Aragonite Group - Aragonite, Strontianite, Witherite & Cerusite
The Aragonite Group consist of a series o f carbonates of the bivalent metals, calcium, strontium, barium and lead, which crystallize in the Orthorhombic System with closely related crystal constants and similar habits of crystallization. All of them appear at times in twin crystals which are pseudohexagonal in character. The members of the group are:
Aragonite
Composition. Calcium carbonate, like calcite, AaCO3 = Carbon dioxide 44, lime 56. May contain a little strontium or lead, rarely zinc.
Crystallization. Orthórhombic. Three prominent habits of crystallization: (1) Acicular pyramidal; consisting of a prism terminated by a combination of a very steep pyramid and brachy-done. Usually in radiating groups of large to very small crystals. (2) Tabular; consisting of prominent brachypinacoid faces modified by a prism and a low brachydome. Often twinned with a prism face as a twinning plane. (3) In pseudohexagonal twins. This type shows hexagonal-like prism terminated by a basal plane, and is formed by a n intergrowth of three individuals with basal plane, and is formed by an intergrowth of three individuals with basal planes in common and their prism faces falling partly in the same plane, and partly with only slightly different positions. The crystals are distinguished from true hexagonal forms by noting that the basal plane is striated in three different directions, and also by the fact that, because the prism angle of the simple crystals is not exactly 60, The composite prism faces for the twin will often show slight reentrant angles.
Structure. In crystals. Also reniform, columnar, stalactitic, etc.
Physical Properties. Vitreous luster. Colorless, white pale yellow and variously tinted. Transparent to translucent. H. = 3.5-4 G. = 2.95 (harder and heavier than calcite).
Test. Infusible. Decrepitates. After intense ignition the powder gives an alkaline reaction on moistened test paper. Fragments fall to powder (change to calcite) when heated at low redness in C. T. Chemical tests same as for calcite: Distinguished from calcite by its lack of cleavage, and the fact that fragments fall to powder when heated in C.T.
Occurrence. Less stable than calcite and much less common in its occurrence. Usually found as a vein mineral. Experiments have shown that carbonated waters containing calcium more often deposit aragonite when they are hot and calcite when they are cold. Some sea shells are composed entirely or in part of aragonite. The pearly layer of many shells is aragonite. It has been noted that the aragonite shells are not readily preserved as fossils, being easily dissolved or disintegrated, or at times apparently slowly changing to calcite. Aragonite is most commonly found associated with beds of gypsum and deposits of iron ore (where it sometimes occurs in forms resembling coral, and is called flos ferry, flower of iron). At times found lining amygdaloidal cavities in basalt, Found frequently with pyrite, chalcopyrite, galena, malachite, etc. Notable localities for the various crystalline types are as follows: Pseudohexagonal twin crystals are found in Aragon, Spain; Bastennes, in the south of France; and at Girgenti, Sicily. The tabular type of crystals is found near Bilin, Bohemis. The acicular type is found at Alston Moor and Cleator Moor, Cumberland, England. Flos ferri is found in the Stryian iron mines. The flos- ferri variety occurs in the Organ Mts., New Mexico and in Bisbee, Arizona.
|
