^ord to the Wise

JOHN RAKOVANDepartment of GeologyMiami UniversityOxford, Ohio 45056rakovajf@muohio.edu

flmygdldes (also known as amygdales) are gas cavities, orvesicles, in igneous rock (usually volcanic or shallowintrusive) that are filled, or partially filled, with second-

ary minerals. These minerals may have a late-stage magmaticorigin or, more commonly, may form from postmagmaticaqueous solutions. Common minerals in amygdules includecalcite, quartz, chalcedony, chlorite, and zeolites. Regionally,other minerals may be predominant. For example, many ofthe datolite nodules found in Michigan's Copper Country(see article by Tom Rosemeyer in this issue) occur as amyg-duies in the Portage Lake Volcanics. Also found in UpperMichigan, particularly in the Keweenaw Peninsula and onIsle Royale, are amygdules of chlorastrolite (a variety ofpumpellyite) known as Michigan greenstones, Michigan'sstate gemstone. Furthermore, one of the important modesof copper formation in the Keweenaw deposits is as amyg-dules. These deposits formed from hydrothermal solutionsthat permeated basaltic lava flows and deposited copperand other minerals by open-space filling (i.e., fractures andvesicles) and replacement.

Vesicles are created by the expansion of gas bubbles orsteam within molten lava; they are, therefore, usually round-ed, elongated, or almond-shaped, like those in figure 1. Ves-icles and hence amygdules are generally more pronouncedin basalts than in other volcanic rock types. This is partlybecause of the lower viscosity of basaltic lavas, which allowsgas bubbles to readily expand before the lava solidifies.Vesicles are typically an inch or two in maximum dimen-sion; however, they can be much larger. The larger ones arecommonly known as pockets or vugs and are less likely to

Dr. John Rakovan, an executive editor of Rocks & Mineralsand a professor of mineralogy and geochemistry at MiamiUniversity in Oxford, Ohio, is currently a Visiting Professor inthe Graduate School of Human and Environmental Studies,Kyoto University, Japan.

Figure 1. The oval features in these photomicrographs (crossedpolars, 20x magnification) are amygdules: formerly open vesiclesthat have heen fdled with secondary minerals that precipitatedfrom low-temperature ground waters that penetrated the hosthasalts. The amygdule comprises zeolites on the top; NinadBondre specimen. The image on the bottom shows an amygdulewith multiple generations and habits of calcite; Brian Curriespecimen.

Figure 2. Copper amygdules in basalt. Wolverine mine, near thevillage of Kearsarge, in Houghton County, Michigan. The speci-men measures 11x7x7 cm, A. E. Seaman Mineral Museumspecimen, George Robinson photo.

202 ROCKS & MINERALS

Figure 3. Mordenite on chalcedony lining a 9-cm-wide gas cavityin hasalt h-om Chinchvad, India. Canadian Museum of Naturespecimen, Jeff Scovil photo.

become completely filled by secondary minerals. If miner-alized, they usually exhibit varying degrees of lining. Thespectacular museum specimens of zeolites from the DeccanTraps in India (Ottens 2003) are found mostly as amygdulesor pocket linings.

In some lavas, rising gas bubbles may stretch out to form

tubular vesicles. Mineralization of these vesicles forms whatare known as tube or pipe amygdules. Large tubular gas pock-ets (often many feet long) lined with amethyst and otheraccessory minerals, colloquially known as "cathedrals," arecommon in the basalts of Rio Grande do Sul, Brazil.

The term amygdule is derived from the Greek word for"almond." There are some ambiguities in the semantics givenabove. Specifically, the dividing line between what we wouldcall an amygdule and a mineralized pocket, and the degreeof vesicle infilling necessary to be called an amygdule, is notprecisely defined—although, as already mentioned, vesiclesand hence amygdules are usually less than 2 inches in size.Igneous rocks with abundant amygdules are said to have anamygdaloidal texture or fabric. These are common terms usedto describe igneous rocks; further information can be foundin most books on igneous petrography and petrology (i.e.,Best 1982).

ACKNOWLEDGMENTSI would like to thank Kendall Hauer and Ninad Bondre for their

helpful reviews and comments, and Brian Currie, Ninad Bondre,George Robinson, and |eff Scovil for the photos.

REFERENCESBest, M. G. 1982. Igneous and metamorphic petrology. New York: W.

H. Freeman and Co.Ottens, B. 2003. Minerals ofthe Deccan Traps, India. Mineraiogical

Record i4:\-82. ^

JOSEPH A. FREILICH L L CWe are pleased to announce the opening of our new website:

www.josephafreilichcollections.com

Fine minerals from the Joseph A. Freilich Collectionas well as contemporary specimens.

lOSEPH A.

.Toseph A Freilich Collections

Volume 80, May/June 2005 203

John RakovanJOSEPH A. FREILICH L L CWe are pleased to announce the opening of our new website:www.josephafreilichcollections.comFine minerals from the Joseph A. Freilich Collectionas well as contemporary specimens.lOSEPH A..Toseph A Freilich Collections