ISSN:
1432-0967
Source:
Springer Online Journal Archives 1860-2000
Topics:
Geosciences
Notes:
Abstract Agates of volcanic origin, containing the different quartz species, fibrous, length-fast chalcedony (CH), granular fine quartz (FQ), and fibrous, length-slow, to lepidospheric quartzine (QN), have been investigated to evaluate possible relations between microstructure, i.e. crystallite size and texture, refractive indices, densities, contents of trace elements and of “water”, as well as dehydration behaviour. By means of near infrared spectroscopy, total “water” contents $$c_{({\text{H}}_{\text{2}} {\text{O}}){\text{tot}}} $$ , could be differentiated quantitatively into contents of molecular “water”, $$c_{{\text{(H}}_{\text{2}} {\text{O)mol}}} $$ , and silanole-group “water”, $$c_{{\text{(H}}_{\text{2}} {\text{O)SiOH}}} $$ . Despite the low total “water” contents of the agates studied ( $$c_{({\text{H}}_{\text{2}} {\text{O}}){\text{tot}}} $$ between 1 and 2 wt.%), near infrared spectroscopy results in reliable data on $$c_{{\text{(H}}_{\text{2}} {\text{O)mol}}} $$ and $$c_{{\text{(H}}_{\text{2}} {\text{O)SiOH}}} $$ . Wall-layering CH consists of fibrous quartz crystals and exhibits higher C-ratios, $${{C = c_{{\text{(H}}_{\text{2}} {\text{O)SiOH}}} } \mathord{\left/ {\vphantom {{C = c_{{\text{(H}}_{\text{2}} {\text{O)SiOH}}} } c}} \right. \kern-\nulldelimiterspace} c}_{{\text{(H}}_{\text{2}} {\text{O)tot}}} $$ , than horizontally layered FQ which consists predominantly of granular quartz crystals (C CH=0.45±0.11 (N=6), C FQ=0.36±0.10 (N=4). This result is interpreted to be due to analogy with the behaviour of C-ratios in fluid phase-deposited opals-AN (hyalithe) and liquid phase-deposited opals-AG (non-crystalline opal) or -CT (common opal) (Langer and Flörke 1974). Translucent layers of CH show mostly lower refractive indices, when measured parallel than when measured perpendicular to the axes of the quartz fibers. The same is true for milky layers of CH. Crystallite sizes are smaller in the latter than in the former. For all samples studied, exists a positive correlation between σ at% (1/2Ca+1/2Mg+Na+K+Li) and σ at% (Al3++Fe3+). This indicates that at least parts of (A13++ Fe3+) substitute for Si in the quartz structure. The charge is balanced by incorporation of di- and mono-valent cations in structural interstices. When the quantity at % H+, as obtained from $$c_{{\text{(H}}_{\text{2}} {\text{O)SiOH}}} $$ , is included into the sum σ at% (1/2 Me2++Me+), the above correlation is destroyed. This result could be indicative for a strong concentration of the Si-OH groups in the surface of the quartz microcrystallites.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00378005
Permalink
