Amphiboles (silicates): Apophyllite

Diagnostic card.
In the photo there are transparent crystals. Below: green crystals of apophyllite on columnar Heylandite. Both samples are from Pune (India).

K Ca ₄ F (Si ₄ O ₁₀ ) ₂ * 8 H ₂ O
Tsingonia tetragonal
Hardness 4,5-5
Specific weight 2.32-2.37
Cleavage perfect along the base
Crack irregular
Color is colorless, differently colored
Color in powder white
Glitter glass, mother-of-pearl

Two extreme members are distinguished in composition: hydroxyl-apophyllite and fluorophilic apophyllite. Apophyllite crystals are very common and very diverse in appearance. They are flattened, some very thin in the form of plates (in the section mostly rectangular), others with corners cut off by four facets, others resemble a cube whose peaks are cut off by small triangular faces of a tetragonal pyramid.

Also quite common are crystals in the form of a square bipyramid with pointed tops resembling an octahedron, or an elongated prism, or pinacoid with a strongly developed base. Crystals can be of considerable size; Be colorless and transparent, or white and translucent and opaque. The most elegant and interesting for collecting are pink, green and azure blue.

Singonia, symmetry class, space group. Tetragonal; 4 / m2 / m2 / m; P4 / mnc. The parameters of a unit cell and the number of formula units. A = 8.960, c = 15.78; Z = 2. Crystal structure. Instead of the right planes formed by the joined six-membered rings that are present in the layered silicates described above, the structure of the apophyllite consists of regular four-membered rings joined together into irregular eight-membered rings that form layers with the formula (Si4O10) 4 usual for phyllosilicates.

Habit. Usually it forms crystals; A combination of {110}, {101} and {001}; The crystals look cubic, composed of a cube and an octahedron, but the correct symmetry breaks the cleavage in one direction and the difference in gloss between basal planes and other facets. Also tabular, flattened parallel {001}; Prism usually with a shading parallel to the c axis. Cleavage. By {001} perfect, by {110} is unclear.

Gloss and light transmission. Pearlescent on {001}, on other planes glass. Transparent to translucent. Optical properties. Anisotropic, biaxial (+). The refractive indices are: no = 1.531-1.536, nе = 1.533 - 1.538. Low birefringence. Diagnostic signs. Shape of crystals, cleavage and association.

Diagnostic signs.
In the flame of the candle it is stratified and melts, forming a light glass; When heated in a glass tube, loses water, which is deposited on the walls. Under the action of hydrochloric acid decomposes, forming a gel-like precipitate. Colorless or white; In some cases slightly colored in pink, green or yellow.

Origin.
Apophyllite is a typical mineral of hydrothermal origin. It mainly occupies voids in the rocks of the basalt and the composition close to it.

Apophyllite, rare earth. Prudyansky K-r, Cherkasy region., Ukraine, CIS. Photo: © А.А. Evseev.

Fluorophosphite (manganese Mn-containing), dark-pink coarse aggregate
In the cavity of the modified andesite. Persia, to the SW of Akhaltsikhe, Georgia, the CIS. Photo: © А.А. Evseev.

Fluoropophyllite. Bear Island, Kandalaksha Bay, White Sea, Russia, CIS.
Rosette-shaped aggregates of lamellar crystals. Photo: © А.А. Evseev.

Place of Birth.
Magnificent specimens, sometimes emerald green, came from the Poona area, adjacent to Bombay, in India. In Italy, notable samples are known from the Alpe de Siusi (Bolzano province) and the surroundings of Moro (province of Trento). Less often, apophyllite in the form of small but elegant crystals is found in the cracks of Montecoño-Majora basalt in Vicentino.

Hydroxyapophyllite. The skin of two dark-green thick-plate crystals of hydroxyapophyllite stained with inclusions of actinolite. Lomontite, okenit. Hydroxyapophyllite. A group of prismatic crystals on a gyrolite crust. Quarry for the town of Maly Kermen (Pervomaisky = Trudolyubovsky kr), near the village of Trudolubovka, to SW from Simferopol, Crimea, Ukraine. Photo: © А.А. Evseev.

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