Almandine is the commonest mineral of the garnet group.
Formula: Fe2+3Al2(SiO4)3
Nesosilicate (insular SiO4 groups), garnet group
Crystal System: Isometric
Specific gravity: 4.318 measured, 4.313 calculated
Hardness: 7 to 7½
Streak: White
Colour: Red, black
Solubility: Insoluble in water, hydrochloric, nitric and sulphuric acid
Common impurities: Mg,Mn,Ca, replacing Fe2+ (Lauf p108)

Plutonic igneous environments
Metamorphic environments (common)

Almandine is the common garnet in metamorphic rocks, typically occuring in mica schist, gneiss and amphibolite, resulting from the regional metamorphism of argillaceous (clay-rich) sediments. It also occurs in contact metamorphic hornfels, and occasionally in plutonic rocks such as diorite, granite and granite pegmatites (Lauf p107). It is stable over a wide range of pressure-temperature conditions. Metamorphic almandine is a mineral of the hornblende-hornfels, amphibolite, granulite, blueschist and eclogite facies.


The Pulsifer pegmatite, West Mount Apatite Mining District, Auburn, Androscoggin county, Maine, USA, is a rare-element granitic pegmatite that intrudes upper amphibolite facies metapelites and biotite schists that are locally interbedded with calc-silicate rocks. The pegmatite exhibits five distinct zonations.
The border zone consists of fine to mediumgrained equi-granular quartz and plagioclase, and minor biotite and almandine.
The wall zone consists of slightly graphic K-feldspar, quartz, biotite and almandine.
The first intermediate zone is characterised by coarse-grained graphic feldspar and plumose muscovite-quartz aggregates. Almandine and schorl are found as accessory minerals, and rare beryl has been observed.
The first intermediate zone grades irregularly into a coarse-grained plagioclase plus quartz plus muscovite second intermediate zone.
The pocket zone assemblage, which lies below the second intermediate zone and immediately above the garnet seam, consists primarily of cleavelandite and quartz, although locally blocky K-feldspar and muscovite are also abundant. Gem tourmaline, beryl, fluorapatite, hydroxylherderite, gahnite, almandine, columbite-(Mn) and cookeite are among the minerals that have been found within the pocket zone.
The garnet seam is a 1 to 5 cm thick layer of 2 mm to 3 cm euhedral almandine plus anhedral smoky quartz.
Just below the garnet seam lies a zone of undetermined thickness that is composed primarily of blocky graphic albite.
Almandine is most prevalent in the garnet seam (R&M 97.1.8-11).

At the Emmons pegmatite, Greenwood, Oxford county, Maine, USA, almandine occurs as crystals to 3 cm. Much of the almandine has been replaced by mica. The Emmons pegmatite is an example of a highly evolved boron-lithium-cesium-tantalum enriched
pegmatite (R&M 94.6.503).

At the Mount Marie Quarries, Paris, Oxford County, Maine, USA, the entire crest of Mount Marie is composed of coarse-grained pegmatites that are frequently rich in schorl, sometimes exhibit aplitic zones, and are wedged between numerous schist, gneiss and calc-silicate xenoliths and roof pendants. The pegmatites are part of a lithium - cesium - boron - (elbaite, lepidolite) enriched pegmatite field. Within the irregularly exposed pegmatite segments are lithium-rich, iron-poor albitic zones protruding around the edges of numerous, discontinuous, white to smoky quartz pods.
The garnets are being recovered adjacent to isolated quartz pods with one side of the crystals in contact with microcline and/or plagioclase feldspar, often associated with diamond-shaped muscovite crystals and the other side, with the sharply formed crystal faces, in contact with the white quartz. Single almandine crystals with a brown-red colour in transmitted light frequently reach 20 cm across. Groups of almandine crystals forming matrix specimens have exceeded 60 cm in height and 60 cm in width.
Rarely, the garnets are found with stepped crystal faces. Single rhombic dodecahedral crystals with stepped faces are typically smaller than the sharply formed trapezohedral garnets, averaging 7–13 cm across. Rare matrix specimens of the stepped crystals can reach 30 cm across.
For both crystal habits, trapezohedral and stepped rhombic dodecahedral, crystallisation was from a mixed silicate melt that was significantly undercooled (cooled below its crystallisation temperature). These garnets are late crystallisation products, which formed near the end stage of the primary pegmatite crystallisation (R&M 98.2.142-147).

At the Spurr mine, Michigamme, Baraga county, Michigan, USA, clinochlore pseudomorphs after almandine have been found (KL p239).


almandine and phlogopite to pyrope and annite
Fe2+3Al2(SiO4)3 + KMg3AlSi3O12(OH)2 ⇌ Mg3Al2Si3O12 + KFe3AlSi3O10(OH)2
Both temperature and pressure affect the equilibrium of this reaction, but temperature is more significant (JVW p 179). This assemblage is commonly formed during amphibolite facies metamorphism of pelitic rocks (KB p129).

calcium-iron amphibole and anorthite to garnet (grossular and almandine), clinozoisite and quartz
Ca2Fe3Si8O22(OH)2 + 6Ca(Al2Si2O8) ⇌ 4/3Ca3Al2(SiO4)3 + 5/3Fe3Al2(SiO4)3 + 2Ca2Al3[SiO7][SiO4]O(OH) + 5SiO2
(MM 48.206)

calcium amphibole, grossular and quartz to diopside- hedenbergite, anorthite, pyrope-almandine and H2O
2Ca2(Mg,Fe2+)3Al4Si6O22(OH)2 + Ca3Al2(SiO4)3 + SiO2 = 3Ca(Fe,Mg)Si2O6 + 4Ca(Al2Si2O8) + (Mg,Fe2+)3Al2(SiO4)3 + 2H2O
Diopside-hedenbergite occurs commonly in regionally metamorphosed calcium-rich sediments and basic igneous rocks belonging to the higher grades of the amphibolite facies, where it may form according to the above reaction (DHZ 2A p272).

chloritoid and quartz to staurolite, almandine and H2O
23Fe2+Al2O(SiO4)(OH)2 + 8SiO2 ⇌ 4Fe2+2Al9Si4O23(OH) + 5Fe2+3Al2(SiO4)3 + 21H2O (DHZ 1A p844)

hornblende, grossular and quartz to Fe-rich diopside, anorthite, almandine and H2O
2Ca2(Mg,Fe2+)3(Al4Si6)O22(OH)2 + Ca3Al2Si3O12 + 2SiO2 = 3Ca(Mg,Fe2+)Si2O6 + 4CaAl2Si2O8 + (Mg,Fe2+)Al2Si3O12 + 2H2O
Fe-rich diopside occurs commonly in regionally metamorphosed calcium-rich sediments and basic igneous rocks belonging to the higher grades of the amphibolite facies. The above reaction is typical (DHZ 2A p272).

muscovite, iron-rich biotite and SiO2 to orthoclase, almandine and H2O
KAl2(AlSi3O10)(OH)2 + KFe2+3(AlSi3O10)(OH)2 + 3SiO2 ⇌ 2KAlSi3O8 + Fe2+3Al2(SiO4)3 + 2H2O
Iron-rich biotite is likely to react at lower PT conditions than iron-poor biotite (DHZ 3 p72).

staurolite and quartz to almandine and sillimanite and H2O
62Fe2+2Al9Si4O23(OH) + 11SiO2 ⇌ 4Fe2+3Al2(SiO4)3 + 23Al2OSiO4 + 3H2O
Increasing temperature favours the forward reaction. At higher pressure kyanite replaces sillimanite in the above reaction (AM61.699-709).

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