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Geology is the Way

Feldspathoids

Tetragonal, hexagonal, cubic

Feldspathoids or foids are a family of rock-forming alkali-bearing aluminosilicates that form only in silica-undersaturated igneous rocks. The name derives from their chemical (and sometimes structural) similarity with feldspars. However, feldspathoids are a ‘petrological group’, since their members have significantly different structures and belong to different mineral groups, such as the zeolite group.

Structure and chemistry
All feldspathoids contain less SiO2 per formula unit than feldspars. For example, leucite has the formula KAlSi2O6 and hence its structure accommodates a K+ and an Al3+ cation every two Si4+, whereas three Si4+ are needed to produce K-feldspar (KAlSi3O8). Consequently, these minerals can crystallize only from magmas with a low SiO2 content (silica undersaturated). This makes feldspathoids chemically incompatible with quartz, which forms when there is silica ‘in excess’. Eventual silica in excess with feldspathoids would react to form feldspars. Feldspathoids, indeed, normally coexist with feldspars. The most common rock-forming feldspathoids that occur in magmatic igneous rocks are:

Nepheline Group
Nepheline Na3(Na,K)[Al4Si4O16]; hexagonal
Kalsilite K[AlSiO4]; hexagonal

Leucite K[AlSi2O6]; tetragonal (pseudo-cubic)

Sodalite Group
Sodalite Na8[Al6Si6O24]Cl2; cubic
Nosean Na8[Al6Si6O24]SO4; cubic
Haüyne (Na,Ca)4-8[Al6Si6O24](SO4,S)1-2; cubic

CancriniteVishnevite (Na,Ca,K)6-8[Al6Si6O24](CO3,SO4,Cl,OH)1-2·1-5H2O; hexagonal

An association of two feldspathoids: extinct sodalite surrounded by birefringent cancrinite in a K-feldspar crystal. Ilímaussaq, Greenland. CPL image. Field of view: 7 mm. Photo by Alessandro Da Mommio.

Properties
Habit: equant, prismatic, acicular
Hardness: 5 – 6
Cleavage: varies from species to species
Twinning: simple and multiple twins
Color: transparent to grey-white, pink, yellow, blue, brown, reddish
Luster: vitreous, greasy, pearly
Streak: white
Alteration: clay minerals, cancrinite, albite
In thin section…
Color: colorless, light blue (sodalite group)
Pleochroism: absent
Birefringence (δ): 0.000 (cubic feldspathoids), 0.001 (leucite), 0.003-0.006 (nepheline, kalsilite), 0.002-0.025 (cancrinite – vishnevite)
Relief: low to negative
Optic sign: + (leucite) or – (nepheline, kalsilite, cancrinite, vishnevite)
[Mindat]

Feldspathoids in the field and under the microscope

Nepheline
Nepheline is the most common feldspathoid. In the field is colorless to white-grey and forms prismatic crystals with a hexagonal base. It is commonly, however, part of the fine-grained groundmass and, hence, difficult to recognize in the field.

Under polarized light, nepheline is transparent at PPL and first-order grey at CPL, sometimes appearing ‘dirty’ due to alteration. It shows poorly developed prismatic and basal cleavage planes ({10-10} and {0001}). The recognition of these cleavage planes and hexagonal sections allow to distinguish nepheline from the optically very similar alkali feldspar. Nepheline crystals can show twinning on {10-10}, {33-65}, and {11-22}.

4-sided euhedral crystal of nepheline (transparent) surrounded by white perlialite, black aegirine, semi-transparent microcline and other minerals in a polished pegmatite slab. Width: 5 cm. Loparskaya Valley, Khibiny Massif, Murmansk Oblast, Russia. Photo by John Sobolewski.

CPL
CPL
CPL
PPL
PPL

Above: prismatic section of nepheline from a foidite, showing rectangular shape, first-order grey colors at CPL, and poor prismatic cleavage. Cape Verde archipelago. Field of view: 2 mm. Photo by Alessandro Da Mommio.

Detail of the hexagonal basal section of nepheline with evident poor prismatic cleavage planes intersecting at 60°. Cape Verde archipelago. PPL image. Field of view: 2 mm. Photo by Alessandro Da Mommio.

Kalsilite
Kalsilite and nepheline have similar structure, habit, and optical properties. This makes the recognition of kalsilite from nepheline extremely difficult without chemical analyses. Kalsilite is a very rare feldspathoid.

CPL
CPL
CPL
PPL
PPL

Above: kalsilite crystal (prismatic section) from a venanzite, San Venanzo, Umbria, Italy. Field of view: 2 mm. Photo by Alessandro Da Mommio.

Euhedral crystal of leucite (replaced by orthoclase). Note the equant trapezohedral habit. Size: 6.4 x 5.7 x 5.5 cm. Oberwiesenthal, Germany. Photo by Robert M. Lavinsky.

Leucite
Leucite occurs in K-rich silica-undersaturated igneous rocks. In the field it forms equant grains with six- or eight-sided sections, ranging in color from transparent to grey-white.

In thin section, leucite is easily recognizable because it is pseudo-cubic. Its equant grains are transparent at PPL and show a very weak (nearly extinct) first-order grey birefringence at CPL. Leucite also shows three sets of repeated twinning that resemble the cross-hatched (tartan) twinning of microcline but intersect at 60°. These twins are related to the transformation from cubic to tetragonal (pseudo-cubic) occurring during cooling. Leucite also commonly includes glass and other minerals in characteristic patterns during its growth.

Volcanic rock (tephrite?) with equant, transparent phenocrystals of leucite, showing the typical six- to eight-sided sections. Rome, Italy. Photo by James St. John.

CPL
CPL
CPL
PPL
PPL

Above: round leucite crystal, transparent at PPL and nearly extinct at CPL, in a tephrite from Vulsini, Latium, Italy. Field of view: 7 mm. Photo by Alessandro Da Mommio.

Differently from microcline, leucite twins are oriented at roughly 60° with respect to each other. Vulsini, Latium, Italy. CPL image. Field of view: 2 mm. Photo by Alessandro Da Mommio.

Sodalite Group
Sodalite, nosean, and haüyne are very similar in the field and under the microscope and can be distinguished only through chemical tests or analyses. They are all cubic minerals that develop equant grains with dodecahedron habit. In the field their color ranges from transparent to grey, green, brown, and blue. Sodalite may also be pale pink or yellow.

Under the microscope, these minerals show six- to ten-sided sections, low to negative relief and colors ranging from colorless to pale blue (sodalite also pale pink). At CPL, the members of the sodalite group are always extinct. Sulfur commonly exsolves from these minerals after crystallizations forming trails of sulphide inclusions and thick black rims. Sodalite, nosean, and haüyne show a poor cleavage on {110} and twinning on {111}.

Euhedral crystals of sodalite (blue) associated with pyrite and other gangue minerals. Ladjuar Medam (lapis-lazuli mine), Koksha Valley, Afghanistan. Photo by Parent Géry.

CPL
CPL
CPL
PPL
PPL

Above: haüyne phenocrystals from a foidite from Melfi, Basilicata, Italy. Field of view: 2 mm. Photo by Alessandro Da Mommio.

CPL
CPL
CPL
PPL
PPL

Above: sodalite (colorless at PPL, extinct at CPL) included in eudialite. The high birefringence mineral on the rim of sodalite is cancrinite. Field of view: 7 mm. Photo by Alessandro Da Mommio.

Extinct sodalite crystal surrounded by cancrinite, showing second-order colors, included in K-feldspar. Ilímaussaq, Greenland. CPL image. Field of view: 7 mm. Photo by Alessandro Da Mommio.

Haüyne phenocrystals from Mt. Vulture, Melfi (Basilicata, Italy). Note the thick, black rim and the light blue color. PPL image. Field of view: 7 mm. Photo by Alessandro Da Mommio.

Cancrinite – Vishnevite
Cancrinite and vishnevite form prismatic to acicular crystals with hexagonal basal sections that may show variable color from colorless to white, grey, light blue or even yellow and reddish.

In thin section, they are colorless at PPL but show a moderate birefringence at CPL ranging from first- to second-order colors. They have a perfect {10-10} and poor {0001} cleavage and show rare lamellar twinning. The moderate birefringence make cancrinite – vishnevite very different from other feldspathoids and feldspar. However, it can be confused with birefringent minerals like epidote or muscovite. Cancrinite occurs as primary phases or, more commonly, as secondary minerals that replace other feldspathoids.

Yellow crystals of cancrinite associated with greenish nepheline, black annite, blue sodalite, and white albite. Dennis Hill, Litchfield, Maine, USA. Width: 1.5 cm. Photo by Douglas Watts.

CPL
CPL
CPL
PPL
PPL

Above: cancrinite crystals (colorless at PPL, high interference colors at CPL) from a nepheline pegmatite. Mt. St. Hilarie, Quebec, Canada. Field of view: 2 mm. Photo by Alessandro Da Mommio.

Occurrence
Feldspathoids form in silica-undersaturated igneous rocks from silica-poor basalts to trachytes, tephrites, phonolites, basanites, and foidites (and their plutonic counterparts).  In general, Na-feldspathoids occur in sodic magmas, whereas leucite and other K-bearing feldspathoids are more common in potassic and ultrapotassic rocks. More rarely, feldspathoids such as haüyne and nepheline may form in contact metamorphosed and metasomatized limestone close to igneous bodies. Feldspathoids may coexist with feldspars, femic minerals, and with rare minerals that form in silica-poor systems such as melilite or primary magmatic carbonates.

References and Further Reading
Bonaccorsi, E., & Orlandi, P. (2003). Marinellite, a new feldspathoid of the cancrinite-sodalite group. European Journal of Mineralogy15(6), 1019-1027.
Hahn, T., & Buerger, M. J. (1954). The detailed structure of nepheline, KNa3Al4Si4O16. Zeitschrift für Kristallographie-Crystalline Materials106(1-6), 308-338.
Hassan, I., & Grundy, H. D. (1984). The crystal structures of sodalite-group minerals. Acta Crystallographica Section B: Structural Science40(1), 6-13.
Hassan, I., & Grundy, H. D. (1991). The crystal structure of basic cancrinite, ideally Na 8 [Al 6 Si 6 O 24](OH) 2. 3H 2 O. The Canadian Mineralogist29(2), 377-383.
Mazzi, F., Galli, E., & Gottardi, G. (1976). The crystal structure of tetragonal leucite. American Mineralogist61(1-2), 108-115.
Palmer, D. C., Putnis, A., & Salje, E. K. (1988). Twinning in tetragonal leucite. Physics and Chemistry of Minerals16(3), 298-303.
Tait, K. T., Sokolova, E., Hawthorne, F. C., & Khomyakov, A. P. (2003). The crystal chemistry of nepheline. The Canadian Mineralogist41(1), 61-70.
Taylor, D. (1967). The sodalite group of minerals. Contributions to Mineralogy and Petrology16(2), 172-188.

        

Special thanks to Alessandro Da Mommio
Alessandro and his website, alexstrekeisen.it, contain the widest collection of feldspathoid microphotos we can find on the web. This site would not exist without the restless work of Alessandro.

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