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Formula:Ca4Al6Si6O24(CO3)
Tectosilicate (framework silicate), scapolite group.
Scapolite is the marialite-meionite
series
Crystal System: Tetragonal
Specific gravity: 2.74 to 2.78 measured, 2.86 calculated
Hardness: 5 to 6
Streak: White
Colour: Colourless, white, grey, pink, violet, blue, yellow, orange-brown, brown
Solubility:
Common impurities: Mg,Na,K,Cl,H2O,S
Environments:
Igneous environments
Pegmatites
Metamorphic environments typical
Meionite occurs typically in regionally metamorphosed rocks,
especially marble, calcareous gneiss,
granulite and greenschist. Also
in skarn, some pegmatites, altered mafic
igneous rocks and ejected volcanic blocks. Associated minerals include plagioclase,
garnet, pyroxene,
amphibole, apatite,
titanite and zircon
(HOM).
Localities
At the type locality, Monte Somma, Somma-Vesuvius Complex, Naples, Campania, Italy, meionite was found in a geode from
volcanic ejecta mainly composed of limestone associated with
pyroxene and leucite
(AM 67.1229-1241).
At Manchester, New Hampshire, USA, meionite has been found as irregular columnar masses in a
biotite schist with narrow calcareous
bands interbedded with granite and pegmatite lenses
(AM 67.1229-1241).
The Purple Diopside Mound, Rose Road, Pitcairn, St. Lawrence county, New York, USA, is situated in
marble. The development of veins of large crystals probably occurred as
a result of fluid penetration from a concurrent intrusion. Many of the minerals of interest to collectors formed during
this primary event, with additional species resulting from the
subsequent alteration of scapolite. There seems to be little, if any,
secondary, late-stage mineralisation present.
Meionite occurs as dark grey, translucent crystals and groups of parallel-growth crystals to 20 cm or more. The
composition is close to the midpoint of the marialite-meionite
isomorphous series. Most commonly, meionite crystals have an external rind of opaque pale tan to white
marialite, but some crystals are lustrous grey prisms with no
marialite overgrowth. Meionite fluoresces dull yellow-orange under
longwave ultraviolet illumination.
(R&M 96.6.550). Massive, yellow-grey meionite that fluoresces bright yellow in longwave ultraviolet
radiation occurs in pegmatitic intergrowths with
diopside and calcite
(R&M 90.5.446).
The overgrowths of marialite on meionite are actually considered to
be marialite
pseudomorphs after meionite, and they are extremely common in the
branched scapolite
vein running up the middle of the mound. Some dark grey, glassy meionite crystals are altered from the surface
inward to a cream-white to pale tan layer of
marialite. The following observations indicate that these
pseudomorphs resulted from later alteration of preexisting
meionite crystals:
No marialite crystals have been found without a meionite core.
The marialite/meionite boundary shows an irregular contact typical
of alteration rather than a sharp boundary resulting from overgrowth.
The ultrastructure of the internal meionite reveals a smooth texture typical
of a single crystal domain, whereas that of the external marialite layer
shows a jumble of small crystallised
regions of varying orientation typical of an alteration structure.
Minute acicular crystals of pyrite extend continuously for long distances in
the meionite interior and are crystallographically oriented; these thin
pyrite filaments extend across the boundary into the
marialite
exterior zone with continuity and the same orientation, but they are discordant with the myriad regions of
marialite of different orientation.
Taken together, these data suggest that the pseudomorphs are the result
of the alteration of meionite to marialite, proceeding from the
crystal surface inward. The altering fluids must have been enriched in sodium and chlorine and capable of dissolving
and removing calcium and carbonate
(R&M 97.5.434-444).
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