APOPHYLLITE

Apophyllite is a relatively common phyllosilicate, typically a secondary mineral associated with basic igneous rocks. It does not constitute ore, but it has acceptance among collectors. For decades there has been collection and commercialization of pieces with apophyllite in India, in the basaltic rocks of the “Traps” of the Deccan.

In reality, “apophyllite” is not a mineral. The name was discredited by the IMA in 1978 and replaced by the terms fluorapophyllite and hydroxyapophyllite, a solid solution series. New developments in the nomenclature, in 1981, 2008 and 2013, ended up subdividing the members of the Apophyllite Group into:

fluorapophyllite-(K),

fluorapophyllite-(Na),

hydroxyapophyllite-(K),

fluorapophyllite-(Cs),

fluorapophyllite-(NH4) and

hydroxymcglassonite-(K).

The last two are new species, respectively recognized in 2019 and 2020. Among the apophyllites, fluorapophyllite-(K) (KCa4(Si8O20)(F,OH)•8H2O) is the most common. Hydroxyapophyllite-(K) (KCa4(Si8O20)(OH,F)•8H2O) is dominant in some occurrences. The others are rare to very rare. Differentiation between apophyllites is not possible by microscopy, it is necessary to use other analytical techniques.

Apophyllite can fluoresce between yellow and white.

1. Characteristics

The characteristics below refer to fluorapophyllite-(K), which is the most common.

Crystal system: Tetragonal, bipiramidal ditetragonal.

Color: Colorless, white, green, blue, pink, yellow and red.

Habit: Short to long prismatic. Tabular thick to narrow, pseudocubic, massive, drusic. Crystals up to 20 cm.

Cleavage: {001} perfect, {110} imperfect. Striations paralell to [001]

Tenacity: Brittle.

Twinning: On {111}, rare.

Fracture: Irregular

Mohs Hardness: 4.5 – 5

Parting: No.

Streak: White.

Lustre: Vitreous.

Diaphaneity: Transparent.

Density (g/cm³): 2,33 – 2,37

 

2. Geology and Deposits

Apophyllite is of hydrothermal origin and usually occurs in cavities and fractures of basaltic volcanic rocks (basalts, diabases) as a secondary mineral. The limited published data on the genesis of apophyllite place temperatures between 150 and 440 degrees Celsius.

It rarely occurs associated with skarns and amphibolites, forming as a product of hydrothermal alteration along fractures. In skarns may occur as a product of alteration of wollastonite.

It is found in cavities in granites and pegmatites.

Apophyllite can occur in low temperature hydrothermal veins in gneisses.

 

3. Mineral Associations

In general, apophyllites are associated with common secondary minerals found in basaltic rocks, such as varieties of silica (quartz, amethyst quartz, chalcedony, opal (hyalite)), calcite, prehnite, datolite, pectolite, clay minerals (including chlorite (chamosite)) and zeolites such as stilbite-(Ca), heulandite-(Ca), scolecite, laumontite, analcime and natrolite.

In amphibolites and skarns it occurs with wollastonite, prehnite, pectolite and datolite.

Also with vesuvianite in alkaline complexes.

 

4. Transmitted Light Microscopy

Refraction indices:  nω: 1.530 – 1.536    nε: 1.532 – 1.538

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless.

Relief: Low to moderate

Cleavage: {001} perfect and {110} imperfect.

Habits: Prismatic crystals formed by prism, bipyramid and pinacoid. It can form pseudocubic crystals. The pinacoid can dominate, generating tabular crystals of variable width; can be thin tabular. Massive, granular, equidimensional.

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: maximum birefringence of 0.003: very dark, almost black interference colors.

Very strong anomalous interference colors can occur, giving cream to brown colors similar to the interference colors of titanite and carbonates.

Extinction: Tends to be parallel to the cleavage and the shape (important!). It is usually undulating and by sectors.

Elongation sign: SE(+), rarely SE(-). It’s not diagnostic.

Twins: Rare, on {111}.

Zoning: No, but sectorizing is very common.

CONVERGENT LIGHT

Character: U(+) or U(-), can be anomalous biaxial.

2V angle: Small if it is anomalous biaxial.

Alterations: Apophyllite can alter or be replaced by clay minerals, calcite, opal or quartz.

May be confused with: few other minerals, considering the paragenesis and the very low or anomalous, pearly, interference colors.

Zeolites have lower relief, slightly higher interference colors (light gray, white), other habits and are biaxial.

Nepheline and scapolite (marialite) occur in other paragenesis.

Gypsum may be similar, but has gray interference colors (not dark nor anomalous) and oblique extinction with the exception of sections parallel to the b-axis, when the extinction is parallel.

 

Apophyllite 01: Apophyllite in a vesicle of a basaltic rock (rock is on the left and below in the image). In PPL, apophyllite is colorless and low-relief and, in this case, does not show its cleavage (section parallel to the {001} cleavage).
Apophyllite 01: ). Same section as in the previous figure. In XPL, apophyllite shows an anomalous interference color similar to that of calcite.
Apophyllite 02: Several apophyllite grains in PPL (left). Two grains on the left show no cleavage, but the grain on the right shows perfect cleavage, which is diagonally in the image.
Apophyllite 02: same section as in the previous figure: In XPL, the interference colors are cream to brown and extinction paralells cleavage.
Apophyllite grains in XPL, with a 2.5x objective. They show irregular boundaries and undulating and sectored extinction.
In XPL, with a 2.5x objective, scolecites (left) with gray/white colors and apophyllite (right) showing sectored, undulating extinction.
Apophyllite grains in XPL and with the 2.5x objective. Basaltic rock below (dark) and apophyllite above showing undulating and sectored extinction, something very characteristic in these large grains grown in vesicles. Both cleaved and non-cleaved sections show undulating extinction. The maximum darkening position corresponds to the position parallel to the cleavage, but is not complete.
Again in XPL and with the 2.5x objective, apophyllite crystals in a vesicle of a basaltic rock, with sectorized and undulating extinction.
In a very altered basaltic rock (almost black in both images), a vesicle with zeolites (gray/white colors) and several apophyllite grains. On the left, some grains show the typical very coarse cleavage. On the right, section parallel to the cleavage, showing no cleavage traces. The interference color of the apophyllites is a light brown, somewhat similar to the colors of titanite and carbonates. In PPL, all minerals are colorless.
In the same very altered basaltic rock of the previous images, another image of the rock (black), zeolites (gray/white) and apophyllite with interference colors of various shades of cream/brown. XPL.

5. Reflected Light Microscopy

Reflected light microscopy is not the recommended analytical method for the identification of apophyllite. However, it is important to make a polished thin section or a polished section to identify the opaque minerals that occur associated with apophyllite.

Sample preparation: polishing of apophyllite is simple, fast and becomes of excellent quality. Some persistent polishing grooves remain, but with great care (if considered necessary) a perfect polish is achieved.

PLANE POLARIZED LIGHT – PPL

Reflection color: Light gray.

Pleochroism: No.

Reflectivity: Low (<<10%)

Bireflectance: No.

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy:  Anisotropy was not perceived.

Internal reflections: Widespread, clear, colorless, milky.

May be confused with: many other transparent minerals of light colors, including many minerals of the same paragenesis like zeolites (heulandite, scolecite, etc.), calcite, datolite, prenhite, etc. There are no diagnostic features in Reflected Light for apophyllite.

Apophyllite 01: in a very weathered basaltic rock (below), apophyllite in PPL, showing the same colors as many other rock-forming minerals like quartz and feldspars.
Apophyllite 01: the same section as in the previous figure, now in XPL. The uniform, widespread clear internal reflections do not allow a conclusive identification of apophyllite.
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