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THOMSONITE
(MESOLITE.)
occurrence
of this mineral, which is used ornamentally to some extent, is
obtained, like chlorastrolite, in the form of waterworn pebbles
weathered out of an amygdaloidal trap. The pebbles are found on the
shores of Lake Superior, near Grand Marais. They are opaque, and
exhibit concentric structure in layers of various shades of color, such
as olive-green, flesh-red, cream, and white. There are often several
centers of structure in a single pebble, giving a unique and pleasing
effect. The pebbles range up to an inch in diameter, and in cutting are
simply rounded so as to best bring out the various colors and centers
of structure. The hardness of the mineral is 5; specific gravity
2.2-2.4. Its luster is vitreous to pearly. Its composition is that of
a hydrous silicate of aluminum, sodium and calcium, and its occurrence
is almost wholly as a secondary mineral filling the cavities of igneous
rocks.
The
mineral at Grand Marais has long been known as thomsonite, and is
generally sold under that name; but Professor N. H.Winchell affirms
that it is in reality the allied mineral mesolite.'
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192
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