Dawsonite

Dawsonite is a rarely occurring mineral from the mineral class of " carbonates and nitrates " (formerly known as carbonates, nitrates and borates, see Classification ). It crystallizes in the orthorhombic crystal system with the composition NaAl [( OH) 2 | CO 3 ], and is therefore in chemical terms, a sodium aluminum carbonate with additional hydroxide ions.

Dawsonite developed fine noble to foliated crystals of up to 3.5 centimeters in length, which are stretched to the c-axis. In general, these are arranged in rosette-shaped, radialstrahligen to wispy or fibrous aggregates.

In its pure form dawsonite is colorless transparent and shows a glass-like shine on the crystal faces. But it can also appear white by multiple refraction due to fibrous feinnadeliger training, with a billowing, shimmering shine similar to the created of silk and decreases the transparency accordingly. Rarely occur by foreign admixtures of chromium pink colored dawsonites.

With a Mohs hardness of 3 dawsonite is one of the medium-hard minerals that can be scratched with a copper coin.

Special Features

Before the blowpipe swelling under intense yellowing of the flame, but not melted [ ... ] In the flask only on strong heating water and carbon dioxide tampering; remains stable up to 140 ° C.

When carbonate is dawsonite already dissolved in very dilute hydrochloric acid with evolution of carbon dioxide. Also in citric acid, ammonia or other alkalis it is soluble. About Here it can be distinguished from the similar looking zeolites.

Etymology and history

Dawsonite was first described in 1874 by Bernard J. Harrington. The mineral was discovered in a Feldspatgang, which was excavated as part of construction work on the grounds of McGill University in Montreal ( Canada). Harrington himself worked at this university and honored with the naming of the mineral John William Dawson, the former director of the University.

Classification

In the now outdated but still in use 8th edition of the mineral classification by Strunz the dawsonite still belonged to the common mineral class of " carbonates, nitrates and borates " and then to the Department of " water clear carbonates with foreign anions ", where he along with Barensit and Tunisite named after him " dawsonite - group " with the system no. V/C.02 formed.

The 9th edition used since 2001 and valid by the International Mineralogical Association (IMA ) of the Strunz'schen Mineral classification assigns the dawsonite in the newly defined class of " carbonates and nitrates " ( here, the borates form a separate class), but there also the department of a " carbonates with other anions, without H2O". However, this is further subdivided according to the conditions prevailing in the connection cations ( mostly metals ), so that the mineral according to its composition in the subdivision is " with alkalis, etc. " to find where it is the only member of the unnamed group 5.BB.10 forms.

The mostly commonly used in English-speaking classification of minerals according to Dana assigns the dawsonite as the old Strunz'sche systematics in the common class of " carbonates, nitrates and borates " and there in the department of " carbonates with hydroxyl or halogen" one. Here he can be found as the only member of the unnamed group 16a.03.08 within the subdivision " carbonates with hydroxyl or halogen, and the general formula ( AB) 2 (XO) 3Zq ".

Education and Locations

Dawsonite can be found on clefts hydrothermally altered feldspar rocks in nepheline and sodium-rich sediments. Accompanying minerals occur among others albite, analcime, aragonite, calcite, dolomite, fluorite, halite, cryolite, pyrite and quartz

A rare mineral formation dawsonite has so far (as of 2012) are detected only at a few localities, of which approximately 80 localities are known. Apart from its type locality McGill University in Montreal, the mineral occurred in Canada still at the Grey River, Newfoundland, at the Princess Sodalite Quarry in Hastings County ( Ontario), the Muskiki Lake in Saskatchewan, as well as at several other discovery locations around Montreal and Montérégie on.

In Austria, the mineral is found so far only in discovered in Loipersdorf bei Furstenfeld thermal spring and in the tunnel " Karl- August " in Fohnsdorf in Styria.

The only known locality in Switzerland is the Simplon Pass near Brig, where the mineral was discovered during the construction of the Simplon Tunnel in the resulting excavated material.

Other localities lie among others in Albania, Algeria, Argentina, Australia, Belgium, Bolivia, China, Italy, Japan, Mexico, New Zealand, Russia, Slovakia, Tanzania, Czech Republic, Hungary and the United States of America (USA).

Crystal structure

Dawsonite crystallizes in the orthorhombic space group Imam ( Raumgruppen-Nr. 74) with the lattice parameters a = 6.76 Å; b = 10.24 Å and c = 5.58 Å and four formula units per unit cell.