn
mineralisation includes fine demantoid specimens. Traces of chromium
(Cr3+) commonly control the deep green colour. Additionally Fe2+-Ti4+ intervalence
charge transfer as well as Fe2+-Fe3+ interactions can play a secondary role in the tint. In the
Belqeys Mountains, demantoid mineralisation occurs in limestone
layers within metamorphosed amphibolite and
ophiolitic rocks
(serpentinite,
dolerite and basalts).
An intrusion with granodiorite composition is believed to be the
source of hydrothermal fluids that formed skarn mineralisation.
Demantoid mineralisation occurs in both pockets and veinlets in
limestone within pyrite,
magnetite, quartz,
calcite and diopside. Matrix is
mostly composed of limestone and
marble covered by fine crystals of
diopside to 10 mm in size. Large demantoid crystals to 8 cm dominated
by trapezohedral and dodecahedral faces, with exceptional deep green colour and concentric zoning
have been found here
(R&M 97.5.448-449).
Andradite from Belqeys Mountain - Image
At Valmalenco, Italy, demantoid is associated with talc and
chrysotile
(Lauf p115).
Andradite from Valmalenco - Image
At Charcas, Charcas Municipality, San Luis Potosí, Mexico,
the primary minerals are
sphalerite, galena,
chalcopyrite, bornite,
tetrahedrite,
arsenopyrite, pyrite and
silver minerals such as jalpaite,
diaphorite and acanthite. In
the host rock, as metamorphic or alteration minerals, danburite,
datolite, hedenbergite,
epidote, chlorite,
andradite, actinolite
and wollastonite have been reported.
Quartz, calcite and
danburite crystallised during the entire life of the systems, throughout
the intrusive emplacement, metamorphism, and mineralising events. With depth, both
sphalerite and galena decrease
while chalcopyrite increases.
Secondary sulphides formed include
bornite, covellite,
digenite and chalcocite.
Native silver, native gold,
hematite and goethite were
deposited after the sulphides
(Minrec 55.6.727-728).
Andradite is very common in the mines of Charcas as a
skarn-forming mineral associated with
wollastonite. It is massive to granular, opaque, and green or yellow
to brown in colour. Rarely observed in well formed crystals, andradite can form crude dodecahedrons up to
1.5 cm on edge. Associated minerals include sphalerite,
chalcopyrite and bornite.
Well crystalised andradite has been found at the La Bufa mine and the Las Eulalias mine. Black andradite
variety melanite was recovered from the Morelos mine when it was in production
(Minrec 55.6.732).
Andradite from the Morelos Mine, Charcas -
Image
At Monmand, Pakistan, andradite has been found with clinochlore
(Lauf p116).
At the Rosario Mabel claim, Pampa Blanca District, Castrovirreyna Province, Huancavelica, Peru, andradite has
been found in
marble
(Lauf p113).
Andradite from the Rosario Mabel claim -
Image
At Ocna de Fier, Caraş-Severin County, Romania, andradite occurs in magnetite
skarn associated with minor calcite
(Lauf p111).
Andradite from Ocna de Fier - Image
At the Wessels Mine, Joe Morolong Local Municipality, John Taolo Gaetsewe District Municipality, Northern Cape, South Africa,
andradite is associated with hematite
(Lauf p115).
Andradite from the Wessels Mine - Image
At the N'Chwaning II Mine, N'Chwaning Mines, Joe Morolong Local Municipality, John Taolo Gaetsewe
Municipality, Northern Cape, South Africa, andradite is associated with
manganite, hausmannite and other
manganese minerals
(Lauf p115). Andradite pseudomorphs after
rhodochrosite have been found here
(KL p219).
Andradite from the N'Chwaning II mine -
Image
At Coed-y-Brenin deposit, Ganllwyd, Gwynedd, Wales, UK, garnet is common but only occurs in the wider
parts of the veins, where it was one of the last minerals to crystallise. It may occur on or in amphibole
or chlorite. The garnet forms bands of crystals, exceptionally
to 4 mm, which vary in colour from pale yellow through orange to deep red-brown and have inclusions of what is probably
epidote. The garnet is most likely andradite, maybe
with some grossular. Many crystals have orange cores and yellow outer zones, so it is possible
that the cores are grossular and the outer zones andradite
(JRS 21.116).
At Stanley Butte, Stanley Mining District, Santa Teresa Mountains, Aravaipa Mining District, Graham County, Arizona,
USA, andradite crystals to 12 mm in size have been found
(Mindat photo).
Andradite from Stanley Butte - Image
In San Benito county, California, USA, the titanium-rich variety melanite together with topazolite is
found in
serpentinite associated with
perovskite, clinochlore,
calcite, diopside and
rare earth-rich
vesuvianite
(Lauf p115).
Andradite from San Banito County -
Image
At the Luck Fairfax Quarry, Centreville, Culpeper Basin, Fairfax county, Virginia, USA, microcrystals of
andradite to 1mm in association with feldspar have been found
(R&M 98.2.124).
Of Extra-Terrestrial Origin: Hydroandradite has been found in two Mighei-type(CM)
carbonaceous chondrites,
Shidian and Kolang.
Shidian fell in 2017 in Yunnan, China. It consists of fine-grained matrix material containing phyllosilicates (sheet
silicates), type I chondrules (form as molten or partially molten droplets in space before being accreted to
their parent) and rare calcium-aluminium-rich inclusions (CAIs). The dominant minerals of the
meteorite are
olivine and serpentine.
Accessory minerals include troilite and
calcite
(Meteoritical Society Bulletin).
Kolang fell in 2020 at Sumatera Utara in Indonesia. The interiors of the stones host common
breccia fragments that protrude from the fracture surfaces. One
fragment shows a large (3 mm) calcium-aluminium-rich inclusion (CAI) with a pinkish hue. Representative pieces from
the bulk matrix are dominated by serpentine, with medium- to
low- intensity reflections for regularly interstratified
tochilinite/cronstedtite,
tochilinite, calcite,
pyrrhotite and pentlandite.
Particularly noticeable in hand specimen are sparsely distributed greenish-grey
breccia clasts (to 2 cm), dominated by two
serpentines, pyrrhotite,
pentlandite and calcite,
together with well-crystallised
smectite. Polished mount of this clast shows abundant chondrule
pseudomorphs and coarse-grained sulphides
(Meteoritical Society Bulletin).
This is the first report of hydroandradite occurring within
meteorites. Hydroandradite forms through aqueous
calc-silicate alteration under specific fluid conditions. The
meteoritic hydroandradite occurs in four
petrographic contexts: layered, perovskite-associated,
sulphide-associated, and spheroidal. Kolang has all four morphologies, while only the sulphide-associated occurs in
Shidian. In Kolang, hydroandradite was likely produced by replacement of
kamacite, titanium-bearing
clinopyroxene in calcium- and aluminum-rich inclusions, and
secondary magnetite
in three distinct alteration events. The formation temperature of
meteoritic hydroandradite was estimated to be
100 to 245°C. Because Kolang and Shidian are the only
meteorites with hydroandradite reported to date
(August 2025), they may be from the same parent body
(AM 110.8.1238–1248).
Alteration
aegirine and CaO to andradite, quartz and
Na2O
2NaFe3+Si2O6 + 3CaO →
Ca3Fe3+2 (SiO4)3 + SiO2 + Na2O
This is a high temperature process
(DHZ 2A p508).
calcite,
hematite and CO2
quartz to andradite and
3CaCO3 + Fe2O3 + 3SiO2 →
Ca3Fe3+2Si3O12 + 3CO2
hematite,
wüstite, quartz and
calcite to andradite,
hedenbergite,
magnetite and CO2
2Fe2O3 + 2FeO + 5SiO2 + 4CaCO3 →
Ca3Fe3+2(SiO4)3 + CaFe2+Si2O6
+Fe2+Fe3+2O4
+4CO2
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