Sanidine

Sanidine
Sanidine from Puy de Sancy, Monts-Dore massif, Puy-de-Dôme, France. Size 5 cm × 4.5 cm (2.0 in × 1.8 in)
General
Category	Feldspar
Formula (repeating unit)	K(AlSi3O8)
IMA symbol	Sa [1]
Strunz classification	9.FA.30
Dana classification	76.01.01.02
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	C2/m
Identification
Color	Colorless to white
Crystal habit	Tabular crystals, may be acicular
Twinning	Carlsbad twinning common
Cleavage	{001} perfect, {010} good
Fracture	Uneven
Tenacity	Brittle
Mohs scale hardness	6
Luster	Vitreous, pearly on cleavage
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	2.52
Optical properties	Biaxial (−)
Refractive index	nα = 1.518–1.525 nβ = 1.523–1.530 nγ = 1.525–1.531
Birefringence	δ = 0.007
2V angle	Measured: 18°–42° (low); 15°–63° (high)
References	[2] [3] [4]

Sanidine is the high temperature form of potassium feldspar with a general formula K(AlSi₃O₈). ^[2] Sanidine is found most typically in felsic volcanic rocks such as obsidian, rhyolite and trachyte. Sanidine crystallizes in the monoclinic crystal system. Orthoclase is a monoclinic polymorph stable at lower temperatures. At yet lower temperatures, microcline, a triclinic polymorph of potassium feldspar, is stable.

Due to the high temperature and rapid quenching, sanidine can contain more sodium in its structure than the two polymorphs that equilibrated at lower temperatures. Sanidine and high albite constitute a solid solution series with intermediate compositions termed anorthoclase. Exsolution of an albite phase does occur; resulting cryptoperthite can best be observed in electron microprobe images.

Occurrence

In addition to its presence in the groundmass of felsic rocks, sanidine is a common phenocryst in rhyolites and, to a lesser extent, rhyodacites. ^[5] Trachyte consists largely of fine-grained sanidine. ^[6]

Fallout ash beds in sedimentary rock of the western United States have been classified in part by whether sanidine phenocrysts are present and, if present, whether they are sodium-enriched. W-type rhyolite ash beds contain sodium-poor sanidine; G-type rhyolite ash beds contain sodium-rich sanidine; and dacite fallout ash beds frequently lack sanidine. Because of their high potassium content, sanidine phenocrysts are also very useful for radiometric dating of rhyolite ash beds by the K–Ar dating method. ^[7]

Composition

Although the ideal composition of sanidine is 64.76 wt% SiO₂, 18.32 wt% Al_sO₃, and 16.72 wt% K₂O, natural sanidine incorporates significant sodium, calcium, and iron(III). Calcium and sodium substitute for potassium (with concurrent substitution of additional aluminum for silicon, in the case of calcium) while ferric iron substitutes for aluminum. A typical natural composition is: ^[8]

Component	Weight %
SiO2	64.03
Al2O3	19.92
Fe2O3	0.62
CaO	0.45
Na2O	4.57
K2O	10.05

At elevated temperature, a complete solid solution exists between sanidine and albite. Rapid cooling of the sanidine freezes the composition, though most sanidine is cryptoperthitic, showing separate layers of low-sodium sanidine and albite at a sub-micron scale that can be detected only by X-ray crystallography or electron microscope methods. ^[9]

Order-disorder transitions

The crystal structure of ideal potassium feldspar has four sets of tetrahedral sites, each capable of accepting either an aluminum or a silicon ion. These are labeled the T₁o, T₁m, T₂o, and T₂m sites. In sanidine, the aluminum and silicon are distributed randomly among all four sites, and the T₁o and T₁m are mirror images of each other, as are the T₂o and T₂m sites. This produces a crystal with monoclinic symmetry. With slow cooling, the aluminum becomes concentrated in the T₁ sites but remains randomly distributed between T₁o and T₁m sites. The resulting orthoclase crystal retains monoclinic symmetry but with different crystal axis lengths. Further cooling causes the aluminum to concentrate in the T₁o sites, breaking the monoclinic symmetry and producing triclinic microcline. Each transition requires exchange of ions between tetrahedral sites, which takes place at measurable rates only at high temperature. ^[10]

Sanidine and genesis of magmas

Pure sanidine melts incongruently at 1150 °C, yielding solid leucite and liquid. A mixture of sanidine with silica in the form of tridymite melts at a eutectic temperature of 990 °C, which defines the "granite" eutectic. ^[11] The temperature at which granite begins to melt is lowered by several hundred degrees by the presence of water. ^[12]

References

1. Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi: 10.1180/mgm.2021.43 . S2CID   235729616.
2. "The New IMA List of Minerals – A Work in Progress – Updated: March 2014" (PDF). Archived from the original (PDF) on March 25, 2014.
3. http://www.mindat.org/min-3521.html Mindat.org
4. http://www.webmineral.com/data/Sanidine.shtml Webmineral data
5. Fisher, Richard V. (1984). Pyroclastic rocks. Berlin: Springer-Verlag. p. 22. ISBN   3540127569.
6. Macdonald, Gordon A. (1983). Volcanoes in the Sea: The Geology of Hawaii (2nd ed.). Honolulu: University of Hawaii Press. p. 128. ISBN   0824808320.
7. Fisher 1984, pp. 355–356.
8. McBirney, Alexander R. (1984). Igneous Petrology. San Francisco, CA: Freeman, Cooper. pp. 104–111. ISBN   0877353239.
9. Klein, Cornelis; Hurlbut, Cornelius S. Jr. (1993). Manual of Mineralogy (after James D. Dana) (21st ed.). New York: Wiley. pp. 535–536, 541. ISBN   047157452X.
10. Nesse, William D. (2000). Introduction to mineralogy. New York: Oxford University Press. pp. 210–211. ISBN   9780195106916.
11. Philpotts, Anthony R.; Ague, Jay J. (2009). Principles of igneous and metamorphic petrology (2nd ed.). Cambridge, UK: Cambridge University Press. pp. 207–208. ISBN   9780521880060.
12. Philpotts & Ague 2009, p. 252.

• Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., Wiley, ISBN   0-471-80580-7