Genthelvite

genthelvite

phenakite

bertrandite

willemite

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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 helvine-group minerals to 4 cm have been found in a hydrothermally altered skarn. Some specimens are entirely genthelvite or helvine, and others are epitactic intergrowths of the two species (R&M 96.1.58-68).

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 Mount Rosa, El Paso county, Colorado, USA, beryl is absent, but late-stage to secondary beryllium-bearing minerals are present. A few genthelvite crystals have been found as epitactic overgrowths on danalite, and genthelvite coatings are also present on some of the exposed danalite faces (R&M 95.3.271-272).

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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