|
|
 |
|||||||||||||||||
|
|||||||||||||||||
 |
|||||||||||||||||
| JOURNAL HOME | HELP | FEEDBACK/COMMNET | SUBSCRIBE | ARCHIVE | SEARCH | TABLE OF CONTENTS |
Articles |
1 55 Common Lane, Auckley, Doncaster, DN9 3HX, England
2 Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, England
* E-mail: tmoxon{at}auck.freeserve.co.uk
** E-mail: rios{at}esc.cam.ac.uk
The crystallite size, moganite, molecular surface water and internal water content of agates from volcanic host rocks ranging in age from 38 
1 100 Ma have been measured. Molecular water was found to be independent of age but maturation produced a general decrease in both the moganite and internal water. Water is shown to be involved on the transformation of moganite into chalcedony and this change is responsible for an internal growth in chalcedony crystallites. The maximum content of moganite found in agate was 14% but after 
410 Ma it is only present in trace amounts. Moganite has not been found in agates from any pre Silurian hosts. After the moganite transformation, the internal water in agate has been found to be constant at 0.4%. The change in composition of internal water is proposed as a method for approximate dating of agates that are from hosts younger than 410 Ma.
Changes in the degree of crystallinity of agates from 10 host rocks < 412 Ma have been determined. The growth in the crystallites from 8 regions suggests that agate genesis would be typically penecontemporaneous with the formation of the host rock. Evidence is offered for hydrothermal activity from a late second volcanic event to account for the formation of agates in the other 2 regions. It is proposed that hydrothermal solutions are the source of the silica in all cases of agate genesis in an igneous environment. With minimal moganite in hosts 
412 Ma, crystallites from these older regions are shown to have achieved their maximum size. Furthermore, measurement of crystallite size, density, internal water and moganite can provide evidence of a later palaeoactivity.
Key-words: agate, moganite, water, age, genesis.
This article has been cited by other articles:
|
|
 |
|
  T. Moxon and M. A. Carpenter Crystallite growth kinetics in nanocrystalline quartz (agate and chalcedony) Mineralogical Magazine, November 13, 2009; 73(4): 551 - 568. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
J. Gotze, R. Mockel, U. Kempe, I. Kapitonov, and T. Vennemann Characteristics and origin of agates in sedimentary rocks from the Dryhead area, Montana, USA Mineralogical Magazine, November 13, 2009; 73(4): 673 - 690. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 J. Comer and P. Ortoleva Coexistence of twisted and untwisted crystals: An impurity/structural order model with implications for agate patterns American Mineralogist, November 1, 2007; 92(11-12): 1952 - 1957. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
T. Moxon, S. J. B. Reed, and M. Zhang Metamorphic effects on agate found near the Shap granite, Cumbria, England: as demonstrated by petrography, X-ray diffraction and spectroscopic methods Mineralogical Magazine, August 1, 2007; 71(4): 461 - 476. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
P. J. Heaney, D. A. McKeown, and J. E. Post Anomalous behavior at the I2/a to Imab phase transition in SiO2-moganite: An analysis using hard-mode Raman spectroscopy American Mineralogist, April 1, 2007; 92(4): 631 - 639. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
T. Moxon and S. J. B. Reed Agate and chalcedony from igneous and sedimentary hosts aged from 13 to 3480 Ma: a cathodoluminescence study Mineralogical Magazine, October 1, 2006; 70(5): 485 - 498. [Abstract] [Full Text] [PDF]
|
|
| JOURNAL HOME | HELP | FEEDBACK/COMMNET | SUBSCRIBE | ARCHIVE | SEARCH | TABLE OF CONTENTS |
