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Good Neighbors

Article reprinted with permission and originally appeared in the April Issue of the NYMC Bulletin - http://www.newyorkmineralogicalclub.org/

 “It’s Elemental” is a series of columns by Bill Shelton written this in year in recognition of the United Nations’ International Year of the Periodic Table of Chemical Elements.

This month, we will focus on three elements that are located next to each other. Starting with Cr, #24, we proceed right to Mn, #25 and Fe, #26. The idea that the periodic table groups similar elements in vertical columns is at the base of why we call it periodic [= repeating]. In some cases, such as this group of three, similarities can exist among neighboring elements. Minerals illustrate numerous examples where iron can be substituted for manganese and vice versa. One common example is rhodochrosite [MnCO3] and siderite [FeCO3]. Chromium is far less common but can be seen to follow a similar procedure in chromite. Iron will replace a percentage of the chromium and this leads to ferrian chromite as we see in Deer et al, 2011.

A periodic-table-printable.jpg

On the periodic table, we can sometimes find information that may be useful in mineral studies. First, the valence helps explain likely substitutions in minerals. When the valence is equal, two elements of similar size are likely to replace each other. Mn 2+ and Fe 2+ are commonly seen; Fe 3+ and Cr 3+ also occur but less frequently. The ionic radius for Mn 2+ is 0.80 and Fe 2+ is 0.74. Fe 3+ is 0.64 while Cr 3+ is 0.63. See Klein and Hurlbut, Jr, 1985. Essentially the same crystal structure is assigned to each element. The common presence of iron, with 1,150 species, and manganese with 581 looks very different from chromium with only 97 species. If we rank order elements, iron is number 4, manganese is number 9 and chromium is number 40 with regard to the number of mineral species.

 Solid solution suggests two or more elements will share and replace one another in a mineral so that we may find an iron-rich species [an end member] and a manganese-rick species such as rhodochrosite and siderite. Analytical results range from pure Mn and no Fe for rhodochrosite to examples where Mn is essentially equal to Fe. The values given are Mn = 1.003 and Fe =0.946. I have seen a piece from Kazakhstan that looks to be brown but the underneath is a bright red core – maybe the crystal started as rhodochrosite and then ended up as siderite. Regarding siderite, nearly pure Fe with a small amount of Mn [0.01] is known and other examples range to where the ratio is 0.68 Mn to 0.98 Fe. Not quite equal but approaching that value, it should become clear that natural samples can and do cover a wide range of values. This situation may be referred to as a complete solid solution series.

 Chromium, being a lot rarer, provides less examples but consider chromite. Here, Fe 3+ replaces Cr 3+ and grades toward ferrian chromite (as we see in Deer et al, 2011.) Here, the chromium values range from 7.8 to 14.3 and the iron values are from 0.1 to 1.7. The wt % shows quite pure chromite with 58 % Cr and 0.5 % Fe. This grades to examples with 45% Cr and 8.5% Fe. I would call this a partial solid solution series.

 If we compare uvarovite garnets, based on molecular end members, they range from 91.2 to 36.3 UV. The GR values range from 0 to 41 while PY ranges from 0.2 to 34.1 SP values are between 0 and 1.9; AN ranges from 0 to 18. AN reflects iron and SP reflects manganese present; UV represents chromium here. A rather more complex example as you can see but it is common to find several elements present in certain minerals as we see here.

 The same three elements affect the color of spinel. V and Co also can play a role but Cr is important in most red examples. Mn is linked to some green crystals and the presence of iron is noted in all colors! See Schmetzer, 1987. Green may be related to zinc or chromium-rich examples. Cr can be the culprit for more that one color. Strange as it may seem, Cr causes red or pink in spinel but very high levels, over 15 %, actually produce green crystals. Olivine group members can have chromium present; it is usually a small amount such as 0.70 % or less. The manganese content can be zero but also has been measured up to nearly 8 %. They share a common structural position and we may find small amounts of Ca, Ni, Al, Ti, K and Na present as well. So, we might decide that the neighbors seem to get along well.