Adularescence in Quartz

[About the author-Jack Lowell is a geologist living in Tempe, Arizona. His special interests are gemology and pegmatite minerals. He can be contacted by email by clicking on "feedback."]

In 1987 this writer received a shipment of 3 tons of amethyst and quartz geodes from Uruguay. A small percentage of the material exhibited adularescence and a sample was sent to the Gemological Institute of America. In the winter, 1987 issue of Gems and Gemology, page240, John Koivula, chief gemologist of the GIA, proposed the optical phenomena be called the "Lowell effect" in amethyst. The news item showed a crystal with the effect, and a request for people interested in studying the material was made. In 1988 the GIA reported that some colorless quartz showing adularescence had been found by two gemologists in India, near Poona. The winter 1988 issue of Gems and Gemology showed a picture of the material, and figure 1 shows a very striking photo taken by John Koivula of this quartz.

Preliminary studies have been done by various persons, and it has been proposed by Dr. Peter Heaney of Princeton that the quartz has polysynthetic twinning. An intensive search of the literature found a paper by the famous physicist, Sir C.V. Raman in the Journal of the Academy of Indian Sciences. (Sir C.V. Raman, 1950) in which Raman describes "irridescent" colors in certain sections of colorless quartz from India. This is the first known mention of the optical effect in scientific literature. To date, quartz with adularescence has also been found by this author in Turkey and Brazil. Recently a sample was sent from Washington state and also identified as being the same phenomena.

This site is created to hopefully further the research on this polysynthetic twinning in quartz by those interested in the scientific community. There will be a section on the observations made by this author while studying a vast amount of quartz, mainly from Uruguay.

Twinning Many of the crystals with the strongest adularescence also show unusual crystal forms, and experts on twinning are invited to look at the material. Study material is available from this author.

Physical structure not fully determined Investigators with access to SEM and TEM and other equipment capable of defining the periodicity and spacing of the the polysynthetic twins are needed. Study material is available to qualified researchers at no cost

Observations of Uruguayan amethyst and colorless quartz showing adularescence.

Photograph courtesy of John I. Koivula

Adularescence in Quartz

2.90 carats, in the collection of Dave Daniel 
Photograph by Tino Hammid

Three photos of a Uruguayan amethyst crystal. Optical phenomenon is found
in colorless zone on outer portion of crystal.
Photos by John Koivula, Gemological Institute of America.

Photo of "X-twin," a form found on quartzes showing the adularescence
Photo by John Koivula of the GIA.
(This may be a new twin law of quartz and researchers interested in studying this can obtain samples by contacting: jacklowell@earthlink.net )

Photo of 2.90 ct cut amethyst showing Lowell effect in amethyst
Photo by Tino Hammid

"Aurora" effect shown well in cut amethyst showing Lowell effect.
Photo by Jeff Scovil, jewelry courtesy of L. Courtland Lee

Observations of the "Lowell effect" in the Artigas, Uruguay Amethyst / Quartz, ~-new variety of quartz.

1) Relatively minor in relation to total specimens produced by Uruguayan mining Companies.

2) Most pieces acquired by CGMG are on very thin agate matrix with caramel and gray, sharply divided color bands, the latter (gray) on exterior.

3) Interior of geode formations exhibits very strong color zonation in *xls purple color in the core and colorless on outer portion of xls.

4) High degree of brilliance shown by all pieces (though frosty and dull surfaces in other pieces may be preventing optical visibility of phenomena).

5) Etching is prominent on the major rhombohedral crystal faces in almost all evident examples of the effect. The etching lines are curved with the dip to the base (or prism face) and are located near the junctions of the minor rhombohedral faces, which are not etched, and are usually strong.

6) The "Lowell effect" is observable on only certain amethyst-quartz specimens, and not on all xls which make up the specimens. It appears as a sheet of light extending from beneath the minor rhom. faces when viewed under any light source. When the xls are not heavily etched at the rhom. face junctions, the color is sometimes blue. Usually the color is whitish. Only at a certain angle to a light source is it possible to see the phenomena. On some specimens / xls the effect is very deep (1/4") under very small rhom. faces, the depth much greater than width of those faces. Blue color shows on these xls. The effect is confined to an outer "skin" in all cases, being in a colorless zone and not in a i zone with purple color, i.e., only in an outer portion of the ! colorless zone and never in amethyst zones. However, the appearance
of the zoned xls is one of amethyst due to the interior color.

7) Penetration twins are observed on crystals of the specimens that exhibit the strongest "Lowell effect".

8) Observance of LE-P is facilitated by orienting the c-axis of a given LE crystal directly at a pinpoint light source while viewing in more subdued light. Reflection on rhombohedral faces is close to angle of LE observance of that same face, the later being a slightly greater angle than the reflection angle. When viewing the LE effect on the junctions, it is sometimes possible to see the LE at "back angles" i.e. where the LE appears in the interior of a rhombohedral face. This is referred to from now on as "reverse" LE.  This has been noted on specimens of LE, or the strongest instances.  (LE-2 is medium LE intensity, LE-3 is weak intensity.) 1st degree,
2nd degree, 3rd degree, as it were.

9) LE has been seen mainly on the type of xls described in #'s 3,4,5, and 7. Some specimens of agate with colorless and almost colorless quartz xls have been found. The xl habit is usually of an elongated (somewhat) prism and with the 6 rhomb, faces more equally developed as far as size is concerned. Little or no etching is seen on the xl faces. The LE is generally not as sharply defined but can be deeply and usually somewhat penetrating into the xl interior, LE-P is often the only noticeable LE in these xls.

[About the author-Jack Lowell is a geologist living in Tempe, Arizona, and owner of Colorado Gem and Mineral Company. His special interests are gemology and pegmatite minerals. He can be contacted by email by clicking on "feedback."]