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About Terlingua Calcite

As published in the UV waves, the newsletter of the Fluorescent Mineral Society

More About Terlingua Calcite

Aside from major works on fluorescent minerals like Fluorescence:  Gems and Minerals Under Ultraviolet Light (Robbins, 1994) and Ultraviolet Light and Fluorescent Minerals (Warren, Gleason, Bostwick & Verbeek, 1995) there is little information about Terlingua calcite to be found without serious searching.  I have encountered many isolated sentences or paragraphs in various publications, including the UV Waves and the Journal of the Fluorescent Mineral Society, and no doubt many others exist that I have not yet discovered, but they usually only echo well known facts.

I have no interest in disputing any previously published comments. I hope only to add to them and to clarify some statements based on my observations of the thousands of Terlingua calcite specimens I've been fortunate to save from destruction (see UV Waves, May/June 2004). It's possible that much of the following - maybe all of it - has been previously noted or may be known by some of the true fluorescent experts.  If you are one of those experts, I hope you stay awake while reading this. For the rest of you, I can only hope you come away with additional appreciation for a truly superb fluorescent mineral.

As always "Terlingua calcite" refers only to that which is typically described as having bright blue fluorescence SW with strong blue phosphorescence, then changing to bright pink fluorescence LW without phosphorescence. It's worth noting that I've seen several references to Terlingua calcite having phosphorescence after LW activation. While I have observed this in a very small number of pieces, my inclination, though I cannot prove it, is that the PH is actually coming from associated (non-Terlingua) calcite that sometimes does PH after LW activation.  (More on this in a future article.)

It's also worth noting that variations in UV lamps can cause variations in fluorescent color in Terlingua calcite, especially LW. Like most of us, I suppose, my first UV lights were the little four-watt portables.  I went through several of these before 'graduating' to a twelve-watt model.  These were all made by well-known companies.  But fairly early in my search for Terlingua calcite, I noticed that hardly any showed the bright pink LW color it was "supposed" to have - it was almost all the orange-pink which is also often described.  This raised several questions in my mind, so one year I took a strongly fluorescent eight-pound piece to a major show and examined it under the lights of a well-known dealer. Out of four (I think, but maybe three) LW lights by the same manufacturer only one showed the bright pink I was looking for.  The others produced the same orange-pink response that my other lights had. This would seem to indicate that extremely minor variations in components could produce different LW responses.  This could also mean that studies, formal or informal, could produce different results even if the lamps used were produced at the same time by the same manufacturer.  The nine watt SW/LW lamp I now use consistently produces the pink response, but I have not examined a single specimen under several different lamps from this manufacturer, so I have no way to know if the results are consistent.

I have samples of several of the Terlingua-like calcites, but have no in-depth knowledge of any of their individual characteristics; therefore my discussion is necessarily limited to only the material from Terlingua and specifically from the Little 38 mine, the (former) primary source.

Probably the most important omission from previous discussions is that of structure. Terlingua calcite has been described as both massive and as crystalline and both descriptions are accurate - sometimes.  As an amateur I hesitate to attempt a precise mineralogical definition of either, so I hope the following is technically correct as well as descriptively sufficient for the purpose of this paper. The crystalline variety of Terlingua calcite typically has euhedral crystals (scalenohedral or modified scalenohedral) exposed above the surface grading into anhedral growth below the surface while the massive variety is just, well, massive - having no external crystal faces.

About two years ago I was pondering the differences between the two structures and decided to determine, the best I could, what percentage of the total each was.  I gathered up most of my larger massive pieces and weighed them.  Having collection records of all (ok, most, not all) Terlingua calcite I had recovered, I calculated that just over three percent was massive.  But being acutely aware that my methodology was very unscientific, since a number of massive green pieces were in ten gallon buckets or stashed under tarps to retain the original green color, I revised my official estimate to "probably about five percent." Close enough

The importance of crystal structure becomes evident when examining what we are all interested in - fluorescent intensity. Fluorescence in Terlingua calcite is usually characterized by highly fluorescent three-dimensional triangular zones.   Some pieces may have only one distinct triangular zone that dominates the overall fluorescence, while others may have several, similar in size and equally fluorescent.  These zones are then surrounded by a much weaker fluorescing area that usually has a few scattered, very small triangular zones as well as several scattered fluorescent lines or "streaks." These lines range from slightly elongated dots to, rarely, streaks over a centimeter in length.  Most are only a few millimeters long and are essentially two-dimensional with a diameter less than .5mm, much like the mark of a "medium" ball-point pen on a piece of paper.

While the above description could be considered accurate for the majority of crystalline pieces, it falls short in the massive pieces.  The difference is in the concentration of fluorescent triangles, lines and streaks. In the massive variety they are highly concentrated - super concentrated might be more descriptive - to the extent that individual lines or triangles often cannot be distinguished within a mass of fluorescence. The result is that massive pieces overall are much more fluorescent than crystalline pieces. The only exceptions that I have observed are in comparisons of small, less than 5 cm, cleavages of exceptionally fluorescent crystalline pieces with equal size massive pieces. Those comparisons show approximately equal fluorescent intensity.

Crystals

It has been written that Terlingua calcite crystals rarely fluoresce.  This is true for a couple of reasons if we are looking at the exposed surfaces of euhedral crystals or crystal clusters.  A secondary reason might be attributed to a final stage of mineralization of what appears to be a mixture of iron-rich clay, coarsely crystalline calcite and gypsum that is present on most crystals, only partially dissolves in acid and is difficult to remove otherwise.  However, night collecting using SW UV revealed the UV waves penetrated fairly thick 'dustings' or thin coatings of clay, producing the typical blue response. Usually this blue fluorescence was weak enough to be extremely difficult to distinguish from other SW responses (pale blue, creamy white, creamy yellow, green, etc.) in the tailings, but the PH intensity showed considerable contrast to the PH of the aragonite and other calcites that PH.  As a result I always used a sweeping motion, looking for PH rather than FL.

These observations lead me to another conclusion that seems to be the primary reason crystals rarely fluoresce.  What appears to be the final stage of calcite deposition (at least nine stages are present in nearby non-Terlingua calcite crystals) or post-deposition alteration resulted in an outer layer of creamy white calcite that does not fluoresce and clearly does not allow penetration of UV waves.  This white layer measured 8 mm thick on one specimen but most are a maximum of 4-5mm grading to zero as one crystal in a cluster approaches another or as it extends subsurface.

Looking at the cleaved portion of a crystal, however, almost always shows the typical triangular FL zones that increase in size - and intensity - in proportion to the distance from the crystal termination and size of the crystal.  Since some 95% of all Terlingua calcite is the crystalline variety, it follows that 95% of specimens owned by collectors are also crystalline and of those specimens it could ! be presumed that many would have at least one surface where this can be observed.

It is also the interior portion of euhedral crystals that the gray daylight color is almost always observed and is where FL lines and streaks are most noticeable.  The gray portion extends from the triangular zones that are usually in the approximate center of the crystal outward to the white rim just described. This gray area is usually very transparent and is the only area where hydrocarbon inclusions are common. On several occasions I have cleaved across an inclusion and watched as tar oozed out.  Within two or three days the tar blob was reduced to a black speck after the more volatile components evaporated.  Most of the hydrocarbon inclusions FL a creamy yellow LW.  Neither hydrocarbon inclusions nor similarly colored gray areas have been observed in the massive variety.

What is undoubtedly one of the most under appreciated properties of the more common fluorescent minerals is the multiple color responses of Terlingua calcite under longwave illumination. In a future paper (in process) for the UV Waves I'll share my observations of both longwave and shortwave color variations, other properties of Terlingua calcite and the variety of other fluorescent responses from the Little 38 Mine.

 If anyone has specific questions or comments about this article, feel free to contact me at fred@terlinguacalcite.com

Fred Gossien

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