In other circles, the handedness of quartz crystals is determined by the"windows". Metaphysical enthusiasts call a left hand crystal the "window to the past" or a right hand crystal the "window to the future"   Index to Quartz Digging Cleaning Worth Fee Pay Mines Types Forms Inclusions Geology Mineralogy Metaphysics Synthetic Gemstones Handedness Experiments Photos from readers

Chapter 1   Page 15 Crystallography and "Handness" of Quartz Recently I had a question concerning how to tell right- from left-handed quartz. This article, although a somewhat lengthy answer, should give the reader an understanding of the problem and how, if you have the proper crystal faces expressed, to determine the "handness" of quartz crystals. To explain such a thing to a person not familiar with crystallography is understandably difficult, so I will endeavor to present a precursory explanation of the crystallography of quartz.       If you will read the Hexagonal System article which my wife and I put together about Crystal Systems and Crystallography , then you will be ready to read the rest of this text. I will be referring to drawings in both the web article and in some books.       Way back in 1816, a naturalist/crystallographer by the name of Weiss discovered a property of crystal forms called enantiomorphism. This word is a mouth full, but pretty easy to understand. Hold your hands out in front of you, thumbs touching. (I assume you have all 8 fingers and 2 thumbs). Each hand is a mirror image or reflection of the other. Some crystal forms or shapes may exhibit this same property. But telling a right-handed one from a left-handed one requires some knowledge of crystal forms, thus our discussion will first have much to do with crystallography, before we look at how you determine if a quartz crystal is right- or left-handed. Internal structure I will explain, however, why quartz is enantiomorphic. It has to do with the internal structure of the mineral. Quartz is composed of silica, SiO2, which occurs in the crystal structure as a tetrahedral form. Tetrahedrons are 4-sided closed geometric figures which are composed of equilateral triangles. You can think of it as a 3-sided pyramid sitting on the 4th side -- its base. All 4 sides are equal in area. Now, to grow a quartz crystal, these tetrahedra link together in chains which are parallel to the C axis, but the chains will have a rotational twist. They may either rotate clockwise along the length of the crystal or counterclockwise. The odds are 50-50 as to which direction they rotate. However, once they begin to form, the crystal's handness has been locked in. Mr. Weiss set forth some external form (morphological) conventions concerning determining whether a crystal is right- or left-handed. But to understand the names of the forms we must know something about crystallography. Hexagonal crystal system Since quartz is hexagonal/trigonal in form, I should start by defining the crystallographic axes for the hexagonal crystal system and inform the listener that all of the silica minerals we define as quartz crystallizes in the trigonal division of this system. You have to know something about trigonal symmetry. Look at the crystallographic axis drawing on the crystallography web site (fig. 6.1) and note the position of the C axis and the 3 a axes, both positive and negative ends. Next, note the drawing of a hexagonal prism, the 6-sided open form which is parallel to the C crystallographic axis. In relatively undistorted, prismatic quartz crystals, this form is exhibited along the length of the crystal. When viewed down the end (parallel) to the C axis, these sides form the edges of the hexagon you see around the outside of the crystal. These faces are designated as m in any crystal drawing. There is a drawing of the prism form in the web article, but it also has an end termination, which in the drawing is noted as the c pinacoid face. You can leave this face off your drawing, so you can visualize that the prism form is open at the ends (see drawing on right. Now that we have a hexagonal open-ended shape, we must define the forms of quartz that compose the termination. If you look at many quartz crystals, you will find that usually there is a set of 3 dominant (larger) faces that alternate with 3 subordinant (smaller) faces on the termination. We will not deal with distorted crystals, only "chalkboard-type ideally formed" quartz crystals. The trigonal division       We have to discuss several forms in the trigonal division. The most simple form to recognize is the positive rhombohedron. It usually forms the dominant set of 3 faces on a terminated quartz crystal. By dominant, I mean that the faces of the positive rhombohedron are larger than the other set of 3 faces. This form is also considered, by convention, to be positive in relation to the 3 a crystallographic axes. The positive rhombohedron faces are designated as r in crystal drawings. The subordinate form on the termination is the negative rhombohedron and is designated as z in drawings. It is easy to confuse which set is the positive rhombohedron, but usually, aside from being larger, the positive rhombohedron presents a higher reflection of light from the surface than does the negative form. Figure 2 on page 10 of Volume III of Frondel's Dana's System of Mineralogy is an excellent drawing to show the relationship of the positive and negative rhombohedral faces to the a crystallographic directions. It's simple, but effective. A mouthful of crystal forms The next form to consider is the trigonal trapezohedron, more specifically the positive right trigonal trapezohedron, which is labeled x on crystal drawings of quartz. This face is not often expressed. When present, it is seen at the junction of 2 prism faces m and the positive rhombohedron r and it displays a trapezohedral planar shape. It is usually bright lustered. The trigonal pyramid, the last form to discuss, is designated as s and is at the junction of the positive rhombohedron r and the prism m which is in line vertically with the negative rhombohedron z. It typically forms an elongated rhombus-shaped face. It is rarely present. Drawings of these forms are present in many editions of Dana's Textbook of Mineralogy. Now, we are finally ready to talk about determining if a quartz crystal is right- or left-handed! [For the demonstration discussed below, you need to look at Figures 7 and 8 from Frondel's Dana's System of Mineralogy, Vol. III, page 14, which are drawings of left- and right-handed quartz crystals. Or see the drawings below.] Orienting the crystal Before you can orient a crystal to determine it's handness with certainty, you must have the forms present that we have just discussed. If the trigonal pyramid s is not present, but the trigonal trapezohedron x is, then we are making an educated guess. Without either two forms present, we cannot tell "handness" by using external form. You start by finding a quartz crystal that has both the pyramid and the trapezohedron present. You will observe them as "unusual extra faces" on your specimen. That is the hard part because most crystals do not display these forms. However, we will assume you have one in your hand. Look at the termination and decide which alternating set of 3 faces is both the largest (dominant) and has the highest luster. This is where we can get into quite a discussion if the specimen's termination exhibits almost equal-size sets of rhombohedral faces! But you must make this determination. When you have, then the set of dominant faces becomes the positive rhombohedron r by convention. Also, the subordinant set must be the negative rhombohedron z. Now, hold the crystal where the termination is pointing up (vertical) and rotate it around horizontally in your hand until the positive rhombohedron r faces you. Are the small faces of the trigonal trapezohedron x and the trigonal pyramid s on the right lower corner of the positive rhombohedron r ? If so, then the crystal is a right-handed crystal by convention (by definition). If these 2 faces are on the lower left of the positive rhombohedron r, then the crystal is a left-handed crystal by definition. Reread these two statements while looking at the drawings below. It's that simple. Not so simple to get to this point, but simple once you are here! Now, as a matter of interest, I must say that all quartz crystals are either right- or left-handed whether we can determine it from their crystal form or not. There are optical and etching methods that confirm this assumption. So, if you don't have the necessary faces present to make an accurate determination, then you can only flip a coin because you have a 50-50 chance of being correct (or wrong) by guessing! Rare specimen Very rarely, you may find a quartz crystal that has the x and s faces on both lower right and lower left edges of the positive rhombohedron r ! If you do, you have a very rare specimen. I have one example for 40 years of collecting and have to admit it was given to me by a fellow collector because I kept raving about how fantastic it was. He just saw that the crystal had a ding on the termination and so it was of no "real value" to him! It is a Brazil twin which is the intergrowth of both a right- and left-hand quartz structure in what appears to be a single crystal. Such twinned quartz crystals are not particularly rare, but to have both sets of x and s faces expressed make the combination of their presence with the twinning most unusual! The advanced collectors are the ones who look at more than just the beauty of the specimen. Next page is experiments with quartz crystals Ch 1, Page 15 Contact the authors of Rockhounding Arkansas revised July 1998 ©Rockhounding Arkansas 1998 http://rockhoundingAR.com