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Formula: Be3Zn4(SiO4)3S
Tectosilicate (framework silicate), helvine group, forms series with
danalite and with helvine,
beryllium-bearing mineral
Specific gravity: 3.44 to 3.70
Hardness: 6 to 6½
Streak: White
Colour: Colourless, white, yellow, green, pink to red, darkens to brown and black on weathering
Solubility: Gelatinises with acids
Luminescence: Nearly pure or end-member genthelvite exhibits strong green fluorescence in long and short wave
ultraviolet radiation, and occasionally short-lived phosphorescence (Dana)
Common impurities: Al,Fe,Mn,Cu
Environments
Plutonic igneous environments
Pegmatites
Hydrothermal environments
The majority of genthelvite occurrences are in alkaline granite
and syenite and associated
pegmatites and hydrothermal veins. More rarely
genthelvite is found in skarn and Alpine-type clefts
(R&M 96.1.58-68). Associated minerals include phenakite,
bertrandite, hambergite,
siderite, sphalerite,
willemite, gahnite,
topaz, zircon,
quartz, microcline and
zeolites
(HOM).
Localities
At the El Criollo mine, Cerro Blanco pegmatite district, Tanti,
San Roque District, Punilla Department, Córdoba Province, Argentina, genthelvite is found in
pegmatites associated with a
monzogranite batholith. It occurs as blue grains and tetrahedral
crystals to 1 cm embedded in massive quartz and specular
hematite. The pale green to deep greenish blue colour of the
genthelvite is
attributed to the presence of copper and
cobalt
(R&M 96.1.58-68).
At the Poudrette quarry, Mont Saint-Hilaire, La Vallée-du-Richelieu RCM, Montérégie, Québec, Canada, excellent crystals
of genthelvite are associated with analcime
(Dana) in feldspathoidal
syenite pegmatites.
Associated minerals include albite,
analcime, aegirine,
serandite and rhodochrosite.
The genthelvite crystals are vivid apple-green in colour, and fluoresce intense green under short and long wave
ultraviolet radiation, and strong to medium orange, yellow or yellowish green under medium wave. A strong but brief
white phosphorescence is also observed
(R&M 96.1.58-68).
At the Huanggang Fe-Sn deposit, Hexigten Banner, Chifeng City, Inner Mongolia, China, outstanding specimens of
At the Ilímaussaq complex, Kujalleq, Greenland, genthelvite has been found in
eudidymite-bearing albitite
veins cutting sodalite syenite.
It occurs as white to pale yellow grains to 1 mm in cavities associated with
aegirine, catapleiite and
neptunite
(R&M 96.1.58-68).
At the Vizze Pass, Sankt Jakob, Pfitsch, Wipptal, South Tyrol, Trentino-Alto Adige, Italy, genthelvite occurs
in fissures in muscovite gneiss
associated with albite, orthoclase variety
adularia, quartz,
rutile epitactic on hematite,
monazite-(Ce) and xenotime-(Y)
(R&M 96.1.58-68).
At Mount Malosa, Zomba, Southern Region, Malawi, genthelvite is found in cavities in
pegmatite veins as yellow-brown crystals to 4.1 cm. It
fluoresces bright green under 365 nm ultraviolet radiation
(R&M 96.1.58-68).
At the Tarraouadji Massif, South Aïr, Aïr Mountains, Agadez, Niger, beryllium
and zinc mineralisation occurs in veins and
breccia in biotite
granite and syenite.
Beryl, bertrandite,
willemite, genthelvite and
berthierite are present, but beryl
and genthelvite do not coexist. It is concluded that genthelvite was precipitated from a low-salinity
aqueous fluid at temperatures higher than 375oC and pressures greater than 220 bars.
There are four reported occurrences of genthelvite.
(1) A vein containing quartz +
sphalerite + galena +
genthelvite is located in a biotite
granite in which late muscovite
has crystallised. Indications are that quartz and
hematite crystallized after genthelvite.
Zircon crystals are also present in this vein.
(2) In a vein containing quartz +
sphalerite + galena +
genthelvite located in biotite
granite genthelvite occurs as beige tetrahedral crystals and
is associated with quartz crystals. A second phase of mineralisation is
responsible for quartz crystallisation on the genthelvite. The second
quartz generation is associated with
galena, sphalerite and
chalcopyrite, the latter often altered to
chalcocite and covellite.
Hematite is the last mineral to crystallise. The last stage of mineralisation
occurs in small cavities as radiating aggregates of small green zincian
berthierite crystals which form a rim on the earlier paragenesis.
(3) An association of willemite +
galena and genthelvite is located in
syenite containing feldspar variety
perthite
and aegirine. Late mineralisation consists of
willemite, galena,
genthelvite, quartz and accessory minerals including
titanite and oxides of iron and
manganese. Genthelvite occurs as crystals coating
willemite and galena, the latter
partly replaced by anglesite.
(4) A breccia containing genthelvite + hematite +
bertrandite is located on a major fault zone. Genthelvite,
bertrandite, clay
minerals, quartz, hematite and rare
cassiterite crystals constitute the cement of
hematite-rich elements. White banded zones contain mainly transparent to
translucent tetrahedra of genthelvite, with marked growth zones containing
quartz inclusions
(AM 75.909-914).
At Jos-Bukuru, Younger ring complex, Jos Plateau, Plateau, Nigeria, genthelvite occurs in
columbite-bearing granite
(Dana).
The country rock consists of albitised
granite. The genthelvite has been found in two different
occurrences.
The first occurrence is within a vein of almost pure albite which intersects
an albite-biotite
granite. This vein contains irregular rounded masses of genthelvite,
usually about four inches in diameter. Occasionally the masses consist of pure genthelvite, but commonly the outer
part or the whole is partially replaced by albite. A selvedge of
mica with accessory thorite invariably
surrounds the nodules, which appear to be xenoliths in the albitic magma.
The second occurrence is in coarse pegmatites containing green
microcline variety amazonite. Most of the genthelvite is coated
superficially and along cracks with bright red laterite. The
mica selvedge is generally, but not invariably, present
(AM 44.1294-1298).
At Baksteval ̊asen, Øvre Eiker, Buskerud, South Norway, the occurrence is related to peralkaline
aegirine and
arfvedsonite-bearing
granite plutons. A microcline variety
amazonite pegmatite
has intruded sediments that are transformed to hornfels. Accessory
minerals include danalite/genthelvite,
phenakite, danburite,
nordenskiöldine and
cassiterite
(CM 58.370).
At Rundemyr, Øvre Eiker, Buskerud, South Norway, the granitic
pegmatite is surrounded by
contact metamorphic sediments. Minerals include
aegirine, galena,
hematite, a pyrochlore-group
mineral, and zircon. Helvine group
minerals mainly occur in quartz cavities but have also been observed as
inclusions in galena. Complete miscibility between
helvine and genthelvite is clearly indicated
(CM 58.370-371).
At Gjerdingselva, Lunner, Oppland, South Norway, tiny crystals of genthelvite to 0.5 mm have been observed in
miarolitic cavities in the pluton. The mineral content of the cavities includes partly decomposed
aegirine, fluorite,
hematite, zircon and
stilpnomelane. The association
genthelvite-hematite-aegirine
implies an alkaline and sulphur-poor environment, which is typical for the
occurrence of genthelvite
(CM 58.371).
At Raikot, Chilas, Diamer District, Gilgit-Baltistan, Pakistan, genthelvite has been reported as dark red
crystals to 5 cm associated with spessartine
(R&M 96.1.58-68).
At Zagi Mountain, Hameed Abad Kafoor Dheri, Peshawar District, Khyber Pakhtunkhwa Province, Pakistan, genthelvite
has been found as yellow to dark reddish brown tetrahedral crystals to 5 cm, with one exceptional 12 cm crystal
(Minrec 353..215-216). The genthelvite is associated with Alpine-type veins in
gneissic alkaline granite
(R&M 96.1.58-68).
At the Western Keivy Massif, Keivy Mountains, Lovozersky District, Murmansk Oblast, Russia, genthelvite crystals
generally to 2 cm are found in the microcline variety amazonite
pegmatite field. Exceptionally an 8 x 12 cm crystal has been
found here and is amongst the largest known for genthelvite
(R&M 96.1.58-68).
At the Ritter pass - Ritterchumme area, Chummibort, Grengiols, Östlich Raron, Valais, Switzerland, genthelvite
is found in small fissures and cavities in
quartz-feldspar boudins in
microcline gneiss. Associated
minerals include albite and rutile
epitactic on
hematite. The genthelvite fluoresces greenish yellow under short and
long wave ultraviolet radiation
(R&M 96.1.58-68).
At Coire an Lochain, Fort William and Ardnamurchan, Highland, Scotland, UK, genthelvite crystals to 1.3 cm
occur in weathered miarolytic cavities in granite, associated with
quartz, feldspar and, less commonly,
bertrandite
(R&M 96.1.58-68).
At St Peters Dome, Cheyenne Mining District, El Paso county, Colorado, USA, the bulk of the
pegmatite material forms a shell about 2 inches thick on the
walls that enclose a central miarolitic cavity. This shell consists of a graphic intergrowth of
microcline-feldspar variety
perthite and quartz, and the
cavity walls are lined with pale buff perthite and smoky
quartz. A few crystals of brown zircon
are embedded in the perthite. The cavity is partly collapsed and filled with
soil, limonitic mud, and smoky quartz
crystals that have become detached from the walls. A genthelvite crystal was found loose in this debris
(AM 38.858-860).
At the type locality, West Cheyenne Canon, Cheyenne Mining District, El Paso county, Colorado, USA, a miarolitic cavity
in a pegmatite has an unusual mineral assemblage that includes
epitaxial genthelvite overgrowths on danalite cores. The cavity was
lined with white microcline crystals with epitaxial
albite overgrowths, clear to slightly smoky
quartz crystals, mica completely altered
to muscovite variety sericite and fluorite crystals
up to 10 cm in size. Prosopite crystals form white coatings on many of the
microcline crystals. Accessory minerals include
columbite-(Fe), ilmenite and
bastnäsite-(Ce) crystals. Late-stage iron minerals, generally
limonite, coat the feldspar,
quartz and muscovite variety sericite. The
genthelvite crystals range in size from 1 to 5 mm and are light rose-coloured. The epitaxial overgrowths of
genthelvite on danalite are unusual because these two minerals do not
coexist in equilibrium, and thus record changing conditions in the cavity
(CM 55.195-206).
At Stove Mountain, El Paso county, Colorado, USA, a genthelvite crystal was taken from a
pegmatite in
granite. Thorium-bearing minerals are common in the dikes, but are rare
in the miarolitic cavities. Correspondingly, thorium-bearing minerals are rare and consist mostly of tabular crystals of
metamict thorite in miaroritic cavities. The form of the
thorite crystals suggests that they may have been the variety
huttonite.
Near Fairview there are two broadly defined zones.
The lower zone
with pegmatites containing
bastnäsite, siderite altered to
hematite, quartz,
feldspar variety perthite, zircon,
thorite variety huttonite(?), and
columbite.
The upper zone has pegmatites containing
bastnäsite, siderite altered to
limonite, quartz,
microcline-feldspar variety
perthite, fluorite,
topaz, phenakite and
genthelvite. The pegmatite from which the
genthelvite was taken is a collapsed miarolitic cavity. The walls of the
pegmatite consist of a graphic intergrowth of
quartz and perthite, set here and
there with small, brown, opaque zircon crystals. Interstices in the walls are
filled with limonite. Well-formed crystals of smoky
quartz and microcline-feldspar
variety perthite project from the walls. Penetrating them are crystals of
bastnäsite and siderite altered to
limonite. The genthelvite crystal was emplanted on a
quartz-microcline-perthite-biotite
intergrowth.
Most of the pegmatites in the upper zone contain
fluorite, but fluorite was not
noted in either of the genthelvite-bearing pegmatites
(AM 42.425-429).
Genthelvite overgrowths on danalite cores occur here. The
pegmatite dike is small and composed primarily of
quartz and feldspar with a surface
alteration to limonite. The
genthelvite/danalite crystals were present in a small miarolitic
cavity that was lined with white microcline crystals, some with epitaxial
albite overgrowths. Microcline
crystals are sometimes covered with mm sized white monoclinic prisms of
prosopite. Quartz crystals to 5 cm
are clear to lightly smoky. Some exhibit a smoky core that is overgrown by milky
quartz. The largest crystals in the pocket are white octahedral
fluorite to 10 cm with light green cores.
Mica minerals are notably absent because they have been altered to
muscovite variety sericite that is stained yellow with late-stage
limonite. Accessory minerals include
ilmenite, columbite-(Fe), and
bastnäsite-(Ce) crystals to 1 cm. Late-stage
iron minerals, generally limonite, coat
the feldspar, quartz and
muscovite variety sericite. Rare, highly altered
limonite after siderite and
hematite are also present. Helvine
group minerals include pale rose-colored genthelvite crystals, A few of which occur as partial to complete
epitaxial overgrowths on tetrahedral danalite cores. If the overgrowth is
incomplete, danalite is exposed as a red triangular face within a
genthelvite crystal face. Late-stage beige genthelvite coatings are present on exposed
danalite faces. This mineral assemblage is, in part, the result of the
alkaline nature of the melt that leads to the formation of helvinegroup
minerals rather than beryl. Decreasing
sulphur activity from early to late pocket-forming stage resulted in the
change from crystallisation of danalite to genthelvite. Increasing
oxygen activity resulted in both the cessation of danalite crystallisation
and a change in crystallising oxide phases, from siderite to
hematite to limonite. Changes in
the late-stage fluid composition are indicated by the shift from earlierformed red genthelvite overgrowths with
Fe greater than Mn to the more Fe-depleted late-stage beige genthelvite coatings. During the final stages of
mineralisation, an extremely latestage fluid may have altered the final mineral assemblage to produce an outer
trace-element-enriched genthelvite rind.
(R&M 93.3.259-260).
At the Iron Mountain Mines, Bartlett, Carroll county, New Hampshire, USA, genthelvite,
helvine and danalite occur in lenses
of magnetite, hematite and
quartz in joints in granite
(R&M 96.1.58-68).
At Sterling Hill, Ogdensburg, Franklin Mining District, Sussex County, New Jersey, USA, genthelvite occurs as
grains and crystals embedded in calcite in a
skarn composed largely of calcite,
rhodonite, actinolite and
albite. In an unusual association crystals of genthelvite were found
embedded in galena. It fluoresces bright green to aqua green under long wave
ultraviolet radiation, weak dark green under short wave and weak greenish grey under medium wave
(R&M 96.1.58-68).
At Washington Pass, Golden Horn Batholith, Okanogan county, Washington, USA, genthelvite is found, very rarely,
in miarolytic cavities in annite
granite as pink to red tetrahedra less than 2 mm
(R&M 96.1.58-68).
Alteration
Genthelvite may overgrow, or be overgrown by, danalite or
helvine, usually in a epitactic relationship
(R&M 96.1.58-68).
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