Apophyllite

apophyllite

datolite

pectolite

zeolite

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Apophyllite refers to a five minerals:
Fluorapophyllite-(Cs) Cs₄Si₈O₂₀F.8H₂O
Fluorapophyllite-(K) KCa₄Si₈O₂₀F.8H₂O
Fluorapophyllite-(Na) NaCa₄Si₈O₂₀F.8H₂O
Fluorapophyllite-(NH₄) NH₄Ca₄Si₈O₂₀F.8H₂O
Hydroxyapophyllite-(K) KCa₄Si₈O₂₀(OH,F).8H₂O
They are all phyllosilicates (sheet silicates)
Specific gravity: 2.3 to 2.4
Hardness: 4½ to 5
Streak: White
Colour: Colourless, white, yellow, green, brown, pink
Solubility: Moderately soluble in hydrochloric acid
Environments:

Plutonic igneous environments
Metamorphic environments
Hydrothermal environments
Basaltic cavities most common

Historically all the apophyllite group minerals were reported simply as "apophyllite", and indeed it requires analysis to differentiate between them, so there is comparatively little information about the individual members of the group, except in recent times.
As a general rule, both fluorapophyllite and hydroxyapophyllite may be present on a single specimen, and frequently are. Aside from Jefferson, North Carolina, and Kimberley, South Africa, only one other locality provides specimens which consistently have OH > F. This locality is Guanajuato, Mexico, which has provided many excellent hydroxyapophyllite exhibit specimens (AM 63.196-202).

Apophyllite occurs chiefly as a late secondary mineral in amygdules and druses in basalt, associated with zeolites (such as stilbite, heulandite, chabazite), datolite, calcite and pectolite. It is found less frequently in cavities in granite and syenite, in metamorphic rocks and limestone and calc-silicate rocks, sometimes as an alteration of wollastonite, and as a late product in hydrothermal ore deposits (Dana, HOM).

Localities

At the Shijiang Shan-Shalonggou mining area, Inner Mongolia, China, the mineral deposits occur predominantly in veins of hydrothermal origin in skarn. Apophyllite occurs as colourless and transparent crystals reaching 3 cm in length. Associated minerals include andradite, diopside, hedenbergite, calcite and olshanskyite, and apophyllite is the last to crystallise in the paragenesis (R&M 96.5.398).

In the Deccan traps, India, in most localities apophyllite crystallised later than heulandite, and at the same time as, or shortly following, stilbite. Rarely, second generation apophyllite has been found (Minrec 34.1.30-36).

At Rahuri, Ahmednagar District, Maharashtra, India, a find in 2000 produced many very large green apophyllite crystals with flat pinacoid terminations and very small pyramid faces, arranged in discrete hemispherical groups on beds of white stilbite crystals (Minrec 34.1.30).

At Savada, Dharangaon Taluka, Jalgaon District, Maharashtra, India, apophyllite occurs in hard, nearly impermeable basalt, almost wholly without channels connecting the cavities. Hence neighbouring cavities often display very different mineralisation. The apophyllite is sometimes colourless, and only very rarely deep green, attributable to inclusions of celadonite and related phyllosilicates (Minrec 34.1.30-36).

At the Pune District, Maharashtra, India, in several locations fine crystals of apophyllite are found, coloured green by vanadium V⁵⁺, with the intensity of the colour increasing proportionately to the vanadium content. Crystals are usually well formed and exhibit faces {110} (prism), {101} (pyramid) and {001} (pinacoid).
In the Pashan quarries the crystals have formed in basalt with particularly large cavities that are widely interconnected so that uniform solutions circulated among them (Minrec 34.1.30-36).

In a quarry in Mahad, Raigad District, Maharashtra, India, pseudomorphs of calcite and fluorite after apophyllite were found (Minrec 34.1.30-36).

At Low Knott Quarry, Forest in Teesdale, County Durham, England, UK, apophyllite crystals up to 1 cm long embedded in calcite have been found in a 2 cm wide calcite-quartz vein in dolerite (JRS 21.7).

At Cambokeels mine, Westgate, Stanhope, County Durham, England, UK, apophyllite crystals up to 7 mm were found in abundance, typically accompanied by analcime crystals. In a few specimens the apophyllite was overgrown by pectolite. The apophyllite is probably fluorapophyllite-(K) (JRS 21.7).

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