--Mildlyoffensive (talk) 06:35, 30 November 2011 (UTC)
Mildlyoffensive (talk) 06:35, 30 November 2011 (UTC)
| Clintonite | |
|---|---|
 Clintonite with spinel on orthoclase matrix from Amity, New York (size: 9.3 x 5.7 x 3.8 cm) | |
| General | |
| Category | Phyllosilicate Mica group |
| Formula (repeating unit) | Ca(Mg,Al)3(Al3Si)O10(OH)2 |
| Crystal system | Monoclinic |
| Space group | Monoclinic 2/m - prismatic |
| Unit cell | a = 5.204Å, b = 9.026Å, c = 9.812Å; β = 100.35°; Z = 2 |
| Identification | |
| Color | Colorless, yellow, orange, red-brown, brown, green |
| Crystal habit | Tabular pseudohexagonal crystals; foliated or lamellar radiated; massive |
| Twinning | Spiral polysynthetic twinning |
| Cleavage | Perfect on {001} |
| Tenacity | Brittle |
| Mohs scale hardness | 3.5 on {001}, 6 at angle to {001} |
| Luster | Vitreous, pearly, submetallic |
| Streak | White, slightly yellow-gray |
| Diaphaneity | Transparent to translucent |
| Specific gravity | 3.0 - 3.1 |
| Optical properties | Biaxial (-) |
| Refractive index | nα = 1.643 - 1.648 nβ = 1.655 - 1.662 nγ = 1.655 - 1.663 |
| Birefringence | δ = 0.012 - 0.015 |
| Pleochroism | X = colorless, pale orange, red-brown; Y = Z = pale brownish yellow, pale green |
| 2V angle | Measured: 2° to 40° |
| References | [1][2][3] [10] |
Clintonite is a calcium magnesium aluminium phyllosilicate mineral. It is a member of the margarite group of micas and the subgroup often referred to as the "brittle" micas.
Composition
editClintonite has the chemical formula: Ca(Mg,Al)3(Al3Si)O10(OH)2. The chemical composition of clintonite has been reported to have remarkable consistency with little substitution outside of the Ca-Mg-Al-Si cations.[10] Samples of clintonite from skarns out of the Predazzo-Monzoni and Adamello Massif area showed similar crystal structure to phlogopite.[9]
Physical Properties
editLike other micas and chlorites, clintonite is monoclinic in crystal form and has a perfect basal cleavage on {001}.[3] The Mohs hardness of clintonite is 6.5, and the specific gravity is 3.0 to 3.1. It occurs as variably colored, colorless, green, yellow, red, to reddish-brown masses and radial clusters.
The brittle micas differ chemically from the micas in containing less silica and no alkalis, and from the chlorites in containing much less water; in many respects, they are intermediate between the micas and chlorites. Clintonite’s brittle nature is caused by the Ca cation’s small atomic radius and charge, leading to an increase in the negative charge of the oxygen atom.[10] Due to the brittle nature of clintonite, the interlayer separation is reduced by 0.6Å on average and it exhibits an inverse relationship with the α angle. As for other types of micas such as phlogopite and biotite, a different relationship is seen.[9] Clintonite and its iron-rich variety xanthophyllite are sometimes considered the calcium analogues of the phlogopites.[9]
There is a correlation between the birefringence seen on the (001) cleavage and the regularity of stacking in the mineral – a decrease in birefringence indicates stacking irregularity. Furthermore, there is a correlation between Md-patterned uniaxial crystals or mosaic-containing crystals with tiny center birefringence and the crystallographic β angle. Specifically, most of the 1M crystals that have twinning perpendicular to the (001) cleave have rotations of ±120º and showed extra spots on photographs. Even though clintonite with the 3T polytype had not been found, some crystals from Amity, New York imitated the form by having volumes of three twins on equal parts.[10]
Structure
editThe typical structure of clintonite is a pseudohexagonal crystal and involves tetrahedral and octahedral sheets with an interlayer. Edge sharing occurs at tetrahedral sites. The Si generally forms the tetrahedral sites, but there is a 70% chance that Al will substitute for it instead. This sort of substitution results in a greater number of tetrahedral sites and increased thickness of the sheets in phlogopite and Al-rich phlogopite. For Clintonite, however, this same substitution phenomenon produces normal and flatter tetrahedrons. Consequently, the lateral dimensions of the tetrahedral sheets increase. Furthermore, the Al substitution pushes the rotational angle α, which links the tetrahedral and octahedral sheets, to its maximum value. Ca and Al form the octahedral sites in clintonite; however, bond lengths are reduced in the presence of Al.[9] Additionally, Al and Ca tend to share edges at octahedral sites due to their small atomic radius and charge.[5]
Geologic Occurrence
editTypical formation occurs in serpentinized dolomitic limestones and contact metamorphosed skarns, with the presence of talc, spinel, grossular, vesuvianite, clinopyroxene, monticellite, chondrodite, phlogopite, chlorite, quartz, calcite and dolomite.[3] Furthermore, high temperature environments are critical to Clintonite formation; if these strict requirements are not met, phlogopite will form instead.[4][5]
Origin of the Name
editDiscovered in 1843 in Orange County, New York, clintonite was named after De Witt Clinton(1769-1828).[2]
See also
editReferences
edit- ^ Webmineral
- ^ a b Mindat with location data
- ^ a b c Handbook of Mineralogy
- ^ a b [Rice, J. M. (1979). Petrology of Clintonite-bearing marbles in Boulder Aureole, Montana.American Mineralogist, 64, 519-526.]
- ^ a b c [Toraya, H. (1981). Distortions of octahedra and octahedral sheets in 1M micas and the relation to their stability. Zeitschrift für Kristallographie, 157, 173–190.]
- ^ [Ulmer, P. (1982). Monticellite-clintonite bearing assemblages at the southern border of the Adamello-Massif. Rendiconti Societa` Italiana di Mineralogia e Petrografia, 38, 617–628.]
- ^ [Annersten, H. and Olesch, M. (1978). Distribution of ferrous and ferric iron in clintonite and the Mössbauer characteristic of ferric iron in tetrahedral coordination. Canadian Mineralogist, 16, 12-60.]
- ^ [Alietti, E., Brigatti, M.F., and Poppi, L. (1995). The crystal structure and chemistry of high aluminium phlogopite. Mineralogical Magazine, 59, 149–157.]
- ^ a b c d e [Alietti, E., Brigatti, M. F. and Poppi, L. (1997). Clintonite-1M: Crystal chemistry and its relationships to closely associated Al-rich Phlogopite. American Mineralogist, 82, 936-945.]
- ^ a b c d [MacKinney, J.A., C.I. Mora, and S.W. Bailey (1988). Structure and crystal chemistry of clintonite. American Mineralogist, 73, 365-375.]
Category:Phyllosilicates Category:Magnesium minerals Category:Calcium minerals Category:Aluminium minerals Category:Monoclinic minerals Category:Mica group