A. Goodletite cabochon (27.94 ct) fashioned from rock from South Island, New Zealand. H. & H. Bracewell collection. (© photo by Grahame Brown)
B. Goodletite from the South Island of New Zealand as viewed with a hand lens - representative circular section was selected from Dr. Brown's original rectangular photo to direct attention to the handlens aspect of the illustration. H. & H. Bracewell collection. (© photo by Grahame Brown)
C. Goodletite cabochon (vertical dimension - 2.0 cm) made from rough from near Hokitika, South Island, New Zealand. This cabochon, actually a doublet with a quartz dome, consists largely of blue sapphire, red ruby and green chromian mica plus minor tourmaline. New Zealand Ruby Rock Ltd. (© photo by Gerry Commandeur)
DESCRIPTION: This gemrock, which I have not seen, has been described differently in different publications. O'Donoghue (1997) describes what he calls "Two types of goodletite" -- 1."green only ... [that] could pass as 'jade' to the unwary" and 2.something roughly similar to that described by Brown and Blackwell (1996), which Brown (personal communication, 1998) has clarified simplified as consisting of "anhedral grains of ruby and sapphire in a matrix of green chrome-rich mica and green tourmaline." Mitchell (1985) gives yet another definition as his #1: "Green-pyroxene or green-amphibole rock containing ruby corundum." Grapes and Palmer (1996) note that the mica content is chromian muscovite and margarite, typically with larger percentages of the muscovite and that some of the rocks contain noteworthy amounts of chromian chlorite, Zn-Mn chromite and Mn-Ti magnetite. To further complicate application of this name, Shipley (1951) notes that it is "A name for Burmese marble forming matrix of rubies."
The following, slightly modified, is based
largely on Brown and Blackwell's data and
description, which seems well founded and relatively complete on the
basis of what I have seen in photographs of the material.
Colors - the green appears to largely, if not wholly, due to green mica (i.e., chrome muscovite) and/or green tourmaline; the red is red corundum(~ruby) and/or pink tourmaline; the blue is blue corundum(sapphire) and/or bluish tourmaline; the pinkish gray is corundum. Properties of these typically present macroscopically visible constituents follow:
H. effective hardness, is due to presence of the corundums -- H. 9; other values are muscovite -- H. 2½ - 3; margarite --H. 3½ - 4½; and tourmaline -- H. 7.
S.G. 3.3 - 3.8, which ranges with the percentages of the different minerals present in any given piece; specific gravity of the chief minerals are: corundum -- 4.0 - 4.1; margarite ~3.1; micas ~ 2.8; tourmaline ~ 3.25.
Light transmission - overall translucent to subtranslucent
Luster - overall subvitreous
Miscellany - Each individual specimen, piece of rough and article fashioned from this rock is likely to look rather different from others. The differences, on the basis of photographs I have seen (and I should add quite what one should expect), are more marked than for nearly all other coarse grained gemrocks.
So far as the appearances of the chief constituents, the following seem noteworthy: Corundum breaks perpendicular to the long dimension of prismatic grains to give virtually perfect to roughly hexagonal flat surfaces that have the appearance of fair to good cleavage -- actually, these surfaces represent a property called parting. Tourmaline grains that occur as crystals commonly appear to be striated parallel to their lengths and have cross-sections that resemble spherical triangles. The fact that fuchsite and other green micas exhibit perfect cleavage into elastic sheets is of little use so far as macroscopic examination because the mica in these rocks typically comprises fine grained masses in which the cleavage is not discernible.
USES: Composite gemstones (doublets and triplets), usually relatively large so all (or most) of the contituent minerals are present (see Figure C) -- however, individual minerals have been highlighted in stones, most of which are cabochons or free forms, used in rings and smaller pieces; also miscellaneous ornaments.OCCURRENCES: This rock has not been found in place -- i.e., as bedrock. It has been found only as cobbles and boulders in glacial moraine debris and in rivers of New Zealand's Southern Alps. On the basis of extensive petrographic and chemical studies Grapes and Palmer (1996) conclude that the rock was formed by "extrene metasomatism of quartzofeldspathic schist enclaves in serpentinite" of the ultrabasic complex east of where the boulders were found. Indeed, at least one boulder had serpentinite spatially associated with this rock.
REMARKS: This gemrock was named after William Goodlett, who isisaid to have taken a sample of the rock for identification and examination to Otago University, Dunedin, New Zealand.
Despite O'Donoghue's previously cited statement implying that goodletite resembles jade, he adds his opinion to the effect that the color resembles "the 'Bonamite' variety of smithsonite more than either of the jade minerals." From the several photographs and other descriptions of these I have seen, this statement leaves me "at loss."
According to Mitchell (1985) the name goodletite
has also been
applied to a rock from Myanmar (formerly Burma) that consists of rubies
and their surrounding marble
matrix. I have been unable to find such application of the
name. In any case, this use should be abandoned. In fact,
one has to wonder if it might be prudent to abandon the name goodletite
-- which, at best, is an informal name for the rock(s?) described in
this entry -- so far as applying it to any rock. But, consider
the following: 1. this New Zealand rock does not seem to fit into
any petrographic pigeon-hole; 2.although its origin has been
hypothesized (see under OCCURRENCE subheading), its geologic occurrence
than as loose boulders) is not known; [and] 3.without such
fit or occurrence information, any at all logical name -- e.g.,
listing its constituent minerals -- would be far too cumbersome for the
marketplace. Thus it appears that at least for the time being
Goodletite is the name to use for these rocks (but NOT for the just
described ruby-bearing marble from Myanmar), eh!?!
SIMULANTS: None that I have seen or seen described.
REFERENCE: Brown and Blackwell, 1996; Grapes and Palmer, 1996; O'Donoghue, 1997.
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