Danalite

danalite

helvine

genthelvite

phenakite

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Formula: Be3Fe2+4(SiO4)3S
Tectosilicate (framework silicate), helvine group, forms a series with genthelvite and with helvine, beryllium-bearing mineral
Specific gravity: 3.28 to 3.46
Hardness: 3½ to 6
Streak: Grey-white
Colour: Pink, grey, yellow, reddish brown, red
Solubility: Gelatinises with acids
Common impurities: Mn,Zn,Ca,S
Environments

Plutonic igneous environments
Pegmatites
Metamorphic environments
Hydrothermal environments

Danalite occurs in granite, granite pegmatites, contact metamorphic and hydrothermal environments, skarn and gneiss. In skarn it is associated with magnetite, garnet and fluorite; in tin-bearing pegmatites it is associated with albite, cassiterite, pyrite and muscovite; in hydrothermal deposits it is associated with arsenopyrite, quartz and chlorite (HOM).

Localities

At Needlepoint Mtn, McDame area, Liard Mining Division, British Columbia, Canada, danalite occurs in skarn in limestone, associated with magnetite, quartz, chlorite, fluorite and native bismuth (AM 65.358).

At the Mihara mine, Mihara City, Hiroshima, Japan, danalite occurs with phenakite in monzonite near a skarn contact (AM 65.358).

At an unspecified locality in Russia, danalite has been reported in pegmatites replacing beryl; it is in turn replaced by bertrandite, phenakite, pyrite and hematite (AM 65.358).

At the Precambrian Russian platform, danalite altered by hematite occurs in quartz veins in altered granite and dolerite. Helvine, phenakite, muscovite, calcite, magnetite and aegirine occur in related rocks (AM 65.358).

In the Russian Far East, in complex tin-rare metal deposits, danalite occurs in plagioclase-clinopyroxene skarn in marble, associated with cassiterite, scheelite, garnet, fluorite and arsenopyrite.
Also, danalite occurs in diopside-magnetite skarn with chrysoberyl, phlogopite, vesuvianite, epidote, garnet, hastingsite, fluorite, cassiterite and chalcopyrite (AM 65.358).

At Lindsay's Leap, Newcastle, Mourne Mountains, County Down, Ulster, Northern Ireland, UK, danalite occurs rarely as small crystals and grains on joint planes and in small cavities with quartz, saponite and stilbite-Ca. It is later in the paragenesis than quartz and feldspars, but earlier than the low-temperature zeolites (MM 25.25, 27).

At the Cheyenne Mining District, El Paso county, Colorado, USA, small genthelvite crystals which sometimes occur as epitaxial overgrowths on danalite have been found in a pegmatite pocket. The genthelvite/danalite crystals were present in a small cavity that was lined with microcline crystals, some with epitaxial albite overgrowths, and sometimes covered with prisms of prosopite to 5 mm. Accessory minerals include ilmenite, columbite-(Fe) and bastnäsite-(Ce). Late-stage genthelvite coatings are present on exposed danalite faces. The mineral assemblage is, in part, the result of the alkaline nature of the melt that leads to the formation of helvine-group minerals rather than beryl. Decreasing sulphur activity from early to late pocket-forming stages 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 (R&M 93.3.259-260).

The type locality is Rockport, Essex county, Massachusetts, USA.

At the Cape Ann Granite Company quarry, Gloucester, Essex county, Massachusetts, USA, danalite occurs in cavities in granite and is altered to siderite, phenakite, quartz, pyrite and sphalerite (AM 65.358).

At the Iron Mountain mines, Bartlett, Carroll county, New Hampshire, USA, danalite occurs in metasomatically replaced granite, danalite-helvine solid solutions are found overgrowing phenakite, with magnetite, pyrite, sphalerite, quartz, galena, fluorite, hematite and pyroxene (AM 65.358).

At Iron Mountain, Sierra county, New Mexico, USA, helvine and danalite occur in a metamorphic contact zone between limestone and rhyolite and granite. The host rock is a dark, banded tactite composed chiefly of magnetite, fluorite, chlorite and diopside. Danalite is associated with fluorite, magnetite, grossular, chlorite, biotite, vesuvianite, epidote, tilleyite and chamosite variety thuringite. (AM 29.163-191, AM 65.358).

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