Macdonaldit
| Macdonaldit | |
|---|---|
 |
|
| General and classification | |
| other names |
IMA 1964-010 |
| chemical formula | BaCa 4 [Si 8 O 18 OH] 2 • 10H 2 O |
|
Mineral class (and possibly department) |
Silicates and Germanates |
|
System no. to Strunz and to Dana |
9.EB.05 ( 8th edition : VIII / H.38) 72.05.01.03 |
| Crystallographic Data | |
| Crystal system | orthorhombic |
| Crystal class ; symbol | orthorhombic, dipyramidal 2 / m 2 / m 2 / m |
| Room group (no.) | Cmcm (No. 63) |
| Lattice parameters | a = 14.08 Å ; b = 13.11 Å; c = 23.56 Å |
| Formula units | Z = 4 |
| Frequent crystal faces | {010}, {001}, {100} |
| Physical Properties | |
| Mohs hardness | 3.5 to 4 |
| Density (g / cm 3 ) | measured: 2.27 (2); calculated: 2.27 |
| Cleavage | perfect after {010}, good after {001}, indistinct or break after {100} |
| colour | colorless, white |
| Line color | White |
| transparency | transparent to translucent |
| shine | Silk gloss to glass gloss |
| Crystal optics | |
| Refractive indices |
n α = 1.518 n β = 1.524 n γ = 1.530 |
| Birefringence | δ = 0.012 |
| Optical character | alternating biaxially |
| Axis angle | 2V = measured: 90 °; calculated: 88 ° |
Macdonaldite is a very rare mineral from the mineral class of " silicates and germanates ". It crystallizes in the orthorhombic crystal system with the composition BaCa 4 [Si 8 O 18 OH] 2 • 10H 2 O, so it is chemically a water-containing barium - calcium- silicate.
Macdonaldit usually develops needle-like crystals up to about six millimeters in length, stretched at right angles to the a-axis , which are usually colorless and transparent and have a glass-like sheen on their surfaces . The mineral can also be found in the form of fibrous, granular or radial mineral aggregates , which appear white due to their multicrystalline formation due to multiple refraction of light, whereby the transparency decreases accordingly and, especially with fibrous aggregates, a surging light similar to that of silk is created.
With a Mohs hardness of 3.5 to 4, Macdonaldite is one of the medium-hard minerals that, like the reference mineral fluorite (4), can be easily scratched with a pocket knife.
Etymology and history
Macdonaldite was first discovered on the Rush Creek and Big Creek Rivers in Fresno County of the US state of California . It was described in 1965 by John T. Alfors, Melvin C. Stinson, Robert A. Matthews and Adolf Pabst , who named the mineral after the American volcanologist Gordon Andrew MacDonald (1911–1978). In addition to the Macdonaldite, their first description included six other barium minerals from the find area in eastern Fresno County: Krauskopfite , Walstromite , Fresnoite , Verplanckite , Muirit and Traskit .
Type material of the mineral is stored among other things in the "California Division of Mines & Geology" in San Francisco (California, USA).
classification
In the meantime outdated, but still in use 8th edition of the mineral classification by Strunz of Macdonaldit belonged to the mineral class of "silicates and Germanates" and then to the Department of " phyllosilicates (phyllosilicates)" where he collaborated with known Cymrite , Delhayelith , Hydrodelhayelith , Kampfit , Lourenswalsit , Monteregianit- (Y) , Rhodesit , Tienshanit and Wickenburgit formed an independent group.
The 9th edition of Strunz's mineral systematics , which has been in effect since 2001 and is used by the International Mineralogical Association (IMA), also assigns Macdonaldite to the class of “silicates and germanates” and there to the “phyllosilicates” section. This section is, however, further subdivided according to the structure of the layers, so that the mineral can be found according to its structure in the sub-section “Double networks with 4 and 6 participating rings”, where the unnamed group 9.EB.05 forms.
The systematics of minerals according to Dana , which is mainly used in the English-speaking world , assigns Macdonaldite to the class of "silicates and Germanates" and there in the department of "layered silicates: two-dimensional unlimited layers with rings other than six-membered". Here it is together with rhodesite, monteregianite- (Y), delhayelite, hydrodelhayelite and seidite- (Ce) in the "rhodesite group" with the system no. 72.05.01 within the sub-section " Layered silicates: two-dimensional unlimited layers with rings other than six-membered: corroded and complex layers ".
Education and Locations
Macdonaldite forms in the form of small veins, brittle crusts or sprinkled in metamorphic rocks containing sanbornite and quartz .
As a very rare mineral formation, Macdonaldite could so far (status: 2011) only be detected in a few samples or a few sites. In addition to its type locality, the Rush Creek and Big Creek rivers in Fresno County, the mineral was also found in the USA in the sanbornite deposit at Trumbull Peak west-northwest of Incline in Mariposa County and at the "Baumann prospect" in Chickencoop Canyon near the city Exeter in Tulare County.
The only other previously known site is Italy, more precisely the Vispi mine near San Venanzo in the province of Terni (Umbria).
Crystal structure
Macdonaldite crystallizes orthorhombically in the space group Cmcm (space group no. 63) with the lattice parameters a = 14.08 Å ; b = 13.11 Å and c = 23.56 Å as well as 4 formula units per unit cell .
See also
Individual evidence
- ↑ a b c d e Hugo Strunz , Ernest H. Nickel: Strunz Mineralogical Tables . 9th edition. E. Schweizerbart'sche Verlagbuchhandlung (Nägele and Obermiller), Stuttgart 2001, ISBN 3-510-65188-X , p. 661 .
- ↑ Webmineral - Macdonaldite (English)
- ↑ a b c Handbook of Mineralogy - Macdonaldite (English, PDF 76.3 kB)
- ↑ a b c d Macdonaldite at mindat.org (engl.)
literature
- John T. Alfors, Melvin C. Stinson, Robert A. Matthews, Adolf Pabst: Seven new barium minerals from eastern Fresno County, California , in: American Mineralogist , Vol. 50 (March – April 1965), pp. 314–340 ( PDF 1.67 MB )
Web links
- Mineral Atlas: Macdonaldit (Wiki)