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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)
Environments:
Plutonic igneous environments
Pegmatites
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.
Localities
At San Shek Wan, Lantau Island, Islands District, New Territories, Hong Kong, China, almandine can be found on the beach,
only at low tide, as small, deep-red crystals embedded within chlorite-rich metamorphic
rock
(AESS).
Almandine from San Shek Wan - Image
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).
Alteration
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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