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Milky quartz is a crystalline quartz that is white and translucent to almost opaque due to numerous evenly distributed gas and/or fluid inclusions.
Specific Properties
As inclusions in a crystal interfere with crystal growth, milky quartz crystals are usually twinned and sometimes also distorted. Often they only show rhombohedral faces and the hexagonal prism and lack accessorial faces as the s and x face altogether.
Crystals of certain growth forms, like artichoke and candle quartz, are always translucent to almost opaque, mostly white, and are never really clear. So most candle quartz qualifies as milky quartz.
Milky quartz crystals can have a waxy instead of vitreous luster and even be dull.
Occurrence
Milky quartz can be found in very different environments. It can be indicative of unsteady growth conditions.
Locations and Specimen
Bolivia
A
milky quartz specimen with split growth from the Mallachuma Mine, Loayza Province, La Paz Department.
Germany
Milky
quartz grown in a pocket in a serpentine gangue. The pocket is very narrow, but the crystals grew parallel to the serpentine fibers. I was surprised to find quartz there, because the surrounding rocks were all mafic or ultramafic (and thus had a very low silica content), but serpentine is already a product of metamorphism, so silica could have been be formed during the alteration of the original rocks. The specimen is from the "Gabbro" quarry, Bad Harzburg, Harz mountains, Germany.
These
intergrown double terminated crystals are not just milky quartz, they are examples of the famous
Suttrop type quartz, named after the village of Suttrop near Warstein in the Sauerland. This specimen is from Hohenlimburg-Oege, Hagen, however. Quartz of this type can be found at many places in Devonian limestone at the northern border of the Sauerland (see
Behr et al., 1979, and
Jahn, 2002). The crystals themselves are younger and formed probably during the late Paleocoic age in a hydrothermal environment. They are characterized by a pseudohexagonal, double terminated habit and a zonar inner structure reflecting a rhythmic growth. Both geyser-like rhythmic movements of water and tectonic events have been discussed as a cause of the zonar pattern. The mineral anhydrite, CaSO
4, was incorporated at relatively high temperatures; later some of it was dissolved at lower temperatures (anhydrite is more soluble at lower temperatures), leaving tiny cavities in the crystal that contribute to its white color. Interestingly, overall the temperature during crystal growth increased with time, starting at about 60-80°C and ending at above 120°C, perhaps even above 300°C (
Behr et al., 1979), which is very different from the typical development of quartz from Alpine-type fissures, for example. The crystals have a very high number of fluid inclusions, consuming up to 10% of their volume. Accordingly, the crystals have a lower specific gravity than usual. They also never show any accessorial faces. Large aggregates of randomly intergrown, equally sized, double-terminated crystals are common. Quartz of the type "Suttrop" should not be confused with the so called "authigenic quartz" also present at and typical for the same locations, as they differ in genesis and properties.
The
term "milky quartz" is occasionally used for white, massive vein quartz. The rock to the right has been found 100 m north of the "Eschbacher Klippen", an outcrop of a large hydrothermal quartz vein that has weathered out of the Taunus mountains northwest of Frankfurt and forms a wall of about 12 m height, 5 m width and about 100 m length. Initially, platy baryte crystals had grown in parts of the vein, but later they were replaced by quartz. The platy morphology is still visible in the rock. The gray-green color in the lower right corner is caused by chlorite inclusions. An image of the Eschbacher Klippen is shown in the section
Occurrence.
Italy
This
specimen is from the Val Ferret, Val D'Aoste, in Italy, but from the southern slope, not the Mont Blanc massive. Iron oxides have precipitated in some of the cracks in the crystal. Because of its resemblance with marble, milky quartz with such a pattern is sometimes called
marble quartz. Note how the crystal has not grown straight, but is bent to the left. Even its rhombohedral faces are not perfectly plane.
These
small, but nice prismatic milky quartz crystals are from the dumps of the Mina Tini, Iglesiente, Sardegna, about 6 km east from the famous Antas temple. They are partially covered with orange and brown iron oxides.
Norway
A
very nice specimen from the Blefjell, Buskerud, that is both a good example of milky as well as of artichoke quartz.
Portugal
Milky
candle and artichoke quartz crystals from the Assunção Mine near Viseu. These crystals are remarkable for being white despite the presence of radioactive elements. There are two small, green crystals of torbernite, an uranium compound, at the lower part of the specimen. Usually you would at least expect the vicinity of the torbernite crystals to be gray or brown, but apparently the large amount of impurities present in the crystals inhibits the formation of color centers.
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