From: Lake George Gem & Mineral Club News, 8/09
(4th Place - AFMS Adult Articles Advanced)

Anyone who is interested in minerals and crystallography will eventually encounter twinned crystals. My introduction to twinning occurred when, as a 12-year-old, I collected staurolite crystals from the classic garnet/staurolite locality at Roxbury Falls, Connecticut. Younger readers may have seen the beautiful staurolite crosses from New Mexico, for sale at the Contin-Tail Show in August, or the Russian staurolites sold at most shows and on the internet.

Closer to home, many Colorado localities, including the Crystal Peak area, produce spectacular feldspar twins. Cripple Creek is noted for twinned crystals of the cold-silver telluride, sylvanite, and Mt. Antero has rare twins of the beryllium mineral phenakite. All of these sell at a premium when available.

New collectors often are unaware of pitfalls they need to avoid when they are offered a twinned crystal. Search eBay and other sources, and you are likely to find dozens of "twins" that are, at best, products of an over-active imagination, or, at worst, attempts to deceive. Minerals commonly sold as twins include the feldspars, staurolite, and phenakite, as well as aragonite, beryl, chrysoberyl, calcite, cinnabar, garnet, pyrite, quartz, rutile, tourmaline, and others. Although some of these form twins, others do not. So, how does one distinguish a true twin from a fake?

First, let's consider a definition. According to Klein (2002), twinning is "a symmetrical intergrowth of two (or more) crystals of the same substance." Notice that if three or four crystals are intergrown, we don't call them "triplets" or "quadruplets." They are still twinned crystals. Note also that twinning is a special kind of intergrowth, and this is where problems commonly arise. Those who attribute magical properties to crystals often seem to think that any two intergrown crystals are twins and so have special "powers." Although I can't comment about crystal magic, I can assure you that for every kind of twin there is a twin law that governs such things as the angular relations of the individual crystals and how their symmetries relate. Twins most definitely are not random intergrowths. Ultimately, all of this relates deeply and precisely to the arrangements of atoms and ions in the crystal structure.

Two broad categories of twins are contact and penetration twins. Contact twins consist of two or more crystals joined along a plane or planes of contact (called composition surfaces). Examples are Manebach and Baveno twins in microcline, Japan twins in quartz and spinel twins in spinel, copper, and galena. You can clearly distinguish the composition surfaces in each of these. Interpenetrating crystals characterize penetration twins.

Twins also can be described as simple, where only two crystals are involved, or multiple, if three or more crystals are intergrown according to the same law. If all of the composition surfaces are parallel, multiple twins are said to be polysynthetic. Examples include plagioclase feldspar, in which polysynthetic twinning causes the striations that provide a near-certain clue to the mineral's identity, and in some calcite that has been subjected to pressure during or after crystal growth . Multiple twinning along non-parallel composition surfaces is called cyclic twinning.

Sometimes twinning is obvious even to the beginner, and, at other times, it is subtle and unlikely to be noticed. Japan-law twins in quartz, the cross-shaped twins in staurolite, and Carlsbad twins in microcline fit into the former category. These are all simple twins, and the two crystals involved are clearly visible. However, most quartz exhibits Dauphine twinning, in which the only obvious clue may be offset striations on the prism faces (the flat surfaces parallel to the crystal's long axis). Only rarely is Dauphine twinning obvious. Many aragonite crystals have a roughly hexagonal outline because they are cyclic twins made up of three crystals that are intergrown. Many beginners think aragonite is hexagonal, but, in reality, it belongs to the orthorhombic crystal system.

Twinned crystals constitute a rare, fascinating category for the mineral collector. Keep your eyes open, and you might find a beautiful twin hidden in a dealer's stock at a show or in one of the gem "pockets" at Crystal Peak. They're well worth the search.