Mixite (#88)

Lustrous bright bluish-green micro sprays of mixite on hard altered limonite matrix.
North America Sioux-Ajax Mine, Tintic District, Juab County, Utah 3 ¼" x 2 ½" x 1 ¾"

 

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Mixite is a rare mineral with the fomlula BiCu6(AsO4)3(OH)6·3H20. We have seen all of the symbols except Bi in other minerals in this display. Bi is the symbol for bismuth, and, if you have lost track of Cu and As, they are copper and arsenic, respectively. AsO4 is in parentheses because arsenic atoms are each surrounded by four oxygen atoms that form a strongly-bonded little structural group, and these are linked together by weaker bonds involving bismuth, copper, hydroxyl ions (OH) and water molecules, hence making this mineral an arsenate.

Mixite is also the mineral for which the mixite group of minerals is named. The mixite group consists of a dozen minerals of different chemical compositions that have very similar crystal structures. All of the members of the group are arsenates or phosphates, and they are visually indistinguishable from one another. So how does one tell them apart, or even know that they are different? Because visual inspection will not help greatly, one uses instrumental techniques, of which mineralogists tend to favor those involving X-rays: (1) A sample of the mineral is placed under a beam of electrons, which knock electrons off of the atoms in the mineral. This generates X-rays with wavelengths characteristic of the elements involved, and a qualitative analysis (what elements are present?) or quantitative analysis (what elements are present and how much of each?) is done. (2) A specimen of the mineral is placed in an X-ray beam, the angle of the beam is varied, and the X-rays interact with the atoms of the mineral in such a way as to produce a pattern of peaks as a function of beam angle. The minerals of the mixite group (for example) will produce similar patterns of peaks, but their exact positions and intensities will reveal which member of the group we have. (For minerals not at all related, the patterns of peaks will typically be very different.)