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--Mildlyoffensive (talk) 06:35, 30 November 2011 (UTC) Mildlyoffensive (talk) 06:35, 30 November 2011 (UTC)

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
Clintonite 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. Samples of clintonite from skarns out of the Predazzo-Monzoni and Adamello Massif area showed similar crystal structure to phlogopite.

Physical Properties
Like other micas and chlorites, clintonite is monoclinic in crystal form and has a perfect basal cleavage on {001}. 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. 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. Clintonite and its iron-rich variety xanthophyllite are sometimes considered the calcium analogues of the phlogopites.

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.

Structure
The 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. Additionally, Al and Ca tend to share edges at octahedral sites due to their small atomic radius and charge.

Geologic Occurrence
Typical 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. Furthermore, high temperature environments are critical to Clintonite formation; if these strict requirements are not met, phlogopite will form instead.

Origin of the Name
Discovered in 1843 in Orange County, New York, clintonite was named after De Witt Clinton(1769-1828).