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
Cancrinite – Vishnevite (Na,Ca,K)6-8[Al6Si6O24](CO3,SO4,Cl,OH)1-2·1-5H2O; hexagonal
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}.
CPL
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.
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
PPL
Above: kalsilite crystal (prismatic section) from a venanzite, San Venanzo, Umbria, Italy. Field of view: 2 mm. Photo by Alessandro Da Mommio.
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.
CPL
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.
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}.
CPL
PPL
Above: haüyne phenocrystals from a foidite from Melfi, Basilicata, Italy. Field of view: 2 mm. Photo by Alessandro Da Mommio.
CPL
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.
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.
CPL
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.
Bonaccorsi, E., & Orlandi, P. (2003). Marinellite, a new feldspathoid of the cancrinite-sodalite group. European Journal of Mineralogy, 15(6), 1019-1027.
Hahn, T., & Buerger, M. J. (1954). The detailed structure of nepheline, KNa3Al4Si4O16. Zeitschrift für Kristallographie-Crystalline Materials, 106(1-6), 308-338.
Hassan, I., & Grundy, H. D. (1984). The crystal structures of sodalite-group minerals. Acta Crystallographica Section B: Structural Science, 40(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 Mineralogist, 29(2), 377-383.
Mazzi, F., Galli, E., & Gottardi, G. (1976). The crystal structure of tetragonal leucite. American Mineralogist, 61(1-2), 108-115.
Palmer, D. C., Putnis, A., & Salje, E. K. (1988). Twinning in tetragonal leucite. Physics and Chemistry of Minerals, 16(3), 298-303.
Tait, K. T., Sokolova, E., Hawthorne, F. C., & Khomyakov, A. P. (2003). The crystal chemistry of nepheline. The Canadian Mineralogist, 41(1), 61-70.
Taylor, D. (1967). The sodalite group of minerals. Contributions to Mineralogy and Petrology, 16(2), 172-188.
Resources
An introduction to the Rock-Forming Minerals. Deer, Howie & Zussmann.
Optical Mineralogy: Principles & Practice. Gribble & Hall.
Transmitted Light Microscopy of Rock-Forming Minerals: An Introduction to Optical Mineralogy (Springer Textbooks in Earth Sciences, Geography and Environment). Schmidt.
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